Odds And Ends:
A Fat Forty Bug
Some machine language programs which work just fine in PETs with BASIC 4.0 and 9-inch screens will yield odd results on the new 12-inch, 40-column machines. Occasionally, it will appear as if the program has responded inaccurately to the key which was pressed.
This is due to a very subtle difference in the ROM. If the machine language program happens to be in decimal mode when an interrupt occurs, the keyboard decode is completely inaccurate. Even if a key is still pressed from a previous character, it will be decoded as a different character, so, instead of getting one character, you get three (the original correct one, one that is wrong, and probably the original one again).
The problem is well illustrated by the program listing attached. If you assemble the program as shown, then from the monitor type: G 0800 you can type on the keyboard and see what character the PET thought was pressed. By the way, to get out of the program, press the RVS key. Now change the SED (set decimal mode) to a NOP. The program gives accurate keyboard decode. Now the STOP key will get you out.
The way to get around this bug, if you must use decimal mode, is to precede the routine with an SEI (disable interrupts) instruction, and follow it with a CLI (enable interrupts).
0010 .BA $0800 0020 .OS 0030 ; TEST KEYBOARD DECODE 0040 ; WITH DECIMAL MODE SET! 0050 GET .DE $FFE4 0060 PRINT .DE $FFD2 0070 ; 0800- F8 0080 STRT SED 0090 ; 0801- CA 0100 DLLOOP DEX 0802- DO FD 0110 BNE DLLOOP 0804- 88 0120 DEY 0805- DO FA 0130 BNE DLLOOP 0140 ; 0807- D8 0150 CLD 0808- 20 E4 FF 0160 JSR GET 080B- C9 00 0170 CMP #0 080D- F0 Fl 0180 BEQ STRT 080F- C9 03 0190 CMP #3 0811- F0 06 0200 BEQ STOP 0813- 20 D2 FF 0210 JSR PRINT 0816- 4C 00 08 0220 JMP STRT 0819- 00 0230 STOP BRK 0240 .EN LABEL FILE: [/ = EXTERNAL] /GET = FFE4 /PRINT = FFD2 STRT = 0800 DLLOOP = 0801 STOP = 0819 //0000, 081A, 081A