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|
;
; Christian Groessler, Jun-2013
;
; This routine is used in preparation to move the screen memory
; in front of the program.
;
; It calculates the value to put into RAMTOP for a subsequent
; "GRAPHICS 0" call, and the lowest address which will be used
; by the screen memory afterwards.
;
; inputs:
; __STARTADDRESS__ - load address of the program
; outputs:
; lodadr - (high byte only) value to
; write into RAMTOP
; lowadr - lowest address occupied by
; screen data
;
; When setting a display mode, the ROM takes the RAMTOP value
; and subtracts the size of the screen memory from it. This will
; become the new screen memory address.
; From this address it subtracts the size of the display list.
; This will become the new display list address.
; Screen memory cannot cross 4K boundaries and a display list
; cannot cross a 1K boundary.
;
; Work out a sane value for RAMTOP to prevent boundary crossing.
; RAMTOP is only one byte, it counts in memory pages.
;
; The ROM doesn't do this boundary checking, since it doesn't
; expect RAMTOP to have (rather) arbitrary values. For a
; "GRAPHICS 0" call and RAMTOP representing the possible physically
; available memory, boundary crossing cannot happen.
SCRBUFSZ = (40 * 24) ; size of mode 0 screen buffer
DLSZ = 32 ; size of mode 0 display list
scrmemtst:
; subtract screen memory size from our load address
lda lodadr
sec
sbc #<SCRBUFSZ
sta tstadr
lda lodadr+1
sbc #>SCRBUFSZ
sta tstadr+1
; check if a 4K boundary is crossed
lda lodadr+1
and #$f0
sta tmp
lda tstadr+1
and #$f0
cmp tmp
beq scrmemok
; if lodadr is at an exact 4K boundary, it's still ok
lda lodadr+1
and #$0f
beq scrmemok
; 4K boundary will be crossed, use this 4K boundary address as lodadr
al4k: lda lodadr+1
and #$f0
sta lodadr+1
bne scrmemtst
; not reached
.ifdef DEBUG
.byte "XLMEMCHK:>"
.endif
lodadr: .word __STARTADDRESS__ & $FF00 ; our program's load address, rounded down to page boundary
tstadr: .res 2
lowadr: .res 2
tmp: .res 1
; subtract display list size from calculated screen address
scrmemok:
lda tstadr
sec
sbc #<DLSZ
sta lowadr
lda tstadr+1
sbc #>DLSZ
sta lowadr+1
.if 0 ; this cannot happen
; check if a 1K boundary is crossed
lda tstadr+1
and #$fc
sta tmp
lda lowadr+1
and #$fc
cmp tmp
bne al4k ; 1K boundary will be crossed, decrease lodadr
.endif
; address of display list is ok
; decrease lowadr by two
lda lowadr
sec
sbc #2
sta lowadr
bcs dec_cont
dec lowadr+1
dec_cont:
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