Fix check whether there is enough memory btw. MEMLO and the program

load address to move screen buffer there.

libsrc/atari/shadow_ram_prepare.s

@@ -107,26 +107,18 @@ sramprep:
 	ora	#2
 	sta	PORTB
 
+.include "xlmemchk.inc"
 
-; ... change system memory variables bla
+	ldx	tstadr2
-
+	stx	MEMTOP
-CMPVAL = 64+255+992		; you may ask, why these values...   @@@ document
+	stx	APPMHI
-
+	lda	tstadr2+1
-sys_ok:	lda	#<__SAVEAREA_LOAD__
-	sec
-	sbc	#<CMPVAL
-	sta	MEMTOP
-	sta	APPMHI
-	lda	#>__SAVEAREA_LOAD__
-	sbc	#>CMPVAL
 	sta	MEMTOP+1
 	sta	APPMHI+1
-
+	lda	lowadr+1
-	lda	#>__SAVEAREA_LOAD__ - 1
 	sta	RAMTOP
 
 
-
 ; ... issue a GRAPHICS 0 call (copied'n'pasted from TGI drivers)
 
 

libsrc/atari/system_check.s

@@ -90,27 +90,15 @@ is_xl:	lda	RAMSIZ
 fail:	jsr	delay
 	jmp	(DOSVEC)
 
-lowadr:	.res	2		; lowest address we need in order to move screen memory down, depending on start address of program
 
-
+sys_ok:
-; system is basically supported, check if there is enough space btw. MEMLO and our start address
+.include "xlmemchk.inc"
-; to move screen memory there
-
-CMPVAL = 64+255+992		; you may ask, why these values...   @@@ document
-
-sys_ok:	lda	#<__SAVEAREA_LOAD__
-	sec
-	sbc	#<CMPVAL
-	sta	lowadr
-	lda	#>__SAVEAREA_LOAD__
-	sbc	#>CMPVAL
-	sta	lowadr+1
 
 	sec
 	lda	MEMLO
-	sbc	lowadr
+	sbc	tstadr2
 	lda	MEMLO+1
-	sbc	lowadr+1
+	sbc	tstadr2+1
 	bcc	memlo_ok
 
 ; load address was too low

libsrc/atari/xlmemchk.inc

@@ -0,0 +1,91 @@
+; ... change system memory variables bla
+
+
+	jmp	cont
+
+lowadr:	.word	__SAVEAREA_LOAD__ & $FF00		; our load address, rounded down to page boundary
+tstadr:	.res	2
+tstadr2:.res	2
+tmp:	.res	1
+
+
+; When setting a display mode, the ROM takes the RAMTOP value
+; and subtracts the size of the screen memory from it. This will
+; become the new screen memory address.
+; From this address it subtracts the size of the display list.
+; This will become the new display list address.
+; Screen memory cannot cross 4K boundaries and a display list
+; cannot cross a 1K boundary.
+; Work out a sane value for RAMTOP to prevent boundary crossing.
+; RAMTOP is only one byte, it counts in memory pages.
+
+cont:
+
+_SCRBUFSZ =	(40 * 24)		; size of mode 0 screen buffer
+_DLSZ	=	32			; size of mode 0 display list
+
+; subtract screen memory size from our load address
+
+	lda	lowadr
+	sec
+	sbc	#<_SCRBUFSZ
+	sta	tstadr
+	lda	lowadr+1
+	sbc	#>_SCRBUFSZ
+	sta	tstadr+1
+
+; check if a 4K boundary is crossed
+
+	lda	lowadr+1
+	and	#$f0
+	sta	tmp
+	lda	tstadr+1
+	and	#$f0
+	cmp	tmp
+	beq	scrmemok
+
+; 4K boundary will be crossed, take 4K boundary address as lowadr
+
+al4k:	lda	lowadr+1
+	and	#$f0
+	tax
+	dex
+	stx	lowadr+1
+	bne	cont
+
+; subtract display list size from calculated screen address
+
+scrmemok:
+	lda	tstadr
+	sec
+	sbc	#<_DLSZ
+	sta	tstadr2
+	lda	tstadr+1
+	sbc	#>_DLSZ
+	sta	tstadr2+1
+
+; check if a 1K boundary is crossed
+
+	lda	tstadr+1
+	and	#$fc
+	sta	tmp
+	lda	tstadr2+1
+	and	#$fc
+	cmp	tmp
+	bne	al4k		; 1K boundary will be crossed, decrease lowadr
+
+; address of display list is ok
+
+dlok:
+
+; decrease tstadr2 by two
+
+	lda	tstadr2
+	sec
+	sbc	#2
+	sta	tstadr2
+	bcs	dec_cont
+	lda	tstadr2+1
+	sbc	#0
+	sta	tstadr2+1
+dec_cont: