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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 03:46] (current) 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 | ||
| Line 52: | Line 52: | ||
| 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 78: | Line 78: | ||
| sta data,x ; restore data | sta data,x ; restore data | ||
| pla ; restore counter (net save 4 bytes) | pla ; restore counter (net save 4 bytes) | ||
| - | beq tsnoram ; no RAM somehow! (if we fixed to find lonely bank 0, returned size would be wrong) | + | beq tsnoram ; no RAM somehow! |
| bcs tsloop ; loop until we find a bank | bcs tsloop ; loop until we find a bank | ||
| adc #1 ; C = 0 from compare | adc #1 ; C = 0 from compare | ||
| + | tsnoram sta numbanks,y | ||
| + | lsr a | ||
| + | sta sizetemp ; sizetemp = upper byte of block count | ||
| + | rts | ||
| + | </code> | ||
| + | |||
| + | We could use a couple of those bytes to fix the bank 0 issue: | ||
| + | |||
| + | <code> | ||
| + | testsize equ * | ||
| + | lda #0 ; zero address reg l/m | ||
| + | sta addrl,x | ||
| + | sta addrm,x | ||
| + | lda #$10 ; start at 1 meg and go down | ||
| + | sec | ||
| + | tsloop sbc #1 ; move down a bank | ||
| + | sta addrh,x | ||
| + | pha ; save counter, +1 byte code added | ||
| + | lda data,x ; save existing data | ||
| + | pha | ||
| + | dec addrl,x ; fix address (undo auto-increment) | ||
| + | lda #$a5 ; common apple check byte | ||
| + | sta data,x ; store it | ||
| + | dec addrl,x ; fix... | ||
| + | eor data,x ; 0 if the data is there | ||
| + | dec addrl,x ; fix... | ||
| + | cmp #1 ; C = 0 if data okay | ||
| + | pla | ||
| + | sta data,x ; restore data | ||
| + | pla ; restore counter (net save 4 bytes) | ||
| + | bcc tsgotram ; RAM found, extra branch net cost 2 bytes | ||
| + | beq tsnoram ; still won't find lonely bank 0 | ||
| + | bne tsloop ; loop until we find a bank | ||
| + | tsgotram adc #1 ; C = 0 from compare | ||
| tsnoram sta numbanks,y | tsnoram sta numbanks,y | ||
| lsr a | lsr a | ||
| Line 108: | Line 142: | ||
| sta data,x ; restore data | sta data,x ; restore data | ||
| pla ; get counter back, sets N and Z, net save 4 bytes over existing code | pla ; get counter back, sets N and Z, net save 4 bytes over existing code | ||
| + | bcc tsgotram ; RAM found, extra branch net cost 2 bytes | ||
| beq tsnoram ; still won't find lonely bank 0 | beq tsnoram ; still won't find lonely bank 0 | ||
| - | bcs tsloop ; loop until we find a bank | + | bra tsloop ; loop until we find a bank |
| - | adc #1 ; C = 0 from compare, could also use inc a | + | tsgotram: adc #1 ; C = 0 from compare, could also use inc a |
| tsnoram: sta numbanks,y | tsnoram: sta numbanks,y | ||
| lsr a | lsr a | ||
| Line 118: | Line 153: | ||
| 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. | ||
| - | |||