Differences
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mg_notes:apple_iic:iic_bugs [2017/11/07 12:59] M.G. [Memory Expansion Firmware] |
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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| If you note the code starting at the indicated line, you can see that the code grabs the high byte of the Slinky address register and uses the low nibble directly as a counter value. | If you note the code starting at the indicated line, you can see that the code grabs the high byte of the Slinky address register and uses the low nibble directly as a counter value. | ||
| - | The problem is that when there is no memory expansion card installed, there is no register, and the value there is floating bus. The only reason the code gets out of the loop is because the floating bus happens to usually have a bunch of bytes streaming by that have 0 in the low byte. | + | The problem is that when there is no memory expansion card installed, there is no register, and the value there is floating bus. The only reason the code gets out of the loop is because the floating bus happens to usually have a bunch of bytes streaming by that have 0 in the low nibble. |
| - | That being said, the current MAME does not float the bus for $C0C0-$C0CF. This results in ROM $03 hanging when the card is first accessed, and, due to changes in in ROM $04, hang at boot for ROM $04. | + | 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: | ||
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| 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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