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Added check that hi-bytes of the screen and charset addresses are in the same vic bank

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mc78 2024-08-27 15:07:30 +02:00
parent 5e530526ba
commit 34e063a2a0
1 changed files with 29 additions and 16 deletions
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45
libsrc/c64/set_viewport.s
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@ -17,23 +17,36 @@
.code
.proc _set_viewport
lsr
lsr
sta tmp1 ; chr_hi
lda #0
sta tmp2 ; bank
jsr popa ; scr_hi
asl
rol tmp2
asl
rol tmp2
ora tmp1
sta VIC_ADDR
sta tmp1 ; chr_hi
ldy #0
eor (sp), y ; eor scr_hi with chr_hi to determine VIC bank compliance
sec
bne @return ; If the upper two bits do not coincide, then the screen and char would be in different VIC banks
sty tmp2 ; store bank bits temporarily, here
lsr tmp1 ; lsr hi-byte into correct bit position for VIC_ADDR (0x08 -> 0x02, 0x10 -> 0x04 ...)
lsr tmp1 ; The vic bank check prevents doing this directly in an effective manner, beforehand
lda CIA2_pra
ora #$03
eor tmp2
sta CIA2_pra
lda (sp), y ; scr_hi
asl ; rol hi-byte into correct bit position for VIC_ADDR (0x04 -> 0x10, 0x08 -> 0x20 ...)
rol tmp2 ; rol hi-bits into bank bits
asl
rol tmp2 ; rol's 0 into carry
ora tmp1 ; combine scr and char bits to the final register value
sta VIC_ADDR
lda CIA2_pra
ora #$03 ; prepare for eor
eor tmp2 ; EOR bank bits, because in the register the bits are reversed
sta CIA2_pra
@return:
jsr incsp1 ; does not influence carry
; return EXIT_SUCCESS, if clc
; return EXIT_FAILURE, if sec
lda #0
tax
bcc :+
adc #0
:
rts
.endproc