92 lines
1.6 KiB
PHP
92 lines
1.6 KiB
PHP
; ... change system memory variables bla
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jmp cont
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lowadr: .word __SAVEAREA_LOAD__ & $FF00 ; our load address, rounded down to page boundary
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tstadr: .res 2
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tstadr2:.res 2
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tmp: .res 1
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; When setting a display mode, the ROM takes the RAMTOP value
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; and subtracts the size of the screen memory from it. This will
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; become the new screen memory address.
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; From this address it subtracts the size of the display list.
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; This will become the new display list address.
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; Screen memory cannot cross 4K boundaries and a display list
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; cannot cross a 1K boundary.
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; Work out a sane value for RAMTOP to prevent boundary crossing.
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; RAMTOP is only one byte, it counts in memory pages.
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cont:
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_SCRBUFSZ = (40 * 24) ; size of mode 0 screen buffer
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_DLSZ = 32 ; size of mode 0 display list
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; subtract screen memory size from our load address
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lda lowadr
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sec
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sbc #<_SCRBUFSZ
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sta tstadr
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lda lowadr+1
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sbc #>_SCRBUFSZ
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sta tstadr+1
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; check if a 4K boundary is crossed
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lda lowadr+1
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and #$f0
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sta tmp
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lda tstadr+1
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and #$f0
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cmp tmp
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beq scrmemok
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; 4K boundary will be crossed, take 4K boundary address as lowadr
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al4k: lda lowadr+1
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and #$f0
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tax
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dex
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stx lowadr+1
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bne cont
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; subtract display list size from calculated screen address
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scrmemok:
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lda tstadr
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sec
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sbc #<_DLSZ
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sta tstadr2
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lda tstadr+1
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sbc #>_DLSZ
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sta tstadr2+1
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; check if a 1K boundary is crossed
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lda tstadr+1
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and #$fc
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sta tmp
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lda tstadr2+1
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and #$fc
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cmp tmp
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bne al4k ; 1K boundary will be crossed, decrease lowadr
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; address of display list is ok
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dlok:
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; decrease tstadr2 by two
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lda tstadr2
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sec
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sbc #2
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sta tstadr2
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bcs dec_cont
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lda tstadr2+1
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sbc #0
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sta tstadr2+1
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dec_cont:
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