From 7d50e73b5b2488fb6a242db359942875529218db Mon Sep 17 00:00:00 2001
From: Oliver Schmidt <ol.sc@web.de>
Date: Tue, 23 May 2017 23:56:57 +0200
Subject: [PATCH] Streamlined display character selection.

The reverse characters of the alternate char set are place in a somewhat strange arrangement in the Apple II character ROM. This is because of backward compatibility with the standard char set. Therefore it seems more efficient and hopefully easier to understand to use a table to select them.
---
 drivers/a2vt100.s | 59 +++++++++++++++++++++++++++++------------------
 1 file changed, 36 insertions(+), 23 deletions(-)

diff --git a/drivers/a2vt100.s b/drivers/a2vt100.s
index 411a0f5..5c2ddbf 100644
--- a/drivers/a2vt100.s
+++ b/drivers/a2vt100.s
@@ -45,8 +45,6 @@ SendStr = telnet_send_string
 
 .include "apple2.inc"
 
-ControlFlags = $1000
-
 ; *************************************
 ; *
 ; * Zeropage
@@ -1038,39 +1036,29 @@ PrnChr  sta xVector ; save char
         lda xVector ; restore char
 
         ; -- $80-$ff -- non-ASCII
-        bpl PC1
-        jmp PCend
-        ; -- $20-$7f -- printable
-PC1     cmp #$20
-        bcc PC2
-        jmp PCrvs
-        ; -- $00-$1f -- control
-PC2     jmp PCend   ; no output
+        bmi PCend   ; no output
 
         ; -- handle reverse mode --
-PCrvs   ora #$80    ; turn on high bit
-        ldy INVFLG
-        cpy #$ff    ; normal character display mode?
-        beq PCput
-        cmp #$e0    ; lowercase?
-        bcc PCmask
-        and #$7f    ; inverse lowercase
+        ldx INVFLG  ; reverse mode?
+        bmi PC1     ; normal:$ff, reverse:$3f
+        tax
+        lda rtsc,x  ; reverse to ScreenCode
         jmp PCput
-PCmask  and INVFLG  ; apply normal, inverse, flash
+PC1     ora #$80    ; normal to ScreenCode
 
 PCput   ldx lbPending   ; need new line?
-        beq PC6         ; no -> skip
+        beq PC2         ; no -> skip
         ldx #$00        ; clear pending
         stx lbPending
         jsr NewLn
-PC6     tax             ; save char
+PC2     tax             ; save char
         lda CH          ; get crsr col
         lsr             ; col DIV 2
         tay
         txa             ; restore char
-        bcs PC7         ; odd col?
+        bcs PC3         ; odd col?
         bit $c055
-PC7     sta (BASL),y    ; char to screen
+PC3     sta (BASL),y    ; char to screen
         bit $c054
         ldy CH          ; get crsr col
 
@@ -1484,10 +1472,35 @@ ExitScr
 
 ; *************************************
 ; *
-; * ASCII tables
+; * ASCII and ScreenCode tables
 ; *
 ; *************************************
 
+; -------------------------------------
+; table reverse to ScreenCode
+;
+; This tabel is used to convert incoming
+; reverse chars.
+; -------------------------------------
+
+rtsc;_0  _1  _2  _3  _4  _5  _6  _7  _8  _9  _a  _b  _c  _d  _e  _f
+
+; --- Control chars - should never appear --------------------------
+.byt $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00  ; 0_
+.byt $00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00,$00  ; 1_
+
+; --- special chars ------------------------------------------------
+.byt $20,$21,$22,$23,$24,$25,$26,$27,$28,$29,$2a,$2b,$2c,$2d,$2e,$2f  ; 2_
+.byt $30,$31,$32,$33,$34,$35,$36,$37,$38,$39,$3a,$3b,$3c,$3d,$3e,$3f  ; 3_
+
+; --- capital letters ----------------------------------------------
+.byt $00,$01,$02,$03,$04,$05,$06,$07,$07,$09,$0a,$0b,$0c,$0d,$0e,$0f  ; 4_
+.byt $10,$11,$12,$13,$14,$15,$16,$17,$18,$19,$1a,$1b,$1c,$1d,$1e,$1f  ; 5_
+
+; --- lower case letters -------------------------------------------
+.byt $60,$61,$62,$63,$64,$65,$66,$67,$68,$69,$6a,$6b,$6c,$6d,$6e,$6f  ; 6_
+.byt $70,$71,$72,$73,$74,$75,$76,$77,$78,$79,$7a,$7b,$7c,$7d,$7e,$7f  ; 7_
+
 ; -------------------------------------
 ; table keyboard to ASCII
 ;