_Run Length Encoding_
by Robert Zigon

[LISTING ONE]

;--------------------------------------------------------------------
; RLE.asm   Run Length Encoding Routines
; Author : Bob Zigon
;--------------------------------------------------------------------
; -------------------------------------------------------------------
; PACK
;    This routine will pack a source buffer into a destination buffer
;    by reducing sequences of identical characters down to a 1 byte
;    repetition count and a 1 byte character.
;    INPUT : DS:SI -- Points to source buffer to pack
;            ES:DI -- Points to destination 
;            DX    -- Number of characters in source buffer to pack
;    OUTPUT: DI    -- Is updated to allow for multiple calls. 
; -------------------------------------------------------------------
pack           proc   near
               push   ax
               push   bx
               push   cx
               lodsb
               mov    bl,al
               xor    cx,cx            ; Counts number of characters packed
               cld                     ; All moves are forward

p10:           lodsb                   ; Get chara into AL
               inc   cx                ; Inc chara count
               sub   dx,1              ; 
               je    p30               ; Exit when DX = 0
               cmp   cx,255            ; 255 characters in this block yet?
               jne   p20               ; If not then jump

               mov   [di],cl           ; output the length
               inc   di
               xor   cx,cx
               mov   [di],bl           ; output the character
               inc   di

p20:           cmp   al,bl             ; Has there been a character transition?
               je    p10               ; NOPE ... so go back for more

               mov   [di],cl           ; output the length
               inc   di
               xor   cx,cx
               mov   [di],bl           ; output the character
               inc   di

               mov   bl,al             ; Move Latest chara into BL
               jmp   p10               ; Loop back for more
;
;  We will get here when DX finally goes to zero.
;
p30:           mov   al,cl             ; Output the length
               stosb
               mov   al,bl             ; Output the character
               stosb

               pop   cx
               pop   bx
               pop   ax
               ret
pack           endp
 
; -------------------------------------------------------------------
; UNPACK
;    This routine will unpack a sequence of characters that were
;    previously PACKed.
;    NOTE  : Source Region must be terminated with a NULL.
;    INPUT : DS:SI -- Source buffer to unpack
;            ES:DI -- Destination buffer to unpack into
;    OUTPUT:  
; -------------------------------------------------------------------
unpack         proc   near
               push   ax
               push   cx
               xor    cx,cx    ; Make sure CH is zero
               cld             ; All moves are forward

unp10:         lodsb           ; Load into AL a character from DS:[SI]
               cmp  al,0       ; Length of zero is end of region
               je   unp40      ; 
               mov  cl,al      ; Length goes into CL
               lodsb           ; AL has chara to repeat

               rep stosb       ; Store AL to [DI] and repeat while CX <> 0
               jmp unp10       ; Loop back forever

unp40:         pop  cx
               pop  ax
               ret
unpack         endp



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