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120 lines
3.1 KiB
PHP
120 lines
3.1 KiB
PHP
;
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; Christian Groessler, Jun-2013
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;
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; This routine is used in preparation to move the screen memory
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; in front of the program.
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;
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; It calculates the value to put into RAMTOP for a subsequent
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; "GRAPHICS 0" call, and the lowest address which will be used
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; by the screen memory afterwards.
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;
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; inputs:
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; __STARTADDRESS__ - load address of the program
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; outputs:
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; lodadr - (high byte only) value to
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; write into RAMTOP
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; lowadr - lowest address occupied by
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; screen data
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;
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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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;
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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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;
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; The ROM doesn't do this boundary checking, since it doesn't
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; expect RAMTOP to have (rather) arbitrary values. For a
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; "GRAPHICS 0" call and RAMTOP representing the possible physically
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; available memory, boundary crossing cannot happen.
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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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scrmemtst:
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; subtract screen memory size from our load address
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lda lodadr
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sec
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sbc #<SCRBUFSZ
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sta tstadr
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lda lodadr+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 lodadr+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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; if lodadr is at an exact 4K boundary, it's still ok
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lda lodadr+1
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and #$0f
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beq scrmemok
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; 4K boundary will be crossed, use this 4K boundary address as lodadr
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al4k: lda lodadr+1
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and #$f0
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sta lodadr+1
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bne scrmemtst
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; not reached
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.ifdef DEBUG
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.byte "XLMEMCHK:>"
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.endif
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lodadr: .word __STARTADDRESS__ & $FF00 ; our program's load address, rounded down to page boundary
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tstadr: .res 2
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lowadr: .res 2
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tmp: .res 1
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; subtract display list size from calculated screen address
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scrmemok:
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lda tstadr
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sec
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sbc #<DLSZ
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sta lowadr
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lda tstadr+1
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sbc #>DLSZ
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sta lowadr+1
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.if 0 ; this cannot happen
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; check if a 1K boundary is crossed
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lda tstadr+1
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and #$fc
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sta tmp
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lda lowadr+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 lodadr
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.endif
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; address of display list is ok
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; decrease lowadr by two
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lda lowadr
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sec
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sbc #2
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sta lowadr
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bcs dec_cont
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dec lowadr+1
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dec_cont:
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