From a5d2f9569decc946c0c2e863ed5aa0c07733ce67 Mon Sep 17 00:00:00 2001
From: "B. Watson" <yalhcru@gmail.com>
Date: Fri, 11 Mar 2016 07:26:11 -0500
Subject: document & clean up 5200 stuff a bit.

---
 crt0_5200.s | 99 +++++++++++++++++--------------------------------------------
 1 file changed, 27 insertions(+), 72 deletions(-)

(limited to 'crt0_5200.s')

diff --git a/crt0_5200.s b/crt0_5200.s
index 5210ec1..cdc882a 100644
--- a/crt0_5200.s
+++ b/crt0_5200.s
@@ -33,10 +33,7 @@
 ;;; machine (the hardware changes don't break compatibility), so the code
 ;;; here comes from the 4-port OS.
 
-; I've gone to a lot of trouble to get the vectors and display list
-; set up correctly for either ROM revision... but the display list, at
-; least, was really a waste of time: conio is going to set up a GR.1
-; display list... at least until I modify it to use GR.0 instead.
+ tmpptr = $11 ; 2 bytes, aka PADDL0 and PADDL1
 
 start:
  ldx #0
@@ -51,95 +48,53 @@ start:
  lda  #$F8
  sta  CHBASE   ;Set Character base to $F800
 
-; determine which OS revision we're running under. This affects
+; Determine which OS revision we're running under. This affects
 ; the locations of the vector table and display list in the ROM,
 ; and is probably the reason a few published 5200 games are
 ; incompatible with the Rev A ROM.
-; rev A has $07 at $fee5, original OS has $61. If there are other
-; OS revisions out there in the wild, this code will likely fail
-; on them.
-
- lda #$fe
- sta $fb
-; sta $fd
-; sta $ff
+; Rev A has $07 at $fee5, original OS has $61. The vector table
+; lives at $fe95 on the original OS and $feab on rev A.
+; If there are other OS revisions out there in the wild, this
+; code will likely fail on them.
+
+ lda #$fe ; either way, the hi byte of the vector table is $fe
+ sta tmpptr+1
  lda $fee5
  cmp #$07
  beq @rev_a
 
- ; set up $fa to point to vector table,
- ; $fc to point to display list start,
- ; $fe to point to DL end.
-
- ; old OS:
- lda #$95
- sta $fa
-; lda #$c8
-; sta $fc
-; lda #$cf
-; sta $fe
- bne @copy_vectors
-
+ ; set up pointer to vector table according to the OS revision we're on.
+ lda #$95 ; the old gods...
+ .byte $2c
 @rev_a:
- lda #$ab
- sta $fa
-; lda #$de
-; sta $fc
-; lda #$e5
-; sta $fe
+ lda #$ab ; ...and the new.
+ sta tmpptr
 
 @copy_vectors:
- ldy  #$0B
+ ldy  #$0B ; 6 vectors, 2 bytes each.
 @cploop:
- lda  ($fa),y
+ lda  (tmpptr),y
  sta  VIMIRQ,y   ;Copy vectors to vector table
  dey
  bpl  @cploop
 
+ ; clear 3K of memory from $3000 to $3c00 (why only 3K?)
+ ; if ROM space gets *really* tight, check & see whether we
+ ; can live without this.
  lda  #$3C   ;Set pointer to $3C00
- sta  $12
+ sta  tmpptr+1
  lda  #$00
- sta  $11
+ sta  tmpptr
 
  ldx  #$0C
- tay
-@vecloop:
- sta  ($11),y
+ tay  ; A still 0
+@memclrloop:
+ sta  (tmpptr),y
  dey
- bne  @vecloop
- dec  $12
+ bne  @memclrloop
+ dec  tmpptr+1
  dex
- bpl  @vecloop
-
-;;; taipan doesn't need the OS-provided display list.
-;;; conio init will set up a GR.1 DL (at least, until
-;;; I modify it to the GR.0 that taipan needs)
-;Build display list
-; lda  #$0D   ;Antic mode 13
-; ldx  #$4D
-;@dl13loop:
-; sta  $2007,x   ;write to display list
-; dex
-; bpl  @dl13loop
-;
-; ldy  #$06   ;Copy start of display list
-;@dlloop:
-; lda  ($fc),y
-; sta  $2000,y
-; dey
-; bpl  @dlloop
-;
-; ldy  #$04   ;Copy end of display list
-;@dlendloop:
-; lda  ($fe),y
-; sta  $2055,y
-; dey
-; bpl  @dlendloop
-;
-; lda  #$00   ;Set Display List pointer to $2000
-; sta  $05
-; lda  #$20
-; sta  $06
+ bpl  @memclrloop
 
  lda  #$22   ;Set DMACTL, dlist on/normal background
  sta  SDMCTL
-- 
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