AppleIIAsm-Collection/disks/disk3_arrays/T.AGET82

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*
*``````````````````````````````*
* AGET82 : INIT 2D ARRAY *
*- -*
* THIS GETS THE ADDRESS OF AN *
* ELEMENT AT THE INDEX OF *
* [:XIDX],[:YIDX]. *
*- -*
* CLOBBERS: *
* *
* FLAGS: ????---- REG: AXYM *
*- -*
* CYCLES: ??? *
* SIZE: *
*- -*
* USAGE: *
* *
* LDA #>$300 ; ARRAY ADDR *
* PHA *
* LDA #<$300 *
* PHA *
* LDA #2 ; X INDEX *
* PHA *
* LDA #3 ; Y INDEX *
* PHA *
* JSR AGET82 *
*- -*
* ENTRY *
* *
* TOP OF STACK *
* *
* LOW BYTE OF RETURN ADDRESS *
* HI BYTE OF RETURN ADDRESS *
* Y INDEX OF ELEMENT *
* X INDEX OF ELEMENT *
* LOW BYTE OF ARRAY ADDRESS *
* HIGH BYTE OF ARRAY ADDRESS *
*- -*
* EXIT *
* *
* TOP OF STACK *
* *
* LOW BYTE OF RETURN ADDRESS *
* HI BYTE OF RETURN ADDRESS *
* *
* .Y = LO BYTE OF ELEM ADDR *
* .X = HI BYTE OF ELEM ADDR *
* .A = ELEMENT/RETURN LENGTH *
* *
* [RETURN] = VALUE FOUND AT *
* SPECIFIED INDEX. *
* *
* [RETLEN] = LENGTH IN BYTES *
* OF THE RETURN VALUE. *
*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
*
AGET82
*
** SAVE RETURN ADDRESS
*
PLA
STA RETADR
PLA
STA RETADR+1
*
** GET PARAMETERS
*
PLA
STA :YIDX
PLA
STA :XIDX
PLA
STA ADDR1
PLA
STA ADDR1+1
*
LDY #2
LDA (ADDR1),Y ; ELEMENT LENGTH
STA :MCAND ; ELEMENT LENGTH
LDA #0
STA :MCAND+1
*
** MULTIPLY X AND Y
*
LDA #0
TAY
STY SCRATCH
BEQ :ENTLP
*
:DOADD
CLC
ADC :XIDX
TAX
*
TYA
ADC SCRATCH
TAY
TXA
*
:LP
ASL :XIDX
ROL SCRATCH
:ENTLP
LSR :YIDX
BCS :DOADD
BNE :LP
*
STX :MLIER
STY :MLIER+1
*
** NOW MULTIPLY BY LENGTH OF ELEMENTS
*
LDA #0
STA :PROD+2
STA :PROD+3
LDX #$10
:SHIFTR LSR :MLIER+1
ROR :MLIER
BCC :ROTR
LDA :PROD+2
CLC
ADC :MCAND
STA :PROD+2
LDA :PROD+3
ADC :MCAND+1
:ROTR
ROR
STA :PROD+3
ROR :PROD+2
ROR :PROD+1
ROR :PROD
DEX
BNE :SHIFTR
*
CLC
INC :PROD ; INCREASE BY 3
INC :PROD
INC :PROD
BCC :NOCAR
INC :PROD+1
:NOCAR
*
** NOW ADD BASE ADDRESS OF ARRAY TO GET
** THE ADDRESS OF THE INDEX VALUE
*
CLC
LDA :PROD
ADC ADDR1
STA :PROD
LDA :PROD+1
ADC ADDR1+1
STA :PROD+1
*
** RETURN ADDRESS IN Y(LOW) AND X(HIGH)
*
LDY :PROD
LDX :PROD+1
*
** RESTORE RETURN ADDRESS
*
LDA RETADR+1
PHA
LDA RETADR
PHA
*
RTS
*
** DATA
*
:XIDX DS 2
:YIDX DS 2
:PROD DS 4
:MLIER DS 2
:MCAND DS 2
*