New Life for the Z8671 BASIC Interperter
by Chris
Howie
Many of us have written mixed BASIC and assembly code for the Zilog Z8671, and have had to abandon it after Zilog discontinued the chip. This application note describes how the BASIC/Debug interpreter from the Z8671 can be installed in EPROM for a system using any other Zilog Z8-series microcontroller, restoring the capability of mixed languages. In addition, it is now possible to use any clock speed instead of being restricted to 7.8 MHz.
The original object code for the interpreter has been altered slightly, and certain SRP (Set Register Pointer) instructions have been corrected since they interfered with the Extended Addressing found in the Z86C93.
The former method for setting the baud rate involved reading address %FFFD and setting the rate according to the data found there. The new routine, which replaces the address-reading section, requires that the baud rate be predetermined and its corresponding division factor be installed in the code at the time the EPROM is created. With this method, the clock frequency is included in the calculation, thereby removing the 7.8-MHz restriction.
The databook relates the baud rate to the values found in the Counter/Timer 0 Register (R244), Prescaler 0 Register (R245), and the crystal frequency. The formula given there is
bit rate = xtal frequency / (2 x 4 x p x t x 16)
where
p = initial value in R245
t = initial value in R244
The factor of interest is the PT product, which is produced by multiplying the prescaler division ratio by the contents of the Timer register. After massaging, these values will be placed into memory locations %97 and %95, respectively, which may be done in the source code or manually during EPROM programming. Rearranging the above formula yields
PT = (xtal frequency) / (128 x baud rate)
For example, for a 20-MHz crystal and a rate of 1200 baud, the PT product will be
PT = (xtal freq) / (128 x baud rate) = (20 x 106) / (128 x 1200) = 130
When the result is less than 255, the prescaler value will be 1. When the PT product is greater than 255, the prescaler value must be chosen so T falls between 1 and 255. Note, however, the prescaler register is organized such that the modulo is contained in the highest six bit locations, so the actual value to put in the register is the desired number multiplied by four. In addition, the first bit must be set in order for the counter to free-run, so the final value to put in R245 will be four times the required prescaler value, plus one. Therefore, in this example, R245 (prescaler) will contain %05, and R244 (timer) will contain %82.
This new routine requires very little room; in fact, less than half the space taken by the %FFFD-reader. The remaining space is padded with NOPs. Listing 1 is an excerpt from the original Zilog code, and Listing 2 shows the code replacement. To use the interpreter code attached to this note, change the %FF values found at addresses %95 and %97 to the values determined by the above formulas. Finally, Listing 3 shows the raw Intel hex format code.
Listing 1:Original code from Zilog’s listing.
008C 31 F0 236 RESET: SRP #RPx ! MAKE =WORK, SO SHORT
008E B0 FB 237 CLR IRQ+1 ! CLEAR INTERRUPT MASK
0090 8C 96 238 LD rP01M,#150 ! RAM, FAST, INT STK
239 ! (10010110)
0092 FC 68 240 LD rSPL,#ISTAK ! STACK POINTER
0094 4C AF 241 LD rT0R,#175 ! INITIALLY 110 BAUD
0096 5C 0D 242 LD rT0Rp1,#13 ! FOR UART (00001101)
0098 7C 41 243 LD rP3M,#65 ! SERIAL I/O (01000001)
009A 1C 03 244 LD rTMR,#0 ! START UART CLOCK (%03)
009C CC 02 245 LD rFLAGS,#2 ! SET XEQ
009E 31 13 246 SRP #AC
00A0 B0 03 247 CLR PORT3
00A2 4C FF 248 LD rADH,#-1 ! POINT TO SWITCHES
00A4 5C FD 249 LD rADL,#-3 ! = FFFD
00A6 C2 24 250 LDC rACX,@rrADH ! GET THEM
251 ! LD rACX,#2 ! USE MUD DEFAULT !(commented out)
00A8 3C 80 252 LD rAC,#128 ! POSITION SINGLE BIT
00AA 90 E3 253 RL rAC ! (THIS MAY TAKE A WHILE
00AC 2A FC 254 DJNZ rACX,#-2 ! BUT WHO CARES?)
00AE A6 E3 20 255 CP rACX,#32 ! IF NOT 110,
00B1 6B 02 256 JR EQ $+4 ! (110 BAUD ALREADY SET)
00B3 39 F4 257 LD T0R,rAC ! THEN FIX UP TIMER
00B5 9F 258 EI ! *&%$#@ UART
00B6 76 03 01 259 TM PORT3,#1 ! IS BREAK SET?
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Listing 2:New code (new comments in lower case).
008C 31 F0 236 RESET: SRP #RPx ! MAKE =WORK, SO SHORT
008E B0 FB 237 CLR IRQ+1 ! CLEAR INTERRUPT MASK
0090 8C 96 238 LD rP01M,#150 ! RAM, FAST, INT STK
239 ! (10010110)
0092 FC 68 240 LD rSPL,#ISTAK ! STACK POINTER
0094 4C FF 241 LD rT0R,#255 ! magic location %95 for T0
0096 5C FF 242 LD rT0Rp1,#255 ! " " %97 for PRE0
0098 7C 41 243 LD rP3M,#65 ! SERIAL I/O (01000001)
009A 1C 03 244 LD rTMR,#0 ! START UART CLOCK (%03)
009C CC 02 245 LD rFLAGS,#2 ! SET XEQ
009E 31 10 246 SRP #AC ! note cleanup of low nybble
00A0 B0 03 247 CLR PORT3
00A2 8D 00 B5 248 JP 00B5 ! jump over NOPs
00A4 FF FF 249 NOP
00A6 FF FF 250 NOP
251
00A8 FF FF 252 NOP
00AA FF FF 253 NOP
00AC FF FF 254 NOP
00AE FF FF FF 255 NOP
00B1 FF FF 256 NOP
00B3 FF FF 257 NOP
00B5 9F 258 EI ! *&%$#@ UART
00B6 76 03 01 259 TM PORT3,#1 ! IS BREAK SET?
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Listing 3:Pure hex code for the interpreter.
1000100310061009100C100F8B7EE33F FEE32FFEAF3C20D60061762007EBF6AF 065020081B0D0AFED600813C05D6063E D60066D60643EBF8B02000ED52355224 AFD6004728E30DB0E24E4A03E3350DC2 34A0E4AF66FB882D101276FA086BFB38 F0202056E37F66FB882D101556FAE739 F076FA106BFBAFD605FAA0E4D6008138 E24E4A03F3530DD23480E4AF31F0B0FB 8CB6FCF04CFF5CFF7C411C03CC023110 B003D2668D00B5FFFFFFFFFFFFFFFFFF FFFFFFFFFF9F7603016B574C07D605BF 7B17EBF94908D605BF7B026BF900E449 0C490AB009E60B200DB00CCC00DC204C 0F5C20D605BF6B0C2E2A09E408045905 490859093C02D6063E480858092CFFD2 24A0E4D224C2242EEB0A023512242904 3905B0FC3100DC68E6F8B2CECA07B0EA BC68E6F8B668EA78EB3110E406FEE407 FF98FC56E902FC80D6063C88E3C23C02 936CFF7CFFC238A0E81C1FA6E30A7B5B A6E3307B18A6E3407B3FB0E052136B04 0219120856E3E0F0E306E3040D023306 E35F2C05C2E2A0E2C23228EED4E23110 8BC376FC026BA2D604DD6B86D6063C88 E3C29CD603926B6AAFC218A0E80C0752 0326E00406E308C0E38BB5023FE303E3 1FF331F3F08BC950E250E350E050E18B 0B50E250E370E970E8D601FAD602558B AFD6000E50E050E170E370E2D601FA8B 4448EA58EBC228A0E8D605D322236BF5 12226B65A8E4B8E58B573100CECA05A6 FF408B08A4E4FEEB03A4E5FF31108B73 50E050E1A40AFEEB03A40BFFFB6750E2 50E3D6026EE4FE06E4FF078BA2D6000E D6026E8B3900EFB1EFC238A0E88B33D6 05FA023512248B2626E12036E0000D80 EA42006B3088E098E1AFD605D3A6E341 7BEDA6E35ABBE8023306E3A0280C00EF F3F200EFF3F3A6EF403B0A440C0CEBD9 A40DEFBBD4D604A0D6063CC22C229232 8338E928E8FC80D604E99C4A8C06D605 5676FC026B0AD6055638172816D604E9 D6002BB0FC8D0129D605D36B4D8B90D6 05D39B8B56E30FD60638D605D39B3B1C 0AD603148BF4D6000EB0E0B0E1EC1122 133202FB0402131202EF11EFFE11EF00 EF10E110E0EAE8AFD60636D605D31B0D A6E3417B0526E3477B0380EAAF1C10D6 03148BE7B0E056E30F8B08E31FFEE30F FEB0E3B0E2EC11CFFEC1EF00EFC1EFFB 0402311220C0E2C0E3EAEDAFD60061DF D605D46BC5A6E3226B2A8BF0D605FDA8 08B80908E618E7D604CF8B03D604DDA2 46EB02A257BBF56B0DC2E8A6EE286B06 78E168E0A0E8AF380E0D38EAA40CE3EB 05A90EB90FAF280E380FD60381A8E2B8 E3AF66FB882B0C3C04D6063EB4F0E302 336BD338FC46E3FE3EEBC9AFD60049EE 3E3AF9AF480E580FB0EED603ACD6000E 58EB48EA80EA80EADF3273326278E3FB 3BEA04ECFD2CFFEE60EED603AC60EE6B 54DB2F58EB48EA08E418E5225E36E4FF D6004FD230A0E0A404147BF4A405157B EF090419052E6B4100E28B29EA01AFEE EEEE5805480418E508E4025E16E400D6 020449045905C23080E0D60087A24ABB F5A25BBBF1D22AA0EAD27A480E580FA0 EAD60049D23A3E3AF6AF00EFE60D67A8 0CBC68200DD6005428E33C03D6063EA2 326B36C23CA2326B3742226BE8A6E27F 6BE3A6E20A6BDE580D480C3C0DB2326D 0027D6007FA60DE7BB084E4AC6A40DEF BBC13C07D600668BBC000DA40DEB3BB5 E60D682C0D8BD0D6000EE3EFFE233FFE 332F1B0AAB0442326B02E0EEE0EEE0EE FB02A0E8E33FFEAFE6FC0280EA80EAC2 6AA0EAC27AA0EAE8E652E7EEAFC23AA0 EA3E3AF98BE9D6000EEC10B0E1B0E012 331222121140E1120040E0EAF200EFF3 F000EFF3F112336B08D605443CEEFE8B 173CEEFE2CF0532FEB0E06E3066B0943 2FEB0506E306FAEC06E3412C0530E2FF E33FD60545E33FD6054400EFE33FD605 45D6000E0DF0E356E30F2C09A22316E3 3040E38D0061C238D60061C238A0E83E DBF4AF01C102550248025101E1025A02 BF02B8039202390235022D000E061303 7A03770627007701D10210034C061C02 3F1018031B02D604A70182101B101E04 E6033F0015002B0556051102F8044A06 2F04DD03B400F404C80112060401B7DF 4E6B0FC23460E3D23460E3C224D23412 32CFAFCFAEAA03E33B0DC23AA0EA7B17 A6E3206BEEA6E3607B0326E320A6E330 1B05A6E33A1B024233AFD6000EE35FFE E34FFEAFD605FAE31FFEE30FFED4E031 108B20D6061C4222DB02A0E8D6000E60 E260E3A0E28B0CD605FDD600418B0428 E638E78D026EB0E3B0E28BF7B0E356ED F042D3C23CDEA6E3FEAF2187204154A0 627A6F24BA27E15BD30F60100D16280E 2A5110110C0016280F600E2C8F47CF9B 54CF351CE010110F601650609D49C634 E884544845CE1C1DC14A41895355C234 FFE028145480C034F07A9B4C45D4A680 BD34ED137184DE3525559AC00C81351E 80BD34DC2E0EE11D83BA3BB082A27B9D 5052494ED483BA2351EE6883BBE71D83 AC225A234182BA4285A221604E748848 4558A83516254434A7170F6324AD6117 20548C5245D4835552CEE015161D6086 53544FD0E02D855255CE3B56854E45D7 E02B845245CD1D9D49CE865055D4A010 63A010E6243FA027E15A81AC34621311 82AC4E3B818C4C4953D428E80CFF170C 7F673B5A3446E00301030216280E230A 740D0B03281C6E280F6A2024A0216324 A25B294A0E0E161D1D34258EBC84BD0B 067684BE0B05710B046E89BE84BD0B03 670B016480BD0B0267340162340297AC 85AD3414176481AB340E85AB340A185A 86AD340417580D2F342485AA34201A5A 92AF170F6734170F711B174E17340F0F 651B4794DC57170F634F170F60538A55 5352A833B480A92E2F89414E44A80C3C 33AA5287C00C4733E50D4C84DE33DF61 A2122F83A5262FC12F80A8339380A92F |