From 6c8ecc649887156a68fd28e9d89a692c5db4e10a Mon Sep 17 00:00:00 2001
From: Peter Ferrie <peter.ferrie@gmail.com>
Date: Thu, 18 Apr 2019 14:43:43 -0700
Subject: [PATCH] build 6-and-2 on-the-fly

---
 src/patchers/nibtable.a | 24 ++++++--------
 src/patchers/rdos13.a   | 70 +++++++++++++++++++++++++----------------
 2 files changed, 52 insertions(+), 42 deletions(-)

diff --git a/src/patchers/nibtable.a b/src/patchers/nibtable.a
index 606c996..5a55e73 100755
--- a/src/patchers/nibtable.a
+++ b/src/patchers/nibtable.a
@@ -6,19 +6,6 @@
 ;-------------------------------
 !zone {
 _nibtable
-         jmp   .checkread
-
-.writedata
-         !byte $96,$97,$9A,$9B,$9D,$9E,$9F,$A6
-         !byte $A7,$AB,$AC,$AD,$AE,$AF,$B2,$B3
-         !byte $B4,$B5,$B6,$B7,$B9,$BA,$BB,$BC
-         !byte $BD,$BE,$BF,$CB,$CD,$CE,$CF,$D3
-         !byte $D6,$D7,$D9,$DA,$DB,$DC,$DD,$DE
-         !byte $DF,$E5,$E6,$E7,$E9,$EA,$EB,$EC
-         !byte $ED,$EE,$EF,$F2,$F3,$F4,$F5,$F6
-         !byte $F7,$F9,$FA,$FB,$FC,$FD,$FE,$FF
-
-.checkread
          bit   gMode      ; nothing to do here in verify-only mode
          bpl   .exit
          lda   gIsRWTS    ; if DOS 3.3 RWTS
@@ -74,11 +61,18 @@ _nibtable
          bne   .common
 
 .fixwrite
+         lda   #$16
+         sta   modsrc
+         lda   #$20
+         ldx   #$00
+         tay
+         jsr   build6and2       ; to $2000
+
          lda   #$40
          sta   .loopcheck+1
          lda   #$29
-         ldx   #<.writedata
-         ldy   #>.writedata
+         ldx   #$00
+         ldy   #$20
 
 .common
          sta   nibdestindex
diff --git a/src/patchers/rdos13.a b/src/patchers/rdos13.a
index 8f043f8..4c379e1 100644
--- a/src/patchers/rdos13.a
+++ b/src/patchers/rdos13.a
@@ -65,33 +65,12 @@
 
          lda   #s_rwtswrite
          jsr   PrintByID
-
-;build 6-and-2 denibbilisation table for reading
-
-         ldx   #$16
-         ldy   #0
---       stx   $48
-         txa
-         asl
-         bit   $48
-         beq   +
-         ora   $48
-         eor   #$ff
-         and   #$7e
--        bcs   +
-         lsr
-         bne   -
-         tya
-         sta   (BASEPAGE * $100) + $6B9 - $16, x
-
-;and 6-and-2 nibbilisation table for writing
-
-         txa
-         ora   #$80
-         sta   (BASEPAGE * $100) + $B70, y
-         iny
-+        inx
-         bpl   --
+         lda   #<((BASEPAGE * $100) + $6B9 - $16)
+         sta   modsrc
+         lda   #>((BASEPAGE * $100) + $6B9 - $16)
+         ldx   #<((BASEPAGE * $100) + $B70)
+         ldy   #>((BASEPAGE * $100) + $B70)
+         jsr   build6and2
 
 ;replace 6-and-2 encoder routine
 ;and introduce DOS-order lookup
@@ -118,6 +97,43 @@
 
          jmp   .exit
 
+;build 6-and-2 denibbilisation table for reading
+
+build6and2
+         sta   modsrc+1
+         stx   moddest
+         sty   moddest+1
+         ldx   #$16
+         ldy   #0
+--       stx   $48
+         txa
+         asl
+         bit   $48
+         beq   +
+         ora   $48
+         eor   #$ff
+         and   #$7e
+-        bcs   +
+         lsr
+         bne   -
+         tya
+         pha
+         txa
+         tay
+         pla
+         sta   (modsrc), y
+         tay
+
+;and 6-and-2 nibbilisation table for writing
+
+         txa
+         ora   #$80
+         sta   (moddest), y
+         iny
++        inx
+         bpl   --
+         rts
+
 .writemod
          !byte $A2,$00     ;LDX #$00
          !byte $A0,$02     ;LDY #$00