6309 RLE decompressor

unrle.as9

; packer from C64 
; https://csdb.dk/release/?id=34685

;  ldx #source_address 
;  ldy #dest_address
;  jsr unrle6309

SUBROUTINE unrle6309 
  ldb #$00
  ; get byte 
  lda ,x+ 
  ; if we're done then exit
  beq .derle_end
  ; if top bit is set then we're a run of data   
  bpl .derle_copy 
  ; else we're a run of a single byte
  ; mask top bit
  anda #$7f 
  ; add 2 to include 0 and length + 1
  adda #2
  ; copy a+b (d) into w
  tfr d,w 
  ; copy contents of X to Y and inc Y 
  tfm x,y+
  ; step to next byte
  leax 1,x
  bra unrle6309
.derle_copy:
  ; store run of data
  tfr d,w 
  ; copy contents of X to Y and inc X & Y 
  tfm x+,y+
  bra unrle6309
.derle_end:
  rts
ENDSUB