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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 16/Times-Roman@0 SF
(The Unix I/O System)237.336 130 Q/F2 10/Times-Italic@0 SF
(Dennis M. Ritc)269.83 159 Q(hie)-.15 E(Bell T)249.035 170 Q
(elephone Labor)-.92 E(atories)-.15 E F0 .494(This paper gi)72 231.4 R
-.15(ve)-.25 G 2.994(sa).15 G 2.994(no)-2.994 G -.15(ve)-3.144 G(rvie)
.15 E 2.994(wo)-.25 G 2.994(ft)-2.994 G .494(he w)-2.994 F .494
(orkings of the Unix I/O system.)-.1 F .493(It w)5.493 F .493
(as written with an e)-.1 F .493(ye to)-.15 F -.1(wa)-.25 G .493(rd pro)
.1 F(viding)-.15 E .46(guidance to writers of de)72 242.4 R .46
(vice dri)-.25 F -.15(ve)-.25 G 2.96(rr).15 G .461
(outines, and is oriented more to)-2.96 F -.1(wa)-.25 G .461
(rd describing the en).1 F .461(vironment and nature of)-.4 F(de)72
253.4 Q(vice dri)-.25 E -.15(ve)-.25 G(rs than the implementation of th\
at part of the \214le system which deals with ordinary \214les.).15 E
.393(It is assumed that the reader has a good kno)72 269.9 R .392
(wledge of the o)-.25 F -.15(ve)-.15 G .392
(rall structure of the \214le system as discussed in the pa-).15 F .265
(per `)72 280.9 R .265(`The Unix T)-.74 F .265(ime-sharing System.)-.35
F 4.245 -.74('' M)-.7 H(oreo).74 E -.15(ve)-.15 G 2.765(rt).15 G .265
(he present document is intended to be used in conjunction with a)-2.765
F(cop)72 291.9 Q 2.5(yo)-.1 G 2.5(ft)-2.5 G
(he system code, since it is basically an e)-2.5 E -.15(xeg)-.15 G
(esis of that code.).15 E/F3 10/Times-Bold@0 SF(De)72 313.9 Q
(vice Classes)-.15 E F0 .924(There are tw)72 330.4 R 3.424(oc)-.1 G .924
(lasses of de)-3.424 F(vice:)-.25 E F2(bloc)3.424 E(k)-.2 E F0(and)3.424
E F2 -.15(ch)3.424 G(ar).15 E(acter)-.15 E(.)-1.11 E F0 .924
(The block interf)5.924 F .924(ace is suitable for de)-.1 F .924
(vices lik)-.25 F 3.423(ed)-.1 G .923(isks, tapes,)-3.423 F .208
(and DECtape which w)72 341.4 R .209(ork, or can w)-.1 F .209
(ork, with addressible 512-byte blocks.)-.1 F .209
(Ordinary magnetic tape just barely \214ts in)5.209 F .699(this cate)72
352.4 R(gory)-.15 E 3.199(,s)-.65 G .699(ince by use of forw)-3.199 F
.699(ard backw)-.1 F .699(ard spacing an)-.1 F 3.199(yb)-.15 G .699
(lock can be read, e)-3.199 F -.15(ve)-.25 G 3.199(nt).15 G .699
(hough blocks can be written)-3.199 F .782(only at the end of the tape.)
72 363.4 R .782(Block de)5.782 F .782
(vices can at least potentially contain a mounted \214le system.)-.25 F
.782(The interf)5.782 F .782(ace to)-.1 F .245(block de)72 374.4 R .245
(vices is v)-.25 F .245(ery highly structured; the dri)-.15 F -.15(ve)
-.25 G .244(rs for these de).15 F .244(vices share a great man)-.25 F
2.744(yr)-.15 G .244(outines as well as a pool of)-2.744 F -.2(bu)72
385.4 S -.25(ff).2 G(ers.).25 E(Character)72 401.9 Q .271(-type de)-.2 F
.271(vices ha)-.25 F .571 -.15(ve a m)-.2 H .271(uch more straightforw)
.15 F .271(ard interf)-.1 F .271(ace, although more w)-.1 F .272
(ork must be done by the dri)-.1 F -.15(ve)-.25 G(r).15 E(itself.)72
412.9 Q(De)72 429.4 Q .653(vices of both types are named by a)-.25 F F2
(major)3.153 E F0 .653(and a)3.153 F F2(minor)3.152 E F0(de)3.152 E .652
(vice number)-.25 F 5.652(.T)-.55 G .652
(hese numbers are generally stored as a)-5.652 F -.1(wo)72 440.4 S .027
(rd with the minor de).1 F .027(vice number as the lo)-.25 F 2.527(wb)
-.25 G .027(yte and the major de)-2.527 F .027
(vice number as the high byte.)-.25 F .028(The major de)5.028 F(vice)
-.25 E .202(number selects which dri)72 451.4 R -.15(ve)-.25 G 2.702(rw)
.15 G .202(ill deal with the de)-2.702 F .202(vice; the minor de)-.25 F
.202(vice number is not used by the rest of the system)-.25 F -.2(bu)72
462.4 S 2.988(ti).2 G 2.989(sp)-2.988 G .489(assed to the dri)-2.989 F
-.15(ve)-.25 G 2.989(ra).15 G 2.989(ta)-2.989 G .489(ppropriate times.)
-2.989 F -.8(Ty)5.489 G .489(pically the minor number selects a subde).8
F .489(vice attached to a gi)-.25 F -.15(ve)-.25 G(n).15 E(controller)72
473.4 Q 2.5(,o)-.4 G 2.5(ro)-2.5 G(ne of se)-2.5 E -.15(ve)-.25 G
(ral similar hardw).15 E(are interf)-.1 E(aces.)-.1 E .3(The major de)72
489.9 R .3(vice numbers for block and character de)-.25 F .299
(vices are used as indices in separate tables; the)-.25 F 2.799(yb)-.15
G .299(oth start at 0)-2.799 F(and therefore o)72 500.9 Q -.15(ve)-.15 G
(rlap.).15 E F3(Ov)72 522.9 Q(er)-.1 E(view of I/O)-.1 E F0 .282
(The purpose of the)72 539.4 R F2(open)2.782 E F0(and)2.782 E F2(cr)
2.782 E(eat)-.37 E F0 .283
(system calls is to set up entries in three separate system tables.)
2.783 F .283(The \214rst of these)5.283 F .276(is the)72 550.4 R .4 LW
106.442 552.9 101.442 552.9 DL F2 5(uo)96.442 550.4 S(\214le)-5 E F0
.276(table, which is stored in the system')2.776 F 2.776(sp)-.55 G(er)
-2.776 E .276(-process data area)-.2 F F2(u.)2.776 E F0 .275
(This table is inde)5.276 F -.15(xe)-.15 G 2.775(db).15 G 2.775(yt)
-2.775 G .275(he \214le descrip-)-2.775 F .317(tor returned by the)72
561.4 R F2(open)2.817 E F0(or)2.817 E F2(cr)2.817 E(eat,)-.37 E F0 .317
(and is accessed during a)2.817 F F2 -.37(re)2.817 G .317(ad, write).37
F(,)-.1 E F0 .318(or other operation on the open \214le.)2.817 F .318
(An entry)5.318 F .157
(contains only a pointer to the corresponding entry of the)72 572.4 R F2
(\214le)2.657 E F0 .157(table, which is a per)2.657 F .156
(-system data base.)-.2 F .156(There is one en-)5.156 F .192(try in the)
72 583.4 R F2(\214le)2.692 E F0 .192(table for each instance of)2.692 F
F2(open)2.692 E F0(or)2.692 E F2(cr)2.692 E(eat.)-.37 E F0 .192
(This table is per)5.192 F .193
(-system because the same instance of an open)-.2 F .312
(\214le must be shared among the se)72 594.4 R -.15(ve)-.25 G .312
(ral processes which can result from).15 F F2(forks)2.812 E F0 .312
(after the \214le is opened.)2.812 F(A)5.312 E F2(\214le)2.812 E F0 .311
(table en-)2.811 F .51
(try contains \215ags which indicate whether the \214le w)72 605.4 R
.511(as open for reading or writing or is a pipe, and a count which is)
-.1 F .187(used to decide when all processes using the entry ha)72 616.4
R .487 -.15(ve t)-.2 H .186
(erminated or closed the \214le \(so the entry can be abandoned\).).15 F
.288(There is also a 32-bit \214le of)72 627.4 R .288
(fset which is used to indicate where in the \214le the ne)-.25 F .288
(xt read or write will tak)-.15 F 2.788(ep)-.1 G 2.788(lace. Fi-)-2.788
F(nally)72 638.4 Q 2.896(,t)-.65 G .395
(here is a pointer to the entry for the \214le in the)-2.896 F F2(inode)
2.895 E F0 .395(table, which contains a cop)2.895 F 2.895(yo)-.1 G 2.895
(ft)-2.895 G .395(he \214le')-2.895 F 2.895(si)-.55 G 2.895
(-node. Notice)-2.895 F .327(that an entry in the)72 649.4 R F2(\214le)
2.827 E F0 .327(table corresponds precisely to an instance of)2.827 F F2
(open)2.827 E F0(or)2.827 E F2(cr)2.827 E(eat;)-.37 E F0 .328
(if the same \214le is opened se)2.827 F -.15(ve)-.25 G(ral).15 E .181
(times, it will ha)72 660.4 R .481 -.15(ve s)-.2 H -2.15 -.25(ev e).15 H
.181(ral entries in this table.).25 F(Ho)5.181 E(we)-.25 E -.15(ve)-.25
G .981 -.4(r, t).15 H .18(here is at most one entry in the).4 F F2
(inode)2.68 E F0 .18(table for a gi)2.68 F -.15(ve)-.25 G 2.68<6e8c>.15
G(le.)-2.68 E .25(Also, a \214le may enter the)72 671.4 R F2(inode)2.751
E F0 .251(table not only because it is open, b)2.751 F .251
(ut also because it is the current directory of some)-.2 F(process or b\
ecause it is a special \214le containing a currently-mounted \214le sys\
tem.)72 682.4 Q .466(An entry in the)72 698.9 R F2(inode)2.966 E F0 .465
(table dif)2.966 F .465(fers some)-.25 F .465(what from the correspondi\
ng i-node as stored on the disk; the modi\214ed and)-.25 F .713(accesse\
d times are not stored, and the entry is augmented by a \215ag w)72
709.9 R .713(ord containing information about the entry)-.1 F 3.214(,a)
-.65 G(count used to determine when it may be allo)72 720.9 Q
(wed to disappear)-.25 E 2.5(,a)-.4 G(nd the de)-2.5 E
(vice and i-number whence the entry came.)-.25 E EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF 2.5(2-U)72 58
S(nix I/O System)-2.5 E F0 .319(During the processing of an)72 105 R F1
(open)2.818 E F0(or)2.818 E F1(cr)2.818 E(eat)-.37 E F0 .318
(call for a special \214le, the system al)2.818 F -.1(wa)-.1 G .318
(ys calls the de).1 F(vice')-.25 E(s)-.55 E F1(open)2.818 E F0 .318
(routine to)2.818 F(allo)72 116 Q 3.568(wf)-.25 G 1.068(or an)-3.568 F
3.568(ys)-.15 G 1.068(pecial processing required \(re)-3.568 F 1.068
(winding a tape, turning on the data-terminal-ready lead of a modem,)
-.25 F 2.87(etc.\). Ho)72 127 R(we)-.25 E -.15(ve)-.25 G 1.169 -.4(r, t)
.15 H(he).4 E F1(close)2.869 E F0 .369(routine is called only when the \
last process closes a \214le, that is, when the i-node table en-)2.869 F
.314(try is being deallocated.)72 138 R .314
(Thus it is not feasible for a de)5.314 F .315
(vice to maintain, or depend on, a count of its users, although)-.25 F
(it is quite possible to implement an e)72 149 Q(xclusi)-.15 E -.15(ve)
-.25 G(-use de).15 E
(vice which cannot be reopened until it has been closed.)-.25 E 1.566
(When a)72 165.5 R F1 -.37(re)4.066 G(ad).37 E F0(or)4.066 E F1(write)
4.066 E F0(tak)4.066 E 1.566(es place, the user')-.1 F 4.066(sa)-.55 G
-.18(rg)-4.066 G 1.566(uments and the).18 F F1(\214le)4.066 E F0 1.565
(table entry are used to set up the v)4.066 F(ariables)-.25 E .4 LW 89.5
179 84.5 179 DL F1 2.5(u.u base)72 176.5 R 3.65(,u)-.1 G 131.38 179
126.38 179 DL 2.5(.u count,)118.88 176.5 R F0(and)3.65 E 195.34 179
190.34 179 DL F1 2.5(u.u of)177.84 176.5 R(fset)-.18 E F0 1.15
(which respecti)3.65 F -.15(ve)-.25 G 1.15
(ly contain the \(user\) address of the I/O tar).15 F 1.15
(get area, the byte-)-.18 F .094(count for the transfer)72 187.5 R 2.594
(,a)-.4 G .094(nd the current location in the \214le.)-2.594 F .093
(If the \214le referred to is a character)5.094 F .093
(-type special \214le, the ap-)-.2 F .324(propriate read or write routi\
ne is called; it is responsible for transferring data and updating the \
count and current lo-)72 198.5 R .371
(cation appropriately as discussed belo)72 209.5 R 4.171 -.65(w. O)-.25
H .37(therwise, the current location is used to calculate a logical blo\
ck number).65 F .316(in the \214le.)72 220.5 R .317(If the \214le is an\
 ordinary \214le the logical block number must be mapped \(possibly usi\
ng an indirect block\))5.316 F .255(to a ph)72 231.5 R .254
(ysical block number; a block-type special \214le need not be mapped.)
-.05 F .254(In an)5.254 F 2.754(ye)-.15 G -.15(ve)-3.004 G .254
(nt, the resulting ph).15 F .254(ysical block)-.05 F
(number is used, as discussed belo)72 242.5 Q 1.3 -.65(w, t)-.25 H 2.5
(or).65 G(ead or write the appropriate de)-2.5 E(vice.)-.25 E/F2 10
/Times-Bold@0 SF(Character de)72 264.5 Q(vice dri)-.15 E -.1(ve)-.1 G
(rs).1 E F0(The)72 281 Q F1(cde)3.241 E(vsw)-.15 E F0 .741
(table speci\214es the interf)3.241 F .741
(ace routines present for character de)-.1 F 3.242(vices. Each)-.25 F
(de)3.242 E .742(vice pro)-.25 F .742(vides \214v)-.15 F 3.242(er)-.15 G
(outines:)-3.242 E .717(open, close, read, write, and special-function.)
72 292 R(An)5.717 E 3.217(yo)-.15 G 3.217(ft)-3.217 G .717
(hese may be missing.)-3.217 F .717
(If a call on the routine should be ig-)5.717 F .724(nored, \(e.g.)72
303 R F1(open)5.724 E F0 .724(on non-e)3.224 F(xclusi)-.15 E 1.024 -.15
(ve d)-.25 H -.25(ev).15 G .725(ices which require no setup\) the).25 F
F1(cde)3.225 E(vsw)-.15 E F0 .725(entry can be gi)3.225 F -.15(ve)-.25 G
3.225(na).15 G(s)-3.225 E F1(nullde)3.225 E(v;)-.15 E F0 .725(if it)
3.225 F(should be considered an error)72 314 Q 2.5(,\()-.4 G(e.g.)-2.5 E
F1(write)5 E F0(on read-only de)2.5 E(vices\))-.25 E F1(node)2.5 E(v)
-.15 E F0(is used.)2.5 E(The)72 330.5 Q F1(open)3.102 E F0 .602
(routine is called each time the \214le is opened with the full de)3.102
F .601(vice number as ar)-.25 F 3.101(gument. The)-.18 F .601(second ar)
3.101 F(gu-)-.18 E(ment is a \215ag which is non-zero only if the de)72
341.5 Q(vice is to be written upon.)-.25 E(The)72 358 Q F1(close)3.001 E
F0 .502(routine is called only when the \214le is closed for the last t\
ime, that is when the v)3.001 F .502(ery last process in which)-.15 F
.315(the \214le is open closes it.)72 369 R .315
(This means it is not possible for the dri)5.315 F -.15(ve)-.25 G 2.814
(rt).15 G 2.814(om)-2.814 G .314(aintain its o)-2.814 F .314
(wn count of its users.)-.25 F .314(The \214rst)5.314 F(ar)72 380 Q .365
(gument is the de)-.18 F .366
(vice number; the second is a \215ag which is non-zero if the \214le w)
-.25 F .366(as open for writing in the process)-.1 F
(which performs the \214nal)72 391 Q F1(close)2.5 E(.)-.15 E F0(When)72
407.5 Q F1(write)2.559 E F0 .059(is called, it is supplied the de)2.559
F .059(vice as ar)-.25 F 2.559(gument. The)-.18 F(per)2.559 E .059
(-user v)-.2 F(ariable)-.25 E 417.996 410 412.996 410 DL F1 2.5
(u.u count)400.496 407.5 R F0 .059(has been set to the num-)2.559 F .167
(ber of characters indicated by the user; for character de)72 418.5 R
.167(vices, this number may be 0 initially)-.25 F(.)-.65 E 464.789 421
459.789 421 DL F1 2.5(u.u base)447.289 418.5 R F0 .167(is the address)
2.667 F .868
(supplied by the user from which to start taking characters.)72 429.5 R
.868(The system may call the routine internally)5.868 F 3.368(,s)-.65 G
3.367(ot)-3.368 G .867(he \215ag)-3.367 F 89.5 443 84.5 443 DL F1 2.5
(u.u se)72 440.5 R(g\215g)-.4 E F0(is supplied which indicates, if)2.5 E
F1(on,)2.5 E F0(that)2.5 E 286.59 443 281.59 443 DL F1 2.5(u.u base)
269.09 440.5 R F0
(refers to the system address space instead of the user')2.5 E(s.)-.55 E
(The)72 457 Q F1(write)4.86 E F0 2.36(routine should cop)4.86 F 4.86(yu)
-.1 G 4.86(pt)-4.86 G(o)-4.86 E 251.751 459.5 246.751 459.5 DL F1 2.5
(u.u count)234.251 457 R F0 2.361(characters from the user')4.861 F
4.861(sb)-.55 G(uf)-5.061 E 2.361(fer to the de)-.25 F 2.361
(vice, decrementing)-.25 F 89.5 470.5 84.5 470.5 DL F1 2.5(u.u count)72
468 R F0(for each character passed.)2.5 E -.15(Fo)5 G 2.5(rm).15 G
(ost dri)-2.5 E -.15(ve)-.25 G(rs, which w).15 E
(ork one character at a time, the routine)-.1 E F1(cpass\( \))122 484.5
Q F0 .808(is used to pick up characters from the user')72 501 R 3.308
(sb)-.55 G(uf)-3.508 E(fer)-.25 E 5.808(.S)-.55 G(uccessi)-5.808 E 1.108
-.15(ve c)-.25 H .808
(alls on it return the characters to be written until).15 F 89.5 514.5
84.5 514.5 DL F1 2.5(u.u count)72 512 R F0 .107
(goes to 0 or an error occurs, when it returns)2.606 F/F3 10/Symbol SF
(-)2.607 E F0(1.)A F1(Cpass)5.107 E F0(tak)2.607 E .107
(es care of interrog)-.1 F(ating)-.05 E 462.416 514.5 457.416 514.5 DL
F1 2.5(u.u se)444.916 512 R(g\215g)-.4 E F0 .107(and updating)2.607 F
89.5 525.5 84.5 525.5 DL F1 2.5(u.u count.)72 523 R F0 .305
(Write routines which w)72 539.5 R .305(ant to transfer a probably lar)
-.1 F .305(ge number of characters into an internal b)-.18 F(uf)-.2 E
.304(fer may also use the)-.25 F(routine)72 550.5 Q F1(iomo)122 567 Q
(ve\(b)-.1 E(uf)-.2 E(fer)-.18 E 2.5(,o)-1.11 G -.18(ff)-2.5 G
(set, count, \215a).18 E(g\))-.1 E F0 .476(which is f)72 583.5 R .476
(aster when man)-.1 F 2.976(yc)-.15 G .477(haracters must be mo)-2.976 F
-.15(ve)-.15 G(d.).15 E F1(Iomo)5.477 E(ve)-.1 E F0 .477
(transfers up to)2.977 F F1(count)2.977 E F0 .477(characters into the)
2.977 F F1 -.2(bu)2.977 G -.18(ff).2 G(er).18 E F0(start-)2.977 E(ing)72
594.5 Q F1(of)3.461 E(fset)-.18 E F0 .961(bytes from the start of the b)
3.461 F(uf)-.2 E(fer;)-.25 E F1<8d61>3.461 E(g)-.1 E F0 .96(should be)
3.461 F 327.22 597 322.22 597 DL F1 5(BW)316.11 594.5 S(RITE)-5 E F0 .96
(\(which is 0\) in the write case.)3.46 F .96(Caution: the)5.96 F 1.118
(caller is responsible for making sure the count is not too lar)72 605.5
R 1.119(ge and is non-zero.)-.18 F 1.119(As an ef)6.119 F(\214cienc)-.25
E 3.619(yn)-.15 G(ote,)-3.619 E F1(iomo)3.619 E(ve)-.1 E F0(is)3.619 E
(much slo)72 616.5 Q(wer if an)-.25 E 2.5(yo)-.15 G(f)-2.5 E F1 -.2(bu)
2.5 G -.18(ff).2 G(er+of).18 E(fset, count)-.18 E F0(or)2.5 E 271.66 619
266.66 619 DL F1 2.5(u.u base)254.16 616.5 R F0(is odd.)2.5 E .465
(The de)72 633 R(vice')-.25 E(s)-.55 E F1 -.37(re)2.965 G(ad).37 E F0
.465(routine is called under conditions similar to)2.965 F F1(write)
2.965 E(,)-.1 E F0 -.15(ex)2.965 G .465(cept that).15 F 416.865 635.5
411.865 635.5 DL F1 2.5(u.u count)399.365 633 R F0 .465
(is guaranteed to be non-)2.965 F 2.5(zero. T)72 644 R 2.5(or)-.8 G
(eturn characters to the user)-2.5 E 2.5(,t)-.4 G(he routine)-2.5 E F1
(passc\(c\))122 660.5 Q F0 .154(is a)72 677 R -.25(va)-.2 G .154
(ilable; it tak).25 F .154(es care of housek)-.1 F .154(eeping lik)-.1 F
(e)-.1 E F1(cpass)2.654 E F0 .154(and returns)2.654 F F3(-)2.654 E F0
2.654(1a)C 2.654(st)-2.654 G .154(he last character speci\214ed by)
-2.654 F 494.702 679.5 489.702 679.5 DL F1 2.5(u.u count)477.202 677 R
F0 .154(is re-)2.654 F .574
(turned to the user; before that time, 0 is returned.)72 688 R F1(Iomo)
5.574 E(ve)-.1 E F0 .574(is also usable as with)3.074 F F1(write;)3.074
E F0 .573(the \215ag should be)3.074 F 514.45 690.5 509.45 690.5 DL F1 5
(BR)503.34 688 S(EAD)-5 E F0 -.2(bu)72 699 S 2.5(tt).2 G
(he same cautions apply)-2.5 E(.)-.65 E(The `)72 715.5 Q
(`special-functions')-.74 E 2.5('r)-.74 G(outine is in)-2.5 E -.2(vo)-.4
G -.1(ke).2 G 2.5(db).1 G 2.5(yt)-2.5 G(he)-2.5 E F1(stty)2.5 E F0(and)
2.5 E F1(gtty)2.5 E F0(system calls as follo)2.5 E(ws:)-.25 E EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF
(Unix I/O System - 3)461.13 58 Q(\(*p\) \(de)122 105 Q 1.48 -.74(v, v)
-.15 H(\)).74 E F0(where)72 121.5 Q F1(p)2.615 E F0 .115
(is a pointer to the de)2.615 F(vice')-.25 E 2.615(sr)-.55 G(outine,)
-2.615 E F1(de)2.615 E(v)-.15 E F0 .115(is the de)2.615 F .115
(vice number)-.25 F 2.615(,a)-.4 G(nd)-2.615 E F1(v)2.615 E F0 .115
(is a v)2.615 F(ector)-.15 E 5.115(.I)-.55 G 2.615(nt)-5.115 G(he)-2.615
E F1(gtty)2.616 E F0 .116(case, the de)2.616 F .116(vice is)-.25 F .554
(supposed to place up to 3 w)72 132.5 R .554
(ords of status information into the v)-.1 F .553
(ector; this will be returned to the caller)-.15 F 5.553(.I)-.55 G 3.053
(nt)-5.553 G(he)-3.053 E F1(stty)3.053 E F0(case,)72 143.5 Q F1(v)2.5 E
F0(is 0; the de)2.5 E(vice should tak)-.25 E 2.5(eu)-.1 G 2.5(pt)-2.5 G
2.5(o3w)-2.5 G(ords of control information from the array)-2.6 E .4 LW
437.6 146 432.6 146 DL F1 2.5(u.u ar)420.1 143.5 R(g[0...2].)-.37 E F0
(Finally)72 160 Q 2.705(,e)-.65 G .205(ach de)-2.705 F .205
(vice should ha)-.25 F .505 -.15(ve a)-.2 H .205
(ppropriate interrupt-time routines.).15 F .206
(When an interrupt occurs, it is turned into a C-)5.205 F .501
(compatible call on the de)72 171 R(vices')-.25 E 3.001(si)-.55 G .501
(nterrupt routine.)-3.001 F .501(The interrupt-catching mechanism mak)
5.501 F .5(es the lo)-.1 F .5(w-order four bits)-.25 F .713(of the `)72
182 R(`ne)-.74 E 3.213(wP)-.25 G(S')-3.213 E 3.213('w)-.74 G .713
(ord in the trap v)-3.313 F .713(ector for the interrupt a)-.15 F -.25
(va)-.2 G .713(ilable to the interrupt handler).25 F 5.714(.T)-.55 G
.714(his is con)-5.714 F -.15(ve)-.4 G(ntionally).15 E .19(used by dri)
72 193 R -.15(ve)-.25 G .19(rs which deal with multiple similar de).15 F
.189(vices to encode the minor de)-.25 F .189(vice number)-.25 F 5.189
(.A)-.55 G .189(fter the interrupt has)-5.189 F(been processed, a retur\
n from the interrupt handler will return from the interrupt itself.)72
204 Q 3.005(An)72 220.5 S .505(umber of subroutines are a)-3.005 F -.25
(va)-.2 G .505(ilable which are useful to character de).25 F .506
(vice dri)-.25 F -.15(ve)-.25 G 3.006(rs. Most).15 F .506
(of these handlers, for e)3.006 F(x-)-.15 E .48
(ample, need a place to b)72 231.5 R(uf)-.2 E .479
(fer characters in the internal interf)-.25 F .479(ace between their `)
-.1 F .479(`top half)-.74 F 1.959 -.74('' \().55 H .479
(read/write\) and `).74 F(`bottom)-.74 E -1.95(half ')72 242.5 R 3.864
('\()-.74 G 1.364(interrupt\) routines.)-3.864 F -.15(Fo)6.364 G 3.864
(rr).15 G(elati)-3.864 E -.15(ve)-.25 G 1.364(ly lo).15 F 3.864(wd)-.25
G 1.364(ata-rate de)-3.864 F 1.364
(vices, the best mechanism is the character queue main-)-.25 F
(tained by the routines)72 253.5 Q F1 -.1(ge)2.5 G(tc).1 E F0(and)2.5 E
F1(putc.)2.5 E F0 2.5(Aq)5 G(ueue header has the structure)-2.5 E F1
(struct {)122 270 Q 203.44 283.5 198.44 283.5 DL 22.94(int c)158 281 R
11.85(cc; /*)5 F -.15(ch)2.5 G(ar).15 E(acter count */)-.15 E -.15(ch)
158 292 S 208.44 294.5 203.44 294.5 DL 15.32(ar *c)167.29 292 R 8.51
(cf; /*)5 F<8c72>2.5 E(st c)-.1 E(har)-.15 E(acter */)-.15 E -.15(ch)158
303 S 208.44 305.5 203.44 305.5 DL 15.32(ar *c)167.29 303 R 8.51(cl; /*)
5 F(last c)2.5 E(har)-.15 E(acter */)-.15 E 2.5(}q)122 314 S(ueue;)-2.5
E F0 2.5(Ac)72 330.5 S(haracter is placed on the end of a queue by)-2.5
E F1(putc\(c, &queue\))122 347 Q F0(where)72 363.5 Q F1(c)2.7 E F0 .2
(is the character and)2.7 F F1(queue)2.7 E F0 .2(is the queue header)2.7
F 5.2(.T)-.55 G .2(he routine returns)-5.2 F/F2 10/Symbol SF(-)2.699 E
F0 2.699(1i)C 2.699(ft)-2.699 G .199
(here is no space to put the charac-)-2.699 F(ter)72 374.5 Q 2.5(,0o)-.4
G 2.5(therwise. The)-2.5 F(\214rst character on the queue may be retrie)
2.5 E -.15(ve)-.25 G 2.5(db).15 G(y)-2.5 E F1 -.1(ge)122 391 S
(tc\(&queue\)).1 E F0(which returns either the \(non-ne)72 407.5 Q -.05
(ga)-.15 G(ti).05 E -.15(ve)-.25 G 2.5(\)c).15 G(haracter or)-2.5 E F2
(-)2.5 E F0 2.5(1i)C 2.5(ft)-2.5 G(he queue is empty)-2.5 E(.)-.65 E
.528
(Notice that the space for characters in queues is shared among all de)
72 424 R .528(vices in the system and in the standard system)-.25 F .344
(there are only some 600 character slots a)72 435 R -.25(va)-.2 G 2.844
(ilable. Thus).25 F(de)2.844 E .344
(vice handlers, especially write routines, must tak)-.25 F 2.844(ec)-.1
G .344(are to)-2.844 F -.2(avo)72 446 S(id gobbling up e).2 E(xcessi)
-.15 E .3 -.15(ve n)-.25 H(umbers of characters.).15 E
(The other major help a)72 462.5 Q -.25(va)-.2 G(ilable to de).25 E
(vice handlers is the sleep-w)-.25 E(ak)-.1 E(eup mechanism.)-.1 E
(The call)5 E F1(sleep\(e)122 479 Q(vent, priority\))-.15 E F0 .087
(causes the process to w)72 495.5 R .087(ait \(allo)-.1 F .087
(wing other processes to run\) until the)-.25 F F1 -.15(ev)2.587 G(ent)
.15 E F0 .087(occurs; at that time, the process is mark)2.587 F(ed)-.1 E
(ready-to-run and the call will return when there is no process with hi\
gher)72 506.5 Q F1(priority)2.5 E(.)-.55 E F0(The call)72 523 Q F1(wak)
122 539.5 Q(eup\(e)-.1 E(vent\))-.15 E F0 .014(indicates that the)72 556
R F1 -.15(ev)2.514 G(ent).15 E F0 .014
(has happened, that is, causes processes sleeping on the e)2.514 F -.15
(ve)-.25 G .013(nt to be a).15 F -.1(wa)-.15 G -.1(ke).1 G 2.513
(ned. The).1 F F1 -.15(ev)2.513 G(ent).15 E F0 .013(is an)2.513 F .622
(arbitrary quantity agreed upon by the sleeper and the w)72 567 R(ak)-.1
E(er)-.1 E 3.123(-up. By)-.2 F(con)3.123 E -.15(ve)-.4 G .623
(ntion, it is the address of some data area).15 F(used by the dri)72 578
Q -.15(ve)-.25 G .8 -.4(r, w).15 H(hich guarantees that e).4 E -.15(ve)
-.25 G(nts are unique.).15 E .051(Processes sleeping on an e)72 594.5 R
-.15(ve)-.25 G .051(nt should not assume that the e).15 F -.15(ve)-.25 G
.051(nt has really happened; the).15 F 2.551(ys)-.15 G .051
(hould check that the con-)-2.551 F
(ditions which caused them to sleep no longer hold.)72 605.5 Q .049
(Priorities can range from 127 to)72 622 R F2(-)2.549 E F0 .049
(127; a higher numerical v)B .05(alue indicates a less-f)-.25 F -.2(avo)
-.1 G .05(red scheduling situation.).2 F 2.55(Ap)5.05 G(ro-)-2.55 E
1.522(cess sleeping at ne)72 633 R -.05(ga)-.15 G(ti).05 E 1.821 -.15
(ve p)-.25 H 1.521(riority cannot be terminated for an).15 F 4.021(yr)
-.15 G 1.521(eason, although it is concei)-4.021 F -.25(va)-.25 G 1.521
(ble that it may be).25 F(sw)72 644 Q .055(apped out.)-.1 F .056
(Thus it is a bad idea to sleep with ne)5.055 F -.05(ga)-.15 G(ti).05 E
.356 -.15(ve p)-.25 H .056(riority on an e).15 F -.15(ve)-.25 G .056
(nt which might ne).15 F -.15(ve)-.25 G 2.556(ro).15 G(ccur)-2.556 E
5.056(.O)-.55 G 2.556(nt)-5.056 G .056(he other)-2.556 F .887
(hand, calls to)72 655 R F1(sleep)3.387 E F0 .887(with non-ne)3.387 F
-.05(ga)-.15 G(ti).05 E 1.187 -.15(ve p)-.25 H .886(riority may ne).15 F
-.15(ve)-.25 G 3.386(rr).15 G .886
(eturn if the process is terminated by some signal in the)-3.386 F 2.558
(meantime. Incidentally)72 666 R 2.558(,i)-.65 G 2.558(ti)-2.558 G 2.558
(sag)-2.558 G .058(ross error to call)-2.558 F F1(sleep)2.558 E F0 .058
(in a routine called at interrupt time, since the process which is)2.558
F .059
(running is almost certainly not the process which should go to sleep.)
72 677 R(Lik)5.058 E -.25(ew)-.1 G .058(ise, none of the v).25 F .058
(ariables in the user area)-.25 F -.74(``)72 688 S F1(u).74 E/F3 10
/Times-Bold@0 SF(.)A F0 3.98 -.74('' s)D
(hould be touched, let alone changed, by an interrupt routine.).74 E
.522(If a de)72 704.5 R .522(vice dri)-.25 F -.15(ve)-.25 G 3.023(rw).15
G .523(ishes to w)-3.023 F .523(ait for some e)-.1 F -.15(ve)-.25 G .523
(nt for which it is incon).15 F -.15(ve)-.4 G .523
(nient or impossible to supply a).15 F F1(wak)3.023 E(eup,)-.1 E F0
(\(for)3.023 E -.15(ex)72 715.5 S(ample, a de).15 E(vice going on-line,\
 which does not generally cause an interrupt\), the call)-.25 E EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF 2.5(4-U)72 58
S(nix I/O System)-2.5 E(sleep\(&lbolt, priority\))122 105 Q F0 .331
(may be gi)72 121.5 R -.15(ve)-.25 G(n.).15 E F1(Lbolt)5.331 E F0 .331
(is an e)2.831 F .331(xternal cell whose address is a)-.15 F -.1(wa)-.15
G -.1(ke).1 G .331(ned once e).1 F -.15(ve)-.25 G .331
(ry 4 seconds by the clock interrupt rou-).15 F(tine.)72 132.5 Q
(The routines)72 149 Q F1(spl4\( \), spl5\( \), spl6\( \), spl7\( \))122
165.5 Q F0(are a)72 182 Q -.25(va)-.2 G
(ilable to set the processor priority le).25 E -.15(ve)-.25 G 2.5(la).15
G 2.5(si)-2.5 G(ndicated to a)-2.5 E -.2(vo)-.2 G(id incon).2 E -.15(ve)
-.4 G(nient interrupts from the de).15 E(vice.)-.25 E(If a de)72 198.5 Q
(vice needs to kno)-.25 E 2.5(wa)-.25 G(bout real-time interv)-2.5 E
(als, then)-.25 E F1(timeout\(func, ar)122 215 Q .2 -.1(g, i)-.37 H
(nterval\)).1 E F0 .066(will be useful.)72 231.5 R .066
(This routine arranges that after)5.066 F F1(interval)2.567 E F0 .067
(sixtieths of a second, the)2.567 F F1(func)2.567 E F0 .067
(will be called with)2.567 F F1(ar)2.567 E(g)-.37 E F0 .067(as ar)2.567
F(gu-)-.18 E(ment, in the style)72 242.5 Q F1(\(*func\)\(ar)122 259 Q
(g\))-.37 E F0 -.35(Ti)72 275.5 S .618(meouts are used, for e).35 F .618
(xample, to pro)-.15 F .618
(vide real-time delays after function characters lik)-.15 F 3.117(en)-.1
G -.25(ew)-3.117 G .617(-line and tab in type-).25 F .889
(writer output, and to terminate an attempt to read the 201 Dataphone)72
286.5 R F1(dp)3.39 E F0 .89
(if there is no response within a speci\214ed)3.39 F .648
(number of seconds.)72 297.5 R .648(Notice that the number of sixtieths\
 of a second is limited to 32767, since it must appear to be)5.648 F
(positi)72 308.5 Q -.15(ve)-.25 G 3.021(,a).15 G .521
(nd that only a bounded number of timeouts can be going on at once.)
-3.021 F .522(Also, the speci\214ed)5.522 F F1(func)3.022 E F0 .522
(is called at)3.022 F(clock-interrupt time, so it should conform to the\
 requirements of interrupt routines in general.)72 319.5 Q F1(An e)72
341.5 Q(xample)-.2 E F0 .304(The dri)72 358 R -.15(ve)-.25 G 2.804(rf)
.15 G .304(or the paper)-2.804 F .304(-tape reader/punch is w)-.2 F .304
(orth e)-.1 F .304(xamining as a f)-.15 F .304(airly simple e)-.1 F .304
(xample of man)-.15 F 2.804(yo)-.15 G 2.803(ft)-2.804 G .303
(he techniques)-2.803 F .434(used in writing character de)72 369 R .434
(vice handlers.)-.25 F(The)5.434 E F1(pc11)2.934 E F0 .435
(structure contains a state \(used for the reader\), an input queue,)
2.934 F .316(and an output queue.)72 380 R 2.816(As)5.316 G .316
(tructure, rather than three indi)-2.816 F .316(vidual v)-.25 F .315
(ariables, w)-.25 F .315(as used to cut do)-.1 F .315
(wn on the number of e)-.25 F(x-)-.15 E
(ternal symbols which might be confused with symbols in other routines.)
72 391 Q .649(When the \214le is opened for reading, the)72 407.5 R F1
(open)3.149 E F0 .649(routine checks to see if its state is not)3.149 F
F1(CLOSED;)3.149 E F0 .65(if so an error is re-)3.149 F .38
(turned since it is considered a bad idea to let se)72 418.5 R -.15(ve)
-.25 G .379(ral people read one tape simultaneously).15 F 5.379(.T)-.65
G .379(he state is set to)-5.379 F F1 -.6(WA)2.879 G(IT).6 E(-)-.74 E
(ING,)72 429.5 Q F0 .485
(the interrupt is enabled, and a character is requested.)2.985 F .485
(The reason for this g)5.485 F .486(ambit is that there is no direct w)
-.05 F(ay)-.1 E .729(to determine if there is an)72 440.5 R 3.228(yt)
-.15 G .728(ape in the reader or if the reader is on-line.)-3.228 F .728
(In these situations an interrupt will occur)5.728 F .16
(immediately and an error indicated.)72 451.5 R .16
(As will be seen, the interrupt routine ignores errors if the state is)
5.16 F F1 -.6(WA)2.66 G(ITING,).6 E F0 -.2(bu)2.66 G(t).2 E .073
(if a good character comes in while in the)72 462.5 R F1 -.6(WA)2.573 G
(ITING).6 E F0 .072(state the interrupt routine sets the state to)2.573
F F1(READING.)2.572 E F0(Thus)5.072 E F1(open)2.572 E F0 .896
(loops until the state changes, meanwhile sleeping on the `)72 473.5 R
.896(`lightning bolt')-.74 F(')-.74 E F1(lbolt.)3.397 E F0 .897
(If it did not sleep at all, it w)5.897 F(ould)-.1 E(pre)72 484.5 Q -.15
(ve)-.25 G .217(nt an).15 F 2.717(yo)-.15 G .217(ther process from runn\
ing until the reader came on-line; if it depended on the interrupt rout\
ine to w)-2.717 F(ak)-.1 E(e)-.1 E(it up, the ef)72 495.5 Q(fect w)-.25
E(ould be the same, since the error interrupt is almost instantaneous.)
-.1 E(The open-write case is much simpler; the punch is enabled and a 1\
00-character leader is punched using)72 512 Q F1(pcleader)2.5 E(.)-1.11
E F0(The)72 528.5 Q F1(close)4.043 E F0 1.544
(routine is also simple; if the reader w)4.043 F 1.544(as open, an)-.1 F
4.044(yu)-.15 G 1.544
(ncollected characters are \215ushed, the interrupt is)-4.044 F .663
(turned of)72 539.5 R .663(f, and the state is set to)-.25 F F1(CLOSED.)
3.163 E F0 .662(In the write case a 100-character trailer is punched.)
5.662 F .662(The routine has a)5.662 F -.2(bu)72 550.5 S 2.727(gi).2 G
2.727(nt)-2.727 G .227(hat if both the reader and punch are open)-2.727
F F1(close)2.727 E F0 .228
(will be called only once, so that either the lefto)2.727 F -.15(ve)-.15
G 2.728(rc).15 G(haracters)-2.728 E .932
(are \215ushed or the trailer is punched, b)72 561.5 R .931
(ut not both.)-.2 F .931(It is hard to see ho)5.931 F 3.431(wt)-.25 G
3.431<6f8c>-3.431 G 3.431(xt)-3.431 G .931(his problem e)-3.431 F .931
(xcept by making the)-.15 F(reader and punch separate de)72 572.5 Q
(vices.)-.25 E(The)72 589 Q F1(pcr)2.704 E(ead)-.37 E F0 .204(routine t\
ries to pick up characters from the input queue and passes them back un\
til the user')2.704 F 2.705(sr)-.55 G .205(ead call is)-2.705 F 2.793
(satis\214ed. If)72 600 R .293
(there are no characters it checks whether the state has gone to)2.793 F
F1(EOF)2.792 E(,)-1.35 E F0 .292(which means that the interrupt rou-)
2.792 F .006(tine detected an error in the)72 611 R F1(READ)2.506 E F0
.007(state \(assumed to indicate the end of the tape\).)2.506 F .007
(If so,)5.007 F F1(pcr)2.507 E(ead)-.37 E F0 .007
(returns; either during)2.507 F .022(this call or the ne)72 622 R .022
(xt one no characters will be passed back, indicating an end-of-\214le.)
-.15 F .021(If the state is still)5.021 F F1(READING)2.521 E F0(the)
2.521 E(routine enables another character by \214ddling the de)72 633 Q
(vice')-.25 E 2.5(sr)-.55 G(eader control re)-2.5 E(gister)-.15 E 2.5
(,p)-.4 G(ro)-2.5 E(vided it is not acti)-.15 E -.15(ve)-.25 G 2.5(,a)
.15 G(nd goes to)-2.5 E(sleep.)72 644 Q .254
(When a reader interrupt occurs and the state is)72 660.5 R F1 -.6(WA)
2.754 G(ITING,).6 E F0 .254(and the de)2.754 F(vice')-.25 E 2.754(se)
-.55 G .254(rror bit is set, the interrupt is ignored; if)-2.754 F .32
(there is no error the state is set to)72 671.5 R F1(READING,)2.82 E F0
.321(as indicated in the discussion of)2.82 F F1(pcr)2.821 E(ead.)-.37 E
F0 .321(If the state is)5.321 F F1(READING)2.821 E F0(and)2.821 E .367
(there is an error)72 682.5 R 2.867(,t)-.4 G .367(he state is set to)
-2.867 F F1(EOF;)2.867 E F0 .367
(it is assumed that the error represents the end of the tape.)2.867 F
.367(If there is no er)5.367 F(-)-.2 E(ror)72 693.5 Q 2.716(,t)-.4 G
.216(he character is pick)-2.716 F .216
(ed up and stored in the input queue.)-.1 F .216(Then, pro)5.216 F .216
(vided the number of characters already in the)-.15 F .261
(queue is less than the high-w)72 704.5 R .261(ater mark)-.1 F F1(PCIHW)
2.761 E -.37(AT)-.6 G(,)-.37 E F0 .261(the reader is enabled ag)2.761 F
.261(ain to read another character)-.05 F 5.261(.T)-.55 G .261
(his strate-)-5.261 F .023(gy k)72 715.5 R .023(eeps the tape mo)-.1 F
.023(ving without \215ooding the input queue with unread characters.)
-.15 F(Finally)5.024 E 2.524(,t)-.65 G .024(he top half is a)-2.524 F
-.1(wa)-.15 G -.1(ke).1 G(ned.).1 E EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF
(Unix I/O System - 5)461.13 58 Q F0 .186(Looking ag)72 105 R .186
(ain at)-.05 F F1(pcr)2.686 E(ead,)-.37 E F0 .186
(notice that the priority)2.686 F(le)5.186 E -.15(ve)-.25 G 2.686(li).15
G 2.686(sr)-2.686 G .185(aised by)-2.686 F F1 .185(spl4\( \))2.685 F F0
.185(to pre)2.685 F -.15(ve)-.25 G .185(nt interrupts during the loop.)
.15 F(This)5.185 E .181
(is done because of the possibility that the input queue is empty)72 116
R 2.681(,a)-.65 G .181
(nd just after the EOF test is made an error interrupt)-2.681 F .021
(occurs because the tape runs out.)72 127 R(Then)5.021 E F1(sleep)2.521
E F0 .02(will be called with no possibility of a)2.521 F F1(wak)2.52 E
(eup.)-.1 E F0 .02(In general the processor)5.02 F .423
(priority should be raised when a routine is about to sleep a)72 138 R
-.1(wa)-.15 G .424
(iting some condition where the presence of the condi-).1 F .454
(tion, and the consequent)72 149 R F1(wak)2.954 E(eup,)-.1 E F0 .454
(is indicated by an interrupt.)2.954 F .454
(The danger is that the interrupt might occur between)5.454 F
(the test for the condition and the call to)72 160 Q F1(sleep,)2.5 E F0
(so that the)2.5 E F1(wak)2.5 E(eup)-.1 E F0(apparently ne)2.5 E -.15
(ve)-.25 G 2.5(rh).15 G(appens.)-2.5 E .77
(At the same time it is a bad idea to raise the processor priority le)72
176.5 R -.15(ve)-.25 G 3.27(lf).15 G .77(or an e)-3.27 F .77
(xtended period of time, since de)-.15 F(vices)-.25 E .183(with real-ti\
me requirements may be shut out so long as to cause an error)72 187.5 R
5.183(.T)-.55 G(he)-5.183 E F1(pcr)2.683 E(ead)-.37 E F0 .183
(routine is perhaps o)2.683 F -.15(ve)-.15 G(rzealous).15 E
(in this respect, although since most de)72 198.5 Q(vices ha)-.25 E .3
-.15(ve a p)-.2 H(riority le).15 E -.15(ve)-.25 G 2.5(lh).15 G
(igher than 4 this dif)-2.5 E(\214culty is not v)-.25 E(ery important.)
-.15 E(The)72 215 Q F1(pcwrite)2.69 E F0 .19
(routine simply gets characters from the user and passes them to)2.69 F
F1(pcoutput,)2.691 E F0 .191(which is separated out so that)2.691 F F1
(pcleader)72 226 Q F0 .833(can call it also.)3.333 F F1(Pcoutput)5.833 E
F0 .833(checks for errors \(lik)3.333 F 3.333(eo)-.1 G .832
(ut-of-tape\) and if none are present mak)-3.333 F .832
(es sure that the)-.1 F .315
(number of characters in the output queue does not e)72 237 R(xceed)-.15
E F1(PCOHW)2.815 E -.37(AT)-.6 G(;)-.28 E F0 .315(if it does,)2.815 F F1
(sleep)2.815 E F0 .316(is called.)2.816 F .316(Then the character)5.316
F .331(is placed on the output queue.)72 248 R .331(There is a small b)
5.331 F .331(ug here in that)-.2 F F1(pcoutput)2.831 E F0 .331
(does not check that the character w)2.831 F .33(as suc-)-.1 F .011
(cessfully put on the queue \(all character)72 259 R .011(-queue space \
might be empty\); perhaps in this case it might be a good idea to)-.2 F
.188(sleep on the lightning-bolt until things quiet do)72 270 R 2.688
(wn. Finally)-.25 F F1(pcstart)2.688 E F0 .187
(is called, which checks to see if the punch is cur)2.688 F(-)-.2 E
(rently b)72 281 Q(usy)-.2 E 2.5(,a)-.65 G(nd if not starts the punchin\
g of the \214rst character on the output queue.)-2.5 E .181
(When punch interrupts occur)72 297.5 R(,)-.4 E F1(pcpint)2.681 E F0
.181(is called; it starts the punching of the ne)2.681 F .182
(xt character on the output queue, and if)-.15 F .695
(the number of characters remaining on the queue is less than the lo)72
308.5 R(w-w)-.25 E .695(ater mark)-.1 F F1(PCOL)3.195 E -1.67 -.6(WA T)
-.55 H F0 .695(it w)3.795 F(ak)-.1 E .695(es up the write)-.1 F
(routine, which is presumably w)72 319.5 Q
(aiting for the queue to empty)-.1 E(.)-.65 E/F2 10/Times-Bold@0 SF
(The Block-de)72 341.5 Q(vice Interface)-.15 E F0 .505
(Handling of block de)72 358 R .506
(vices is mediated by a collection of routines which manage a set of b)
-.25 F(uf)-.2 E .506(fers containing the im-)-.25 F .282
(ages of blocks of data on the v)72 369 R .282(arious de)-.25 F 2.782
(vices. The)-.25 F .282
(most important purpose of these routines is to assure that se)2.782 F
-.15(ve)-.25 G(ral).15 E .612
(processes which access the same block of the same de)72 380 R .612
(vice in multiprogrammed f)-.25 F .612(ashion maintain a consistent vie)
-.1 F(w)-.25 E .299(of the data in the block.)72 391 R 2.799(As)5.299 G
.299(econdary b)-2.799 F .299
(ut still important purpose is to increase the ef)-.2 F(\214cienc)-.25 E
2.798(yo)-.15 G 2.798(ft)-2.798 G .298(he system by k)-2.798 F(eep-)-.1
E .007(ing in-core copies of blocks which are being accessed frequently)
72 402 R 5.007(.T)-.65 G .008
(he main data base for this mechanism is the table)-5.007 F .578(of b)72
413 R(uf)-.2 E(fers)-.25 E F1 -.2(bu)3.078 G -.15(f.).2 G F0 .578
(Each b)5.728 F(uf)-.2 E .578(fer header contains a pair of pointers)
-.25 F .4 LW 337.308 415.5 332.308 415.5 DL F1 2.5(\(b forw)323.978 413
R 370.485 415.5 365.485 415.5 DL -3.654 3.077(,b b)354.908 413 T(ac)
-3.077 E(k\))-.2 E F0 .577(which maintain a doubly-link)3.077 F .577
(ed list)-.1 F .818(of the b)72 424 R(uf)-.2 E .819
(fers associated with a particular block de)-.25 F .819
(vice, and a pair of pointers)-.25 F 411.843 426.5 406.843 426.5 DL F1
2.5(\(av forw)394.073 424 R 3.319(,a)-.74 G 449.702 426.5 444.702 426.5
DL 5(vb)440.262 424 S(ac)-5 E(k\))-.2 E F0 .819(which generally)3.319 F
.839(maintain a doubly-link)72 435 R .839(ed list of blocks which are `)
-.1 F(`free,)-.74 E 2.319 -.74('' t)-.7 H .839
(hat is, eligible to be reallocated for another transaction.).74 F(Buf)
72 446 Q .458(fers which ha)-.25 F .758 -.15(ve I)-.2 H .458
(/O in progress or are b).15 F .459
(usy for other purposes do not appear in this list.)-.2 F .459(The b)
5.459 F(uf)-.2 E .459(fer header also)-.25 F .493(contains the de)72 457
R .493(vice and block number to which the b)-.25 F(uf)-.2 E .492
(fer refers, and a pointer to the actual storage associated with)-.25 F
1.207(the b)72 468 R(uf)-.2 E(fer)-.25 E 6.207(.T)-.55 G 1.207
(here is a w)-6.207 F 1.208(ord count which is the ne)-.1 F -.05(ga)-.15
G(ti).05 E 1.508 -.15(ve o)-.25 H 3.708(ft).15 G 1.208(he number of w)
-3.708 F 1.208(ords to be transferred to or from the)-.1 F -.2(bu)72 479
S -.25(ff).2 G .182(er; there is also an error byte and a residual w).25
F .182(ord count used to communicate information from an I/O routine to)
-.1 F .36(its caller)72 490 R 5.36(.F)-.55 G(inally)-5.36 E 2.86(,t)-.65
G .36(here is a \215ag w)-2.86 F .36
(ord with bits indicating the status of the b)-.1 F(uf)-.2 E(fer)-.25 E
5.361(.T)-.55 G .361(hese \215ags will be discussed be-)-5.361 F(lo)72
501 Q -.65(w.)-.25 G .163
(Six routines constitute the most important part of the interf)72 517.5
R .162(ace with the rest of the system.)-.1 F(Gi)5.162 E -.15(ve)-.25 G
2.662(nad).15 G -.25(ev)-2.662 G .162(ice and block).25 F(number)72
528.5 Q 2.529(,b)-.4 G(oth)-2.529 E F1(br)2.529 E(ead)-.37 E F0(and)
2.529 E F1 -.1(ge)2.529 G(tblk).1 E F0 .029(return a pointer to a b)
2.529 F(uf)-.2 E .029(fer header for the block; the dif)-.25 F .029
(ference is that)-.25 F F1(br)2.529 E(ead)-.37 E F0 .03(is guaran-)2.529
F .188(teed to return a b)72 539.5 R(uf)-.2 E .187
(fer actually containing the current data for the block, while)-.25 F F1
-.1(ge)2.687 G(tblk).1 E F0 .187(returns a b)2.687 F(uf)-.2 E .187
(fer which contains)-.25 F .165(the data in the block only if it is alr\
eady in core \(whether it is or not is indicated by the)72 550.5 R
435.156 553 430.156 553 DL F1 5(BD)424.046 550.5 S(ONE)-5 E F0 .166
(bit; see belo)2.666 F 2.666(w\). In)-.25 F .236(either case the b)72
561.5 R(uf)-.2 E(fer)-.25 E 2.736(,a)-.4 G .236(nd the corresponding de)
-2.736 F .236(vice block, is made `)-.25 F(`b)-.74 E(usy)-.2 E -.7(,')
-.65 G 2.736('s)-.04 G 2.736(ot)-2.736 G .235
(hat other processes referring to it are)-2.736 F .566(obliged to w)72
572.5 R .566(ait until it becomes free.)-.1 F F1(Getblk)5.566 E F0 .566
(is used, for e)3.066 F .566
(xample, when a block is about to be totally re)-.15 F .567(written, so)
-.25 F .161(that its pre)72 583.5 R .161
(vious contents are not useful; still, no other process can be allo)-.25
F .161(wed to refer to the block until the ne)-.25 F 2.66(wd)-.25 G(ata)
-2.66 E(is placed into it.)72 594.5 Q(Gi)72 611 Q -.15(ve)-.25 G 2.584
(nap).15 G .084(ointer to a b)-2.584 F(uf)-.2 E(fer)-.25 E 2.584(,t)-.4
G(he)-2.584 E F1(br)2.584 E(else)-.37 E F0 .084(routine mak)2.584 F .084
(es the b)-.1 F(uf)-.2 E .084(fer ag)-.25 F .084(ain a)-.05 F -.25(va)
-.2 G .084(ilable to other processes.).25 F .085(It is called, for e)
5.085 F(x-)-.15 E .489(ample, after data has been e)72 622 R .489
(xtracted follo)-.15 F .489(wing a)-.25 F F1(br)2.989 E(ead.)-.37 E F0
.489(There are three subtly-dif)5.489 F .488
(ferent write routines, all of which)-.25 F(tak)72 633 Q 3.061(eab)-.1 G
(uf)-3.261 E .561(fer pointer as ar)-.25 F .561
(gument, and all of which logically release the b)-.18 F(uf)-.2 E .562
(fer for use by others and place it on the)-.25 F .01(free list.)72 644
R F1(Bwrite)5.01 E F0 .01(puts the b)2.51 F(uf)-.2 E .01
(fer on the appropriate de)-.25 F .01(vice queue, w)-.25 F .009
(aits for the write to be done, and sets the user')-.1 F 2.509(se)-.55 G
-.2(r-)-2.509 G(ror \215ag if required.)72 655 Q F1(Bawrite)72 677 Q F0
.463(places the b)2.963 F(uf)-.2 E .463(fer on the de)-.25 F(vice')-.25
E 2.964(sq)-.55 G .464(ueue, b)-2.964 F .464(ut does not w)-.2 F .464
(ait for completion, so that errors cannot be re\215ected)-.1 F .315
(directly to the user)72 688 R(.)-.55 E F1(Bdwrite)5.314 E F0 .314
(does not start an)2.814 F 2.814(yI)-.15 G .314(/O operation at all, b)
-2.814 F .314(ut merely marks the b)-.2 F(uf)-.2 E .314
(fer so that if it happens)-.25 F(to be grabbed from the free list to c\
ontain data from some other block, the data in it will \214rst be writt\
en out.)72 699 Q F1(Bwrite)72 715.5 Q F0 .142(is used when one w)2.642 F
.142(ants to be sure that I/O tak)-.1 F .142(es place correctly)-.1 F
2.642(,a)-.65 G .143(nd that errors are re\215ected to the proper us-)
-2.642 F .881(er; it is used, for e)72 726.5 R .881
(xample, when updating i-nodes.)-.15 F F1(Bawrite)5.881 E F0 .88
(is useful when more ef)3.381 F(\214cienc)-.25 E 3.38(yi)-.15 G 3.38(sd)
-3.38 G .88(esired \(because no)-3.38 F EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF 2.5(6-U)72 58
S(nix I/O System)-2.5 E F0 -.1(wa)72 105 S .517
(it is required for I/O to \214nish\) b).1 F .517
(ut when it is reasonably certain that the write is really required.)-.2
F F1(Bdwrite)5.518 E F0 .518(is used)3.018 F .595
(when there is doubt that the write is needed at the moment.)72 116 R
-.15(Fo)5.595 G 3.094(re).15 G(xample,)-3.244 E F1(bdwrite)3.094 E F0
.594(is called when the last byte of a)3.094 F F1(write)72 127 Q F0 .071
(system call f)2.571 F .072
(alls short of the end of a block, on the assumption that another)-.1 F
F1(write)2.572 E F0 .072(will be gi)2.572 F -.15(ve)-.25 G 2.572(ns).15
G .072(oon which will)-2.572 F .077(re-use the same block.)72 138 R .077
(On the other hand, as the end of a block is passed,)5.077 F F1(bawrite)
2.577 E F0 .076(is called, since probably the block)2.577 F
(will not be accessed ag)72 149 Q(ain soon and one might as well start \
the writing process as soon as possible.)-.05 E .661(In an)72 165.5 R
3.161(ye)-.15 G -.15(ve)-3.411 G .661(nt, notice that the routines).15 F
F1 -.1(ge)3.161 G(tblk).1 E F0(and)3.161 E F1(br)3.161 E(ead)-.37 E F0
.662(dedicate the gi)3.161 F -.15(ve)-.25 G 3.162(nb).15 G .662(lock e)
-3.162 F(xclusi)-.15 E -.15(ve)-.25 G .662(ly to the use of the caller)
.15 F(,)-.4 E .161(and mak)72 176.5 R 2.661(eo)-.1 G .161(thers w)-2.661
F .161(ait, while one of)-.1 F F1(br)2.661 E(else)-.37 E 2.661(,b)-.1 G
(write)-2.661 E 2.661(,b)-.1 G(awrite)-2.661 E(,)-.1 E F0(or)2.661 E F1
(bdwrite)2.661 E F0 .161(must e)2.661 F -.15(ve)-.25 G .161
(ntually be called to free the block for).15 F(use by others.)72 187.5 Q
.63(As mentioned, each b)72 204 R(uf)-.2 E .63
(fer header contains a \215ag w)-.25 F .63
(ord which indicates the status of the b)-.1 F(uf)-.2 E(fer)-.25 E 5.63
(.S)-.55 G .63(ince the)-5.63 F 3.13(yp)-.15 G(ro)-3.13 E(vide)-.15 E
.824(one important channel for information between the dri)72 215 R -.15
(ve)-.25 G .824
(rs and the block I/O system, it is important to understand).15 F
(these \215ags.)72 226 Q(The follo)5 E(wing names are manifest constant\
s which select the associated \215ag bits.)-.25 E .4 LW 83.11 250.5
78.11 250.5 DL F1 5(BR)72 248 S(EAD)-5 E F0 .345
(This bit is set when the b)72 264.5 R(uf)-.2 E .345
(fer is handed to the de)-.25 F .345(vice strate)-.25 F .345
(gy routine \(see belo)-.15 F .346(w\) to indicate a read operation.)
-.25 F(The)5.346 E(symbol)72 275.5 Q 115.517 278 110.517 278 DL F1 5(BW)
104.407 275.5 S(RITE)-5 E F0 .457
(is de\214ned as 0 and does not de\214ne a \215ag; it is pro)2.957 F
.456(vided as a mnemonic con)-.15 F -.15(ve)-.4 G .456
(nience to callers of).15 F(routines lik)72 286.5 Q(e)-.1 E F1(swap)2.5
E F0(which ha)2.5 E .3 -.15(ve a s)-.2 H(eparate ar).15 E
(gument which indicates read or write.)-.18 E 83.11 311 78.11 311 DL F1
5(BD)72 308.5 S(ONE)-5 E F0 .935
(This bit is set to 0 when a block is handed to the the de)72 325 R .935
(vice strate)-.25 F .936(gy routine and is turned on when the operation)
-.15 F .216(completes, whether normally as the result of an error)72 336
R 5.216(.I)-.55 G 2.716(ti)-5.216 G 2.716(sa)-2.716 G .216
(lso used as part of the return ar)-2.716 F .216(gument of)-.18 F F1 -.1
(ge)2.716 G(tblk).1 E F0 .215(to indi-)2.715 F
(cate if 1 that the returned b)72 347 Q(uf)-.2 E
(fer actually contains the data in the requested block.)-.25 E 83.11
371.5 78.11 371.5 DL F1 5(BE)72 369 S(RR)-5 E(OR)-.4 E F0 .418
(This bit may be set to 1 when)72 385.5 R 207.004 388 202.004 388 DL F1
5(BD)195.894 385.5 S(ONE)-5 E F0 .418
(is set to indicate that an I/O or other error occurred.)2.918 F .419
(If it is set the)5.419 F 519.34 388 514.34 388 DL F1 5(be)509.34 385.5
S(rr)-5 E(or)-.45 E F0 .862(byte of the b)72 396.5 R(uf)-.2 E .862
(fer header may contain an error code if it is non-zero.)-.25 F(If)5.862
E 380.78 399 375.78 399 DL F1 5(be)370.78 396.5 S(rr)-5 E(or)-.45 E F0
.861(is 0 the nature of the error is not)3.362 F 2.62
(speci\214ed. Actually)72 407.5 R .12(no dri)2.62 F -.15(ve)-.25 G 2.62
(ra).15 G 2.62(tp)-2.62 G .12(resent sets)-2.62 F 259.91 410 254.91 410
DL F1 5(be)249.91 407.5 S(rr)-5 E(or;)-.45 E F0 .121(the latter is pro)
2.62 F .121(vided for a future impro)-.15 F -.15(ve)-.15 G .121
(ment whereby a more).15 F(detailed error)72 418.5 Q
(-reporting scheme may be implemented.)-.2 E 83.11 443 78.11 443 DL F1 5
(BB)72 440.5 S(USY)-5.1 E F0 1.301(This bit indicates that the b)72 457
R(uf)-.2 E 1.3
(fer header is not on the free list, i.e. is dedicated to someone')-.25
F 3.8(se)-.55 G(xclusi)-3.95 E 1.6 -.15(ve u)-.25 H 3.8(se. The).15 F
-.2(bu)72 468 S -.25(ff).2 G .426
(er still remains attached to the list of blocks associated with its de)
.25 F .426(vice, ho)-.25 F(we)-.25 E -.15(ve)-.25 G 4.026 -.55(r. W).15
H(hen).55 E F1 -.1(ge)2.926 G(tblk).1 E F0(\(or)2.926 E F1(br)2.927 E
(ead,)-.37 E F0(which)2.927 E .181(calls it\) searches the b)72 479 R
(uf)-.2 E .181(fer list for a gi)-.25 F -.15(ve)-.25 G 2.681(nd).15 G
-.25(ev)-2.681 G .18(ice and \214nds the requested block with this bit \
on, it sleeps until the bit).25 F(clears.)72 490 Q 83.11 514.5 78.11
514.5 DL F1 5(BW)72 512 S(ANTED)-5.6 E F0 .434
(This \215ag is used in conjunction with the)72 528.5 R 249.362 531
244.362 531 DL F1 5(BB)238.252 528.5 S(USY)-5.1 E F0 2.934(bit. Before)
2.934 F .434(sleeping as described just abo)2.934 F -.15(ve)-.15 G(,).15
E F1 -.1(ge)2.935 G(tblk).1 E F0 .435(sets this \215ag.)2.935 F(Con)72
539.5 Q -.15(ve)-.4 G(rsely).15 E 3.19(,w)-.65 G .69
(hen the block is freed and the b)-3.19 F .69(usy bit goes do)-.2 F .69
(wn \(in)-.25 F F1(br)3.19 E(else\))-.37 E F0(a)3.19 E F1(wak)3.19 E
(eup)-.1 E F0 .69(is gi)3.19 F -.15(ve)-.25 G 3.19(nf).15 G .69
(or the block header)-3.19 F(whene)72 550.5 Q -.15(ve)-.25 G(r).15 E
125.127 553 120.127 553 DL F1 5(BW)114.017 550.5 S(ANTED)-5.6 E F0 1.047
(is on.)3.547 F 1.048(This strate)6.047 F 1.048(gem a)-.15 F -.2(vo)-.2
G 1.048(ids the o).2 F -.15(ve)-.15 G 1.048(rhead of ha).15 F 1.048
(ving to call)-.2 F F1(wak)3.548 E(eup)-.1 E F0 -2.15 -.25(ev e)3.548 H
1.048(ry time a b).25 F(uf)-.2 E 1.048(fer is)-.25 F
(freed on the chance that someone might w)72 561.5 Q(ant it.)-.1 E 83.11
586 78.11 586 DL F1 5(BA)72 583.5 S(SYNC)-5 E F0 .001
(This bit is set by)72 600 R F1(bawrite)2.501 E F0 .001
(to indicate to the appropriate de)2.501 F(vice dri)-.25 E -.15(ve)-.25
G 2.5(rt).15 G(hat the b)-2.5 E(uf)-.2 E
(fer should be released when the write)-.25 E .307
(has been \214nished, usually at interrupt time.)72 611 R .308(The dif)
5.308 F .308(ference between)-.25 F F1(bwrite)2.808 E F0(and)2.808 E F1
(bawrite)2.808 E F0 .308(is that the former starts I/O,)2.808 F -.1(wa)
72 622 S .522(its until it is done, and frees the b).1 F(uf)-.2 E(fer)
-.25 E 5.521(.T)-.55 G .521
(he latter merely sets this bit and starts I/O.)-5.521 F .521
(The bit indicates that)5.521 F F1 -.37(re)3.021 G(lse).37 E F0
(should be called for the b)72 633 Q(uf)-.2 E(fer on completion.)-.25 E
83.11 657.5 78.11 657.5 DL F1 5(BD)72 655 S(EL)-5 E(WRI)-.55 E F0 .058
(This bit is set by)72 671.5 R F1(bdwrite)2.558 E F0 .058
(before releasing the b)2.558 F(uf)-.2 E(fer)-.25 E 5.058(.W)-.55 G(hen)
-5.058 E F1 -.1(ge)2.558 G(tblk,).1 E F0 .058
(while searching for a free block, disco)2.558 F -.15(ve)-.15 G .058
(rs the bit).15 F(is 1 in a b)72 682.5 Q(uf)-.2 E(fer it w)-.25 E(ould \
otherwise grab, it causes the block to be written out before reusing it\
.)-.1 E 83.11 707 78.11 707 DL F1 5(BX)72 704.5 S(MEM)-5 E F0 .146(This\
 is actually a mask for the pair of bits which contain the high-order t\
w)72 721 R 2.646(ob)-.1 G .146(its of the ph)-2.646 F .146
(ysical address of the origin)-.05 F EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF
(Unix I/O System - 7)461.13 58 Q F0(of the b)72 105 Q(uf)-.2 E
(fer; these bits are an e)-.25 E(xtension of the 16 address bits else)
-.15 E(where in the b)-.25 E(uf)-.2 E(fer header)-.25 E(.)-.55 E .4 LW
83.11 129.5 78.11 129.5 DL F1 5(BR)72 127 S(ELOC)-5 E F0 .929
(This bit is currently unused; it pre)72 143.5 R .929
(viously indicated that a system-wide relocation constant w)-.25 F .93
(as to be added to the)-.1 F -.2(bu)72 154.5 S -.25(ff).2 G 1.186
(er address.).25 F 1.186(It w)6.186 F 1.186(as needed during a period w\
hen addresses of data in the system \(including the b)-.1 F(uf)-.2 E
1.185(fers\) were)-.25 F(mapped by the relocation hardw)72 165.5 Q
(are to a ph)-.1 E(ysical address dif)-.05 E
(fering from its virtual address.)-.25 E/F2 10/Times-Bold@0 SF(Block De)
72 187.5 Q(vice Dri)-.15 E -.1(ve)-.1 G(rs).1 E F0(The)72 204 Q F1(bde)
2.5 E(vsw)-.15 E F0(table contains the names of the interf)2.5 E
(ace routines and that of a table for each block de)-.1 E(vice.)-.25 E
1.155(Just as for character de)72 220.5 R 1.155(vices, block de)-.25 F
1.155(vice dri)-.25 F -.15(ve)-.25 G 1.156(rs may supply an).15 F F1
(open)3.656 E F0 1.156(and a)3.656 F F1(close)3.656 E F0 1.156
(routine called respecti)3.656 F -.15(ve)-.25 G 1.156(ly on).15 F .355
(each open and on the \214nal close of the de)72 231.5 R 2.855
(vice. Instead)-.25 F .354
(of separate read and write routines, each block de)2.854 F .354
(vice dri)-.25 F -.15(ve)-.25 G(r).15 E .306(has a)72 242.5 R F1(str)
2.806 E(ate)-.15 E(gy)-.4 E F0 .306
(routine which is called with a pointer to a b)2.806 F(uf)-.2 E .307
(fer header as ar)-.25 F 2.807(gument. As)-.18 F .307(discussed, the b)
2.807 F(uf)-.2 E .307(fer header)-.25 F .624
(contains a read/write \215ag, the core address \(including e)72 253.5 R
.623(xtended-memory bits\), the block number)-.15 F 3.123(,a\()-.4 G(ne)
-3.123 E -.05(ga)-.15 G(ti).05 E -.15(ve)-.25 G 3.123(\)w).15 G(ord)
-3.223 E .615(count, and the major and minor de)72 264.5 R .616
(vice number)-.25 F 5.616(.T)-.55 G .616(he ro)-5.616 F .835(^l)-4.165 G
3.116(eo)-.835 G 3.116(ft)-3.116 G .616(he strate)-3.116 F .616
(gy routine is to carry out the operation as re-)-.15 F .779
(quested by the information in the b)72 275.5 R(uf)-.2 E .779
(fer header)-.25 F 5.779(.W)-.55 G .779
(hen the transaction is complete the)-5.779 F 439.616 278 434.616 278 DL
F1 5(BD)428.506 275.5 S(ONE)-5 E F0 .778(\(and possibly the)3.278 F
83.11 289 78.11 289 DL F1 5(BE)72 286.5 S(RR)-5 E(OR\))-.4 E F0 .22
(bits should be set.)2.72 F .22(Then if the)5.22 F 256.12 289 251.12 289
DL F1 5(BA)245.01 286.5 S(SYNC)-5 E F0 .221(bit is set,)2.72 F F1(br)
2.721 E(else)-.37 E F0 .221(should be called; otherwise,)2.721 F F1(wak)
2.721 E(eup.)-.1 E F0 .221(In cases)5.221 F 1.503(where the de)72 297.5
R 1.503(vice is capable, under error)-.25 F 1.503
(-free operation, of transferring fe)-.2 F 1.502(wer w)-.25 F 1.502
(ords than requested, the de)-.1 F(vice')-.25 E(s)-.55 E -.1(wo)72 308.5
S 1.04(rd-count re).1 F 1.04
(gister should be placed in the residual count slot of the b)-.15 F(uf)
-.2 E 1.04(fer header; otherwise, the residual count)-.25 F .563
(should be set to 0.)72 319.5 R .562(This particular mechanism is reall\
y for the bene\214t of the magtape dri)5.563 F -.15(ve)-.25 G .562
(r; when reading this de-).15 F 1.231(vice records shorter than request\
ed are quite normal, and the user should be told the actual length of t\
he record.)72 330.5 R([Ho)72 341.5 Q(we)-.25 E -.15(ve)-.25 G 2.658(rt)
.15 G .158(he mechanism has not been inte)-2.658 F .158
(grated into normal I/O e)-.15 F -.15(ve)-.25 G 2.657(no).15 G 2.657(nm)
-2.657 G .157(agtape and is used only in `)-2.657 F(`ra)-.74 E(w')-.15 E
2.657('I)-.74 G .157(/O as)-2.657 F(discussed belo)72 352.5 Q -.65(w.)
-.25 G(]).65 E 1.078(Notice that although the most usual ar)72 369 R
1.078(gument to the strate)-.18 F 1.078(gy routines is a genuine b)-.15
F(uf)-.2 E 1.079(fer header allocated as dis-)-.25 F .526(cussed abo)72
380 R -.15(ve)-.15 G 3.026(,a).15 G .526
(ll that is actually required is that the ar)-3.026 F .525
(gument be a pointer to a place containing the appropriate in-)-.18 F
3.081(formation. F)72 391 R .581(or e)-.15 F .581(xample the)-.15 F F1
(swap)3.081 E F0 .581(routine, which manages mo)3.081 F -.15(ve)-.15 G
.581(ment of core images to and from the sw).15 F .582(apping de-)-.1 F
.51(vice, uses the strate)72 402 R .51(gy routine for this de)-.15 F
3.01(vice. Care)-.25 F .51(has to be tak)3.01 F .51(en that no e)-.1 F
.51(xtraneous bits get turned on in the \215ag)-.15 F -.1(wo)72 413 S
(rd.).1 E .467(The de)72 429.5 R(vice')-.25 E 2.967(st)-.55 G .467
(able speci\214ed by)-2.967 F F1(bde)2.967 E(vsw)-.15 E F0 .467
(has a byte to contain an acti)2.967 F .767 -.15(ve \215)-.25 H .468
(ag and an error count, a pair of links which).15 F .78
(constitute the head of the chain of b)72 440.5 R(uf)-.2 E .78
(fers for the de)-.25 F(vice)-.25 E 319.54 443 314.54 443 DL F1 2.5
(\(b forw)306.21 440.5 R 352.92 443 347.92 443 DL -4.06 3.28(,b b)337.14
440.5 T(ac)-3.28 E(k\),)-.2 E F0 .78
(and a \214rst and last pointer for a de)3.28 F(vice)-.25 E 2.874
(queue. Of)72 451.5 R .374(these things, all are used solely by the de)
2.874 F .375(vice dri)-.25 F -.15(ve)-.25 G 2.875(ri).15 G .375(tself e)
-2.875 F .375(xcept for the b)-.15 F(uf)-.2 E(fer)-.25 E .375
(-chain pointers.)-.2 F -.8(Ty)5.375 G(pically).8 E .354
(the \215ag encodes the state of the de)72 462.5 R .354
(vice, and is used at a minimum to indicate that the de)-.25 F .354
(vice is currently eng)-.25 F .354(aged in)-.05 F .212
(transferring information and no ne)72 473.5 R 2.712(wc)-.25 G .212
(ommand should be issued.)-2.712 F .213
(The error count is useful for counting retries when)5.212 F .2
(errors occur)72 484.5 R 5.2(.T)-.55 G .2(he de)-5.2 F .2
(vice queue is used to remember stack)-.25 F .2
(ed requests; in the simplest case it may be maintained as a)-.1 F .403
(\214rst-in \214rst-out list.)72 495.5 R .403(Since b)5.403 F(uf)-.2 E
.403(fers which ha)-.25 F .703 -.15(ve b)-.2 H .403(een handed o).15 F
-.15(ve)-.15 G 2.903(rt).15 G 2.904(ot)-2.903 G .404(he strate)-2.904 F
.404(gy routines are ne)-.15 F -.15(ve)-.25 G 2.904(ro).15 G 2.904(nt)
-2.904 G .404(he list of free)-2.904 F -.2(bu)72 506.5 S -.25(ff).2 G
.42(ers, the pointers in the b).25 F(uf)-.2 E .42
(fer which maintain the free list)-.25 F 340.11 509 335.11 509 DL F1 2.5
(\(av forw)322.34 506.5 R 2.92(,a)-.74 G 377.57 509 372.57 509 DL 5(vb)
368.13 506.5 S(ac)-5 E(k\))-.2 E F0 .42
(are also used to contain the point-)2.92 F(ers which maintain the de)72
517.5 Q(vice queues.)-.25 E 2.5(Ac)72 534 S(ouple of routines are pro)
-2.5 E(vided which are useful to block de)-.15 E(vice dri)-.25 E -.15
(ve)-.25 G(rs.).15 E F1 2.5(iodone\(bp\) ,)122 550.5 R F0 .75
(arranges that the b)72 567 R(uf)-.2 E .75(fer to which)-.25 F F1(bp)
3.25 E F0 .751(points be released or a)3.25 F -.1(wa)-.15 G -.1(ke).1 G
.751(ned, as appropriate, when the strate).1 F .751(gy module has)-.15 F
.434(\214nished with the b)72 578 R(uf)-.2 E(fer)-.25 E 2.934(,e)-.4 G
.434(ither normally or after an error)-2.934 F 5.434(.\()-.55 G .434
(In the latter case the)-5.434 F 407.578 580.5 402.578 580.5 DL F1 5(BE)
396.468 578 S(RR)-5 E(OR)-.4 E F0 .433(bit has presumably been)2.933 F
(set.\))72 589 Q(When the de)72 605.5 Q(vice conforms to some rather lo\
ose standards adhered to by certain DEC hardw)-.25 E(are, the routine)
-.1 E F1(de)122 622 Q(vstart\(bp, de)-.15 E(vloc, de)-.15 E
(vblk, hbcom\))-.15 E F0 .52(is useful.)72 638.5 R(Here)5.52 E F1(bp)
3.02 E F0 .52(is the address of the b)3.02 F(uf)-.2 E .52(fer header)
-.25 F(,)-.4 E F1(de)3.02 E(vloc)-.15 E F0 .521
(is the address of the slot in the de)3.02 F .521(vice re)-.25 F .521
(gisters which)-.15 F .212(accepts a perhaps-encoded de)72 649.5 R .212
(vice block number)-.25 F(,)-.4 E F1(de)2.712 E(vblk)-.15 E F0 .212
(is the block number)2.712 F 2.712(,a)-.4 G(nd)-2.712 E F1(hbcom)2.711 E
F0 .211(is a quantity to be stored in)2.711 F .769
(the high byte of the de)72 660.5 R(vice')-.25 E 3.269(sc)-.55 G .769
(ommand re)-3.269 F(gister)-.15 E 5.769(.I)-.55 G 3.269(ti)-5.769 G
3.269(su)-3.269 G .77(nderstood, when using this routine, that the de)
-3.269 F .77(vice re)-.25 F(gisters)-.15 E(are laid out in the order)72
671.5 Q(command re)122 688 Q(gister)-.15 E -.1(wo)122 699 S(rd count).1
E(core address)122 710 Q(block \(or track or sector\))122 721 Q EP
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/F0 10/Times-Roman@0 SF(-)72 11 Q/F1 10/Times-Italic@0 SF 2.5(8-U)72 58
S(nix I/O System)-2.5 E F0 .279
(where the address of the last corresponds to)72 105 R F1(de)2.779 E
(vloc.)-.15 E F0(Moreo)5.278 E -.15(ve)-.15 G 1.078 -.4(r, t).15 H .278
(he de).4 F .278(vice should correspond to the RP)-.25 F 2.778(,R)-1.11
G .278(K, and RF)-2.778 F(de)72 116 Q
(vices with respect to its layout of e)-.25 E
(xtended-memory bits and structure of read and write commands.)-.15 E
(The routine)72 132.5 Q F1 -.1(ge)122 149 S(terr).1 E(or\(bp\))-.45 E F0
.092(can be used to e)72 165.5 R .092(xamine the error bit in a b)-.15 F
(uf)-.2 E .092(fer header and arrange that an)-.25 F 2.593(ye)-.15 G
.093(rror indication found therein is re\215ected)-2.593 F .55
(to the user)72 176.5 R 5.55(.I)-.55 G 3.05(tm)-5.55 G .55
(ay be called only in the non-interrupt part of a dri)-3.05 F -.15(ve)
-.25 G 3.049(rw).15 G .549(hen I/O has completed)-3.049 F .4 LW 474.472
179 469.472 179 DL F1 2.5(\(B DONE)460.032 176.5 R F0 .549(has been)
3.049 F(set\).)72 187.5 Q F1(An e)72 209.5 Q(xample)-.2 E F0 .697
(The RF disk dri)72 226 R -.15(ve)-.25 G 3.197(ri).15 G 3.197(sw)-3.197
G .697(orth studying as the simplest e)-3.297 F .697
(xample of a block I/O de)-.15 F 3.197(vice. Its)-.25 F F1(str)3.197 E
(ate)-.15 E(gy)-.4 E F0 .697(routine checks to)3.197 F .405
(see if the requested block lies be)72 237 R .404
(yond the end of the de)-.15 F .404
(vice; the size of the disk, in this instance, is indicated by the)-.25
F .042(minor de)72 248 R .042(vice number)-.25 F 5.042(.I)-.55 G 2.542
(ft)-5.042 G .042(he request is plausible, the b)-2.542 F(uf)-.2 E .043
(fer is placed at the end of the de)-.25 F .043
(vice queue, and if the disk is)-.25 F(not running,)72 259 Q F1(rfstart)
2.5 E F0(is called.)2.5 E F1(Rfstart)72 275.5 Q F0 .349
(merely returns if there is nothing to do, b)2.849 F .349
(ut otherwise sets the de)-.2 F(vice-acti)-.25 E .649 -.15(ve \215)-.25
H .349(ag, loads the address e).15 F(xtension)-.15 E(re)72 286.5 Q
(gister)-.15 E 2.5(,a)-.4 G(nd calls)-2.5 E F1(de)2.5 E(vstart)-.15 E F0
(to perform the remaining tasks attendant on be)2.5 E
(ginning a data transfer)-.15 E(.)-.55 E .637
(When a completion or error interrupt occurs,)72 303 R F1(r\214ntr)3.137
E F0 .637(is called.)3.137 F .637
(If an error is indicated, and if the error count has not)5.637 F -.15
(ex)72 314 S .324(ceeded 10, the same transaction is reattempted; other\
wise the error bit is set.).15 F .323(If there w)5.323 F .323
(as no error or if 10 f)-.1 F(ailing)-.1 E(transfers ha)72 325 Q .3 -.15
(ve b)-.2 H(een issued the queue is adv).15 E(anced and)-.25 E F1
(rfstart)2.5 E F0(is called to be)2.5 E(gin another transaction.)-.15 E
/F2 10/Times-Bold@0 SF(Raw Block-de)72 347 Q(vice I/O)-.15 E F0 2.963
(As)72 363.5 S .463(cheme has been set up whereby block de)-2.963 F .463
(vice dri)-.25 F -.15(ve)-.25 G .463(rs may pro).15 F .464
(vide the ability to transfer information directly be-)-.15 F .72
(tween the user')72 374.5 R 3.219(sc)-.55 G .719(ore image and the de)
-3.219 F .719(vice without the use of b)-.25 F(uf)-.2 E .719
(fers and in blocks as lar)-.25 F .719(ge as the caller requests.)-.18 F
.275(The method in)72 385.5 R -.2(vo)-.4 G(lv).2 E .275
(es setting up a character)-.15 F .275
(-type special \214le corresponding to the ra)-.2 F 2.776(wd)-.15 G -.25
(ev)-2.776 G .276(ice and pro).25 F(viding)-.15 E F1 -.37(re)2.776 G(ad)
.37 E F0(and)2.776 E F1(write)72 396.5 Q F0 .336
(routines which set up what is usually a pri)2.836 F -.25(va)-.25 G .335
(te, non-shared b).25 F(uf)-.2 E .335
(fer header with the appropriate information and)-.25 F .055
(call the de)72 407.5 R(vice')-.25 E 2.555(ss)-.55 G(trate)-2.555 E .055
(gy routine.)-.15 F .055(If desired, separate)5.055 F F1(open)2.555 E F0
(and)2.555 E F1(close)2.555 E F0 .055(routines may be pro)2.555 F .056
(vided b)-.15 F .056(ut this is usually un-)-.2 F(necessary)72 418.5 Q 5
(.A)-.65 G(special-function routine might come in handy)-2.5 E 2.5(,e)
-.65 G(specially for magtape.)-2.5 E 2.9(Ag)72 435 S .399
(reat deal of w)-2.9 F .399(ork has to be done to generate the `)-.1 F
.399(`appropriate information')-.74 F 2.899('t)-.74 G 2.899(op)-2.899 G
.399(ut in the ar)-2.899 F .399(gument b)-.18 F(uf)-.2 E .399
(fer for the)-.25 F(strate)72 446 Q .46(gy module; the w)-.15 F .46
(orst part is to map relocated user addresses to ph)-.1 F .46
(ysical addresses.)-.05 F .46(Most of this w)5.46 F .46(ork is done)-.1
F(by)72 457 Q F1(physio\(str)122 473.5 Q(at, bp, de)-.15 E 1.48 -.74
(v, r)-.15 H(w\)).74 E F0 .162(whose ar)72 490 R .162
(guments are the name of the strate)-.18 F .162(gy routine)-.15 F F1
(str)2.662 E(at,)-.15 E F0 .162(the b)2.662 F(uf)-.2 E .162(fer pointer)
-.25 F F1(bp,)2.662 E F0 .162(the de)2.662 F .162(vice number)-.25 F F1
(de)2.662 E -.74(v,)-.15 G F0 .162(and a read-)3.402 F .165
(write \215ag)72 501 R F1(rw)2.665 E F0 .165(whose v)2.665 F .166
(alue is either)-.25 F 224.278 503.5 219.278 503.5 DL F1 5(BR)213.168
501 S(EAD)-5 E F0(or)2.666 E 274.6 503.5 269.6 503.5 DL F1 5(BW)263.49
501 S 2.666(RITE. Physio)-5 F F0(mak)2.666 E .166
(es sure that the user')-.1 F 2.666(sb)-.55 G .166
(ase address and count)-2.666 F .242(are e)72 512 R -.15(ve)-.25 G 2.742
(n\().15 G .242(because most de)-2.742 F .242(vices w)-.25 F .242
(ork in w)-.1 F .242(ords\) and that the core area af)-.1 F .241
(fected is contiguous in ph)-.25 F .241(ysical space; it de-)-.05 F .323
(lays until the b)72 523 R(uf)-.2 E .323(fer is not b)-.25 F(usy)-.2 E
2.823(,a)-.65 G .323(nd mak)-2.823 F .323(es it b)-.1 F .323(usy while \
the operation is in progress; and it sets up user error return)-.2 F
(information.)72 534 Q .177(The magtape dri)72 550.5 R -.15(ve)-.25 G
2.677(ri).15 G 2.677(st)-2.677 G .177
(he only one which as of this writing pro)-2.677 F .177(vides a ra)-.15
F 2.676(wI)-.15 G .176(/O capability; gi)-2.676 F -.15(ve)-.25 G(n).15 E
F1(physio,)2.676 E F0 .176(the w)2.676 F .176(ork in-)-.1 F -.2(vo)72
561.5 S(lv).2 E .988(ed is tri)-.15 F .989(vial, and amounts to passing\
 back to the user information on the length of the record read or writt\
en.)-.25 F .01
(\(There is some funniness because the magtape, uniquely among DEC de)72
572.5 R .01(vices, w)-.25 F .01(orks in bytes, not w)-.1 F 2.51
(ords.\) Putting)-.1 F(in)2.51 E(ra)72 583.5 Q 3.251(wI)-.15 G .751
(/O for disks should be equally tri)-3.251 F .751(vial e)-.25 F .752
(xcept that the disk address has to be carefully check)-.15 F .752
(ed to mak)-.1 F 3.252(es)-.1 G .752(ure it)-3.252 F(does not o)72 594.5
Q -.15(ve)-.15 G(r\215o).15 E 2.5(wf)-.25 G(rom one logical de)-2.5 E
(vice to another on which the caller may not ha)-.25 E .3 -.15(ve w)-.2
H(rite permission.).15 E EP
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