Shortenned the VIC-20's cputc() by 17 bytes.

Changed to a modified table look-up method to convert PetSCII to screen-codes.

libsrc/vic20/cputc.s

@@ -1,5 +1,6 @@
 ;
-; Ullrich von Bassewitz, 06.08.1998
+; 1998-08-06, Ullrich von Bassewitz
+; 2020-10-09, Greg King
 ;
 ; void cputcxy (unsigned char x, unsigned char y, char c);
 ; void cputc (char c);
@@ -39,69 +40,64 @@ _cputcxy:
         jsr     gotoxy          ; Set cursor, drop x and y
         pla                     ; Restore C
 
-; Plot a character - also used as internal function
+; Plot a character -- also used as an internal function
 
-_cputc: cmp     #$0A            ; CR?
-        bne     L1
-        lda     #0
-        sta     CURS_X
-        beq     plot            ; Recalculate pointers
-
-L1:     cmp     #$0D            ; LF?
+_cputc: cmp     #$0D            ; Is it CBM '\n'?
         beq     newline         ; Recalculate pointers
+        cmp     #$0A            ; Is it CBM '\r'?
+        beq     cr
 
-; Printable char of some sort
+; Printable char. of some sort
+; Convert it from PetSCII into a screen-code
 
-        cmp     #' '
-        bcc     cputdirect      ; Other control char
+        cmp     #$FF            ; BASIC token?
+        bne     convert
+        lda     #$DE            ; Pi symbol
+convert:
         tay
-        bmi     L10
-        cmp     #$60
-        bcc     L2
-        and     #$DF
-        bne     cputdirect      ; Branch always
-L2:     and     #$3F
+        lsr     a               ; Divide by 256/8
+        lsr     a
+        lsr     a
+        lsr     a
+        lsr     a
+        tax                     ; .X = %00000xxx
+        tya
+        eor     pet_to_screen,x
 
 cputdirect:
         jsr     putchar         ; Write the character to the screen
 
-; Advance cursor position
+; Advance the cursor position
 
 advance:
         iny
         cpy     #XSIZE
         bne     L3
-        jsr     newline         ; new line
-        ldy     #0              ; + cr
+        jsr     newline         ; Wrap around
+
+cr:     ldy     #$00            ; Do carriage-return
 L3:     sty     CURS_X
         rts
 
+
+; Move down by one full screen-line.  Note: this routine doesn't scroll.
+;
+; (Both screen RAM and color RAM are aligned to page boundaries.
+; Therefore, the lower bytes of their addresses have the same values.
+; Shorten the code by taking advantage of that fact.)
+
 newline:
         clc
         lda     #XSIZE
         adc     SCREEN_PTR
         sta     SCREEN_PTR
-        bcc     L4
-        inc     SCREEN_PTR+1
-        clc
-L4:     lda     #XSIZE
-        adc     CRAM_PTR
         sta     CRAM_PTR
         bcc     L5
+        inc     SCREEN_PTR+1
         inc     CRAM_PTR+1
 L5:     inc     CURS_Y
         rts
 
-; Handle character if high bit set
-
-L10:    and     #$7F
-        cmp     #$7F            ; PI?
-        bne     L11
-        lda     #$5E            ; Load screen code for PI
-L11:    ora     #$40
-        bne     cputdirect
-
-
 
 ; Set cursor position, calculate RAM pointers
 
@@ -111,14 +107,19 @@ plot:   ldy     CURS_X
         jmp     PLOT            ; Set the new cursor
 
 
-
-; Write one character to the screen without doing anything else, return X
-; position in Y
+; Write one character to the screen without doing anything else,
+; return the X position in .Y
 
 putchar:
         ora     RVS             ; Set revers bit
         ldy     CURS_X
-        sta     (SCREEN_PTR),y  ; Set char
+        sta     (SCREEN_PTR),y  ; Set char.
         lda     CHARCOLOR
         sta     (CRAM_PTR),y    ; Set color
         rts
+
+
+        .rodata
+pet_to_screen:
+        .byte   %10000000,%00000000,%01000000,%00100000 ; PetSCII -> screen-code
+        .byte   %01000000,%11000000,%10000000,%10000000