📄 chap.8.interrupts.ps
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1740(\256nished,)X2098(and)X2267(then)X2463(immediately)X2976(return)X3236(to)X3341(the)X3489(caller.)X3757(Thus,)X4002(another)X776 5699(subroutine)N1217(\(an)X1375(interrupt)X1742(service)X2050(routine)X2358(in)X2468(fact\))X2680(must)X2902(be)X3028(written)X3336(and)X3511(initialized)X3934(such)X4146(that)X776 5811(when)N1022(the)X1178(printer)X1473(interrupts)X1880(the)X2035(CPU,)X2282(it)X2373(is)X2474(invoked)X2821(and)X2997(will)X3184(print)X3403(the)X3558(next)X3761(character)X4152(and)X64 p%%Page: 64 712 s 0 xH 0 xS 1 f3 f576 400(57:017)N872(CIE)X1 f2329(-)X2385(64)X2505(-)X3 f3717(Lecture)X4062(Notes)X1 f776 803(enable)N1052(the)X1194(next)X1384(interrupt.)X576 943(The)N750(Printer)X1036(interface)X1398(is)X1486(assumed)X1841(to)X1940(consist)X2231(of:)X10 f576 1083(g)N1 f634(A)X727(Display)X1050(Control)X1368(and)X1531(Status)X1790(Register)X2135(\(DSPCS\))X776 1223(:)N872(A)X968(value)X1204(of)X1311(0x00)X1530(written)X1830(by)X1953(the)X2098(CPU)X2311(to)X2413(this)X2579(register)X2895(disables)X3232(the)X3378(printer)X3663(from)X3878(both)X4077(print-)X872 1335(ing)N1019(and)X1182(generating)X1613(interrupts.)X776 1475(:)N872(A)X965(value)X1199(of)X1304(0x41)X1521(tells)X1707(the)X1850(device)X2127(to)X2227(print)X2434(the)X2577(character)X2956(currently)X3329(in)X3429(DSPDATA,)X3919(and)X4083(when)X872 1587(done,)N1107(to)X1206(generate)X1557(an)X1672(interrupt)X2028(back)X2234(to)X2333(the)X2475(CPU.)X10 f576 1727(g)N1 f634(A)X727(Display)X1050(Data)X1256(Register)X1601(\(DSPDATA\))X776 1867(:)N872(This)X1068(register)X1382(is)X1471(written)X1769(to)X1869(by)X1990(the)X2133(CPU)X2344(and)X2508(will)X2683(contain)X2992(the)X3135(ASCII)X3411(code)X3619(for)X3757(the)X3901(next)X4093(char-)X872 1979(acter)N1084(to)X1183(be)X1298(printed)X1595(by)X1715(the)X1857(device.)X576 2119(We)N734(assume)X1041(the)X1183(two)X1351(registers)X1701(above)X1955(are)X2097(mapped)X2426(to)X2525(memory)X2870(locations)X3242(as)X3346(follows:)X776 2231(DSPCS)N1092(-)X1148(0x00FFF005)X776 2343(DSPDATA)N1241(-)X1297(0x00FFF007)X576 2483(Figure)N854(8.5)X1001(shows)X1267(the)X1412(entire)X1659(program)X2012(and)X2178(the)X2323(interface)X2689(needed)X2990(to)X3093(implement)X3534(the)X3680(overlapped)X4136(ver-)X576 2595(sion)N763(of)X7 f904(strout)X1 f1252(.)X1327(Note)X1541(\256rst,)X1741(that)X1913(the)X2057(main)X2276(program)X2628(is)X2718(nearly)X2985(identical)X3344(to)X3445(the)X3589(one)X3754(shown)X4030(in)X4131(Fig-)X576 2707(ure)N728(8.4.)X901(The)X1080(only)X1280(different)X1642(is)X1736(in)X1841(line)X2016(22,)X2166(where)X2431(the)X2579(interrupt)X2941(vector)X3212(table)X3430(location)X3771(for)X3913(this)X4082(inter-)X576 2819(rupt)N755(is)X843(initialized.)X1302(The)X1476(interrupt)X1832(vector)X2097(being)X2335(used)X2535(in)X2634(this)X2797(example)X3148(is)X3236(the)X3378(one)X3541(at)X3635(memory)X3980(location)X576 2931(0x00000020.)N776 3071(The)N951(declarations)X1442(and)X1606(de\256nes)X1903(in)X2003(lines)X2210(1-4)X2363(are)X2 f2506(global)X1 f2752(.)X2801(That)X3003(is,)X3116(they)X3308(are)X3452(accessible)X3869(and)X4034(visible)X576 3183(to)N679(all)X804(functions,)X1214(as)X1322(well)X1515(as)X1622(the)X1767(main)X1987(program.)X2364(The)X2541(variables)X7 f2950(sbusy)X1 f3240(,)X7 f3325(Ibufpnt)X1 f3731(,)X3782(and)X3948(the)X4093(array)X7 f576 3295(ibuf)N1 f833(are)X976(used)X1177(for)X1314(communication)X1939(between)X2286(the)X2430(two)X2600(functions)X2984(and)X3149(the)X3293(main)X3512(program.)X3912(The)X4088(func-)X576 3407(tion)N7 f795(strout)X1 f1177(``busy)X1456(waits'')X1757(on)X7 f1921(sbusy)X1 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f3894(will)X4077(block)X576 3967(unless)N852(this)X1027(entire)X1283(string)X1538(has)X1702(been)X1920(printed.)X2277(\()X7 f2309(sbusy)X1 f2635(is)X2735(cleared)X3050(only)X3257(in)X7 f3403(strint)X1 f3788(after)X4002(the)X4157(last)X576 4079(character)N957(in)X7 f1092(ibuf)X1 f1350(has)X1504(been)X1712(printed\).)X2067(Finally,)X2390(in)X2491(lines)X2699(10-11,)X2973(the)X3117(character)X3497(to)X3598(be)X3715(printed)X4014(is)X4104(writ-)X576 4191(ten)N733(to)X847(the)X1004(display)X1321(data)X1521(register,)X1873(and)X2051(the)X2208(value)X2456(0x41)X2687(is)X2790(written)X3102(to)X3216(the)X3373(display)X3690(control)X4002(register)X576 4303(which)N848(tells)X1046(the)X1201(printer)X1494(to)X1605(begin)X1855(printing)X2196(the)X2350(character,)X2764(and)X2939(also)X3130(enables)X3455(an)X3582(interrupt)X3950(from)X4173(the)X576 4415(printer)N857(when)X1089(it)X1167(\(the)X1341(printer\))X1654(completes)X2069(printing)X2398(the)X2540(character)X2918(stored)X3177(in)X3276(DSPDATA.)X7 f776 4555(strint)N1 f1156(is)X1252(the)X1402(actual)X1665(interrupt)X2030(service)X2336(routine)X2642(that)X2820(is)X2917(invoked)X3260(everytime)X3679(the)X3830(printer)X4120(gen-)X576 4667(erates)N826(an)X942(interrupt)X1299(to)X1399(the)X1542(CPU.)X1777(The)X1952(routine)X2250(in)X2350(fact)X2520(only)X2716(checks)X3003(to)X3103(see)X3250(if)X3333(there)X3550(is)X3638(yet)X3780(another)X4093(char-)X576 4779(acter)N796(to)X903(print,)X1141(and)X1312(if)X1403(there)X1628(is,)X1748(it)X1834(does)X2042(so.)X2183(Otherwise,)X2634(it)X2720(disables)X3062(printing)X3400(and)X3572(clears)X7 f3864(sbusy)X1 f4154(.)X4211(In)X576 4891(line)N746(13,)X891(the)X1034(pointer)X1332(to)X1432(the)X1575(next)X1766(character)X2145(is)X2234(updated.)X2612(In)X2717(line)X2887(14,)X3032(the)X3175(character)X3554(is)X3643(checked)X3984(to)X4084(see)X4232(if)X576 5003(it)N657(is)X748(the)X893(NUL)X1117(\(string)X1395(termination\))X1899(character.)X2305(If)X2397(not,)X2572(then)X2766(there)X2987(is)X3079(another)X3396(character)X3778(to)X3881(print.)X4115(Oth-)X576 5115(erwise,)N889(the)X7 f1079(else)X1 f1349(clause)X1628(of)X1746(lines)X1966(18-19)X2228(sends)X2479(the)X2635(command)X3052(to)X3164(the)X3319(printer)X3613(\(0x00\))X3906(to)X4018(disable)X576 5227(printing)N905(and)X1068(further)X1354(interrupts,)X1771(and)X1934(also)X2113(clears)X7 f2396(sbusy)X1 f2686(.)X65 p%%Page: 65 812 s 0 xH 0 xS 1 f3 f576 400(57:017)N872(CIE)X1 f2329(-)X2385(65)X2505(-)X3 f3717(Lecture)X4062(Notes)X7 f576 851(1)N1152(#define)X1728 0.4062(STRINTVECaddr)AX2880 0.4167(0x00000020)AX576 1075(2)N1152(int)X1728(sbusy=0;)X576 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