Exercise in self modifying memory copy routine, 6502 ASM

Question

Below is my self modifying routine for memory copy on Commodore 64.

I wrote char codes and number of repeats in a table and filled the screen_ram the with this routine.

I'm looking for suggestions for optimization. My priority is memory in this case.

memCopy:    
  sourceAddress=*+1 ; mark self modifying addrres
fetchNewData:
  lda data_table    ; read char value into A
  ldx data_table+1  ; read repeat value into x
  inc sourceAddress     
  inc sourceAddress 

  cpx #00           ; if X=0 
  beq end           ; finish copying

  destination=*+1
- sta SCREEN_RAM
  inc destination
  dex
  bne -

  jmp fetchNewData

end:
  rts   

; data format:  <char>,<number of repeats>,[<char>,<number of repeats>,...],00,00

data_table: 
!by 01,03,02,02,......,00,00

Answer 1

Correct increment of instruction's address should be made like this:

address=*+1
    lda self_modifying_address
    inc address+0
    bne *+5
    inc address+1

thus probably neglecting all memory savings for self-modified code.

I suggest another approach, that includes self-modifying instruction addresses only where absolulety necessary and also stores memory variables in the instructions.

.loop
fetch_ptr=*+1
    ldx #0
    lda filler_bytes,x ;have two tables, first contains only filler bytes,
    ldy repeat_bytes,x ;second only repeat counts
    beq .exit
    inc fetch_ptr      ;this way you save 1 increment

fill_ptr=*+1
    ldx #0
.fill
    sta SCREEN_RAM,x
    inx
    bne +
    inc .fill+2 ;only self-modify high byte of address in the instruction
+   dey
    bne .fill

    stx fill_ptr

    jmp .loop
.exit
    rts


filler_bytes !byte 1,2,3,4,5,4,3,2,1
repeat_bytes !byte 4,4,5,5,6,6,5,5,4,0

Answer 2

JMP fetchNewData -> BEQ fetchNewData. Move INC sourceAddress after BEQ end and there is no need from CPX #0 (after LDX). 3 bytes less.