*``````````````````````````````*
* MEMFILL :: FILL MEMORY LOC   *
*-                            -*
* FILLS A BLOCK OF MEMORY WITH *
* THE SPECIFIED VALUE; USED    *
* OFTEN TO CLEAR LARGE BLOCKS. *
*-                            -*
* CLOBBERS:                    *
*                              *
* FLAGS:  ????----  REG: AXYM  *
*-                            -*
* CYCLES: ???                  *
* SIZE:                        *
*-                            -*
* USAGE:                       *
*                              *
*     LDA  #>$6A00             *
*     PHA                      *
*     LDA  #<$6A00             *
*     PHA                      *
*     LDA  #>1024              *
*     PHA                      *
*     LDA  #<1024              *
*     PHA                      *
*     LDA  #0                  *
*     PHA                      *
*     JSR  MEMFILL             *
*-                            -*
* ENTRY                        *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
* FILL VALUE FOR MEMORY        *
* ARRAY SIZE                   *
* LOW BYTE OF STARTING POINT   *
* HIGH BYTE OF STARTING POINT  *
*-                            -*
* EXIT                         *
*                              *
* TOP OF STACK                 *
*                              *
* LOW BYTE OF RETURN ADDRESS   *
* HI BYTE OF RETURN ADDRESS    *
*                              *
*   Y = COUNTER; TRASH         *
*   X = COUNTER; TRASH         *
*   A = LOW BYTE OF RET ADDR   *
*-                            -*
* ADAPTED FROM LEVANTHAL AND   *
* WINTHROP'S /6502 ASSEMBLY    *
* LANGUAGE ROUTINES/.          *
* AS SUCH, THIS MAY NOT FALL   *
* UNDER THE APACHE 2.0 LICENSE *
* AGREEMENT, SINCE THE BOOK    *
* WAS WRITTEN BEFORE THE       *
* LICENSE!                     *
*                              *
* THAT SAID, LEVENTHAL WROTE   *
* THAT THE PURPOSE OF THE BOOK *
* WAS TO COPY THE SOURCE CODE, *
* AS REINVENTING THE WHEEL IS  *
* TEDIOUS, AND HE PLACED NO    *
* RESTRICTIONS ON ITS USE.     *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
MEMFILL
*
** SAVE RETURN ADDRESS
*
         PLA
         STA   :RETADR
         PLA
         STA   :RETADR+1
*
** GET PARAMETERS
*
         PLA
         STA   :VALUE
         PLA
         STA   :ARYSZ
         PLA
         STA   :ARYSZ+1
         PLA
         STA   ADDR1      ; ZERO PAGE POINTER
         PLA              ; DEFINED IN
         STA   ADDR1+1    ; DECS
*
** FILL WHOLE PAGES FIRST
*
         LDA   :VALUE     ; GET VAL FOR FILL
         LDX   :ARYSZ+1   ; X=# OF PAGES TO DO
         BEQ   :PARTPG    ; BRANCH IF HIGHBYTE OF SZ = 0
         LDY   #0
:FULLPG
         STA   (ADDR1),Y
         INY              ; INC TO NEXT BYTE
         BNE   :FULLPG    ; BRANCH IF NOT DONE W/ PAGE
         INC   ADDR1+1    ; ADVANCE TO NEXT PAGE
         DEX
         BNE   :FULLPG    ; BRANCH IF NOT DONE W/ PAGES
*
** DO THE REMAINING PARTIAL PAGE
** REGISTER A STILL CONTAINS VALUE
*
:PARTPG
         LDX   :ARYSZ     ;GET # OF BYTES IN FINAL PAGE
         BEQ   :EXIT      ; BRANCH IF LOW BYTE = 0
         LDY   #0
:PARTLP
         STA   (ADDR1),Y  ; STORE VAL
         INY              ; INCREMENT INDEX
         DEX              ; DECREMENT COUNTER
         BNE   :PARTLP    ; BRANCH IF NOT DONE
:EXIT
*
** RESTORE RETURN ADDRESS
*
         LDA   :RETADR+1
         PHA
         LDA   :RETADR
         PHA
         RTS
*
** DATA
*
:VALUE   DS    1          ; FILL VALUE
:ARYSZ   DS    2          ; ARRAY SIZE
:RETADR  DS    2