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mg_notes:apple_iic:iic_bugs [2017/11/10 02:08] M.G. fix link, not markdown |
mg_notes:apple_iic:iic_bugs [2017/11/10 03:37] M.G. |
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| lda addrh,x ; <-- SEE COMMENTS BELOW | lda addrh,x ; <-- SEE COMMENTS BELOW | ||
| and #$0f ; only lower nibble valid | and #$0f ; only lower nibble valid | ||
| - | beq tsnoram ; no RAM somehow! | + | beq tsnoram ; no RAM somehow! [note: if bank 0, skips checking result of RAM test] |
| - | bcs tsloop ; loop until we find a bank | + | bcs tsloop ; loop until we find a bank [carry set = no RAM in bank] |
| adc #1 ; C = 0 from compare | adc #1 ; C = 0 from compare | ||
| tsnoram sta numbanks,y | tsnoram sta numbanks,y | ||
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| That being said, MAME before [[https://github.com/mamedev/mame/commit/bdcb98307822e79c242782ee67bef966bf4c25de|bdcb983]] did not float the bus for $C0C0-$C0CF. A non-floating bus results in ROM $03 hanging when the card is first accessed, and, due to changes made by Apple, a hang at boot for ROM $04 (and presumably IIc Plus ROM $05). | That being said, MAME before [[https://github.com/mamedev/mame/commit/bdcb98307822e79c242782ee67bef966bf4c25de|bdcb983]] did not float the bus for $C0C0-$C0CF. A non-floating bus results in ROM $03 hanging when the card is first accessed, and, due to changes made by Apple, a hang at boot for ROM $04 (and presumably IIc Plus ROM $05). | ||
| - | Another minor issue is that the code wouldn't find a theoretical Slinky that had only one 64K bank built into it, as the loop terminates when the bank hits 0. | + | Another minor issue is that the code wouldn't find a theoretical Slinky that had only one 64K bank built into it, as the loop terminates when the bank hits 0 (beq tsnoram) even if the carry flag is clear because RAM was found in bank 0. |
| The code could be fixed by using the stack to save the counter and comes out a few bytes shorter: | The code could be fixed by using the stack to save the counter and comes out a few bytes shorter: | ||
| Line 118: | Line 118: | ||
| If it was undesirable to use the stack, we overwrite sizetemp anyway when the routine exits, so we could replace the pha/pla with sta sizetemp/lda sizetemp at the cost of 4 bytes. | If it was undesirable to use the stack, we overwrite sizetemp anyway when the routine exits, so we could replace the pha/pla with sta sizetemp/lda sizetemp at the cost of 4 bytes. | ||
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