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

load address to move screen buffer there.

diff --git a/libsrc/atari/shadow_ram_prepare.s b/libsrc/atari/shadow_ram_prepare.s
index e3be4799df9b9c72088d34d90327456ba22fd040..81b05ece3eb5b7d5f712cf6e11ffe26f7e065f27 100644 (file)
--- a/libsrc/atari/shadow_ram_prepare.s
+++ b/libsrc/atari/shadow_ram_prepare.s
@@ -107,26 +107,18 @@ sramprep:
        ora     #2
        sta     PORTB
 
+.include "xlmemchk.inc"
 
-; ... change system memory variables bla
-
-CMPVAL = 64+255+992            ; you may ask, why these values...   @@@ document
-
-sys_ok:        lda     #<__SAVEAREA_LOAD__
-       sec
-       sbc     #<CMPVAL
-       sta     MEMTOP
-       sta     APPMHI
-       lda     #>__SAVEAREA_LOAD__
-       sbc     #>CMPVAL
+       ldx     tstadr2
+       stx     MEMTOP
+       stx     APPMHI
+       lda     tstadr2+1
        sta     MEMTOP+1
        sta     APPMHI+1
-
-       lda     #>__SAVEAREA_LOAD__ - 1
+       lda     lowadr+1
        sta     RAMTOP
 
 
-
 ; ... issue a GRAPHICS 0 call (copied'n'pasted from TGI drivers)
 
 

diff --git a/libsrc/atari/system_check.s b/libsrc/atari/system_check.s
index d197dfdb62b43331af34642c832c9ec9d222002d..bb0fe483e3045396300511de3b3719f455c03aec 100644 (file)
--- a/libsrc/atari/system_check.s
+++ b/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
 
-
-; system is basically supported, check if there is enough space btw. MEMLO and our start address
-; 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
+sys_ok:
+.include "xlmemchk.inc"
 
        sec
        lda     MEMLO
-       sbc     lowadr
+       sbc     tstadr2
        lda     MEMLO+1
-       sbc     lowadr+1
+       sbc     tstadr2+1
        bcc     memlo_ok
 
 ; load address was too low

diff --git a/libsrc/atari/xlmemchk.inc b/libsrc/atari/xlmemchk.inc
new file mode 100644 (file)
index 0000000..a4207aa
--- /dev/null
+++ b/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: