mirror of
https://github.com/nathanriggs/AppleIIAsm-Collection.git
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157 lines
4.0 KiB
Plaintext
157 lines
4.0 KiB
Plaintext
*
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*``````````````````````````````*
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* MULT16 :: MULTIPLY WORDS *
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*- -*
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* PERFORM MULTIPLICATION USING *
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* THE SHIFT AND ADD ALGORITHM *
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* *
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* NOTE: WHILE THIS DOES PASS *
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* THE FULL 32BIT PRODUCT BACK *
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* VIA X AND Y, THIS WOULD *
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* RETURN FALSE RESULTS IF ONE *
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* OF THE PARAMETERS HAD BEEN *
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* SIGNED. THUS, USUALLY ONLY *
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* THE LOW WORD IS USED. *
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*- -*
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* CLOBBERS: *
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* *
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* FLAGS: ????---- REG: AXYM *
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*- -*
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* CYCLES: ??? *
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* SIZE: *
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*- -*
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* USAGE: *
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* *
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* LDA #>1000 *
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* PHA *
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* LDA #<1000 ; MCAND *
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* PHA *
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* LDA #>3 *
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* PHA *
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* LDA #<3 ; MLIER *
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* PHA *
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* JSR MUL16 ; = 3000 *
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*- -*
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* ENTRY *
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* *
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* TOP OF STACK *
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* *
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* LOW BYTE OF RETURN ADDRESS *
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* HI BYTE OF RETURN ADDRESS *
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* LOW BYTE OF MULTIPLIER *
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* HIGH BYTE OF MULTIPLIER *
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* LOW BYTE OF MULTIPLICAND *
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* HIGH BYTE OF MULTIPLICAND *
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*- -*
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* EXIT *
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* *
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* TOP OF STACK *
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* *
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* LOW BYTE OF RETURN ADDRESS *
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* HI BYTE OF RETURN ADDRESS *
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* LOW BYTE OF PRODUCT *
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* HIGH BYTE OF PRODUCT (16BIT) *
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* *
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* .Y = LOW BYTE OF HIGH WORD *
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* .X = HIGH BYTE OF HIGH WORD *
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* .A = LOW BYTE OF RET ADDR *
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* *
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* [RETURN] = RESULT (2 BYTES) *
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* [RETLEN] = 2 (RESULT LENGTH) *
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*- -*
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* ADAPTED FROM LEVANTHAL AND *
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* WINTHROP'S /6502 ASSEMBLY *
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* LANGUAGE ROUTINES/. *
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*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
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*
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MULT16
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*
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** GET RETURN ADDRESS
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*
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PLA
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STA RETADR
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PLA
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STA RETADR+1
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*
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** GET PARAMETERS
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*
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PLA
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STA :MLIER
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PLA
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STA :MLIER+1
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PLA
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STA :MCAND
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PLA
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STA :MCAND+1
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*
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** PERFORM MULTIPLICATION USING THE SHIFT
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** AND ADD ALGORITHM, WHICH PRODUCES AN
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** UNSIGNED 32-BIT PRODUCT IN :HIPROD AND
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** :MLIER WITH :HIPROD BEING THE HIGH WORD.
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*
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LDA #0
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STA :HPROD ; ZERO HIGH WORD
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STA :HPROD+1
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LDX #17 ; # OF BITS IN MPLIER
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; PLUS 1. EXTRA LOOP IS
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; TO MOVE LAST CARRY INTO
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; THE PRODUCT.
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CLC ; CLEAR CARRY FOR 1ST TIME
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; THROUGH LOOP.
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:MULLP
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*
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** IF NEXT BIT = 1 THEN
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** HIPROD = HIPROD+MCAND
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*
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ROR :HPROD+1
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ROR :HPROD
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ROR :MLIER+1
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ROR :MLIER
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BCC :DECCNT ; BR IF NEXT BIT OF
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; MULTIPLIER IS 0
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CLC ; NEXT BIT=1 SO ADD MCAND
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; TO PRODUCT
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LDA :MCAND
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ADC :HPROD
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STA :HPROD
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LDA :MCAND+1
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ADC :HPROD+1
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STA :HPROD+1 ; CARRY = OVERFLOW
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; FROM ADD
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:DECCNT
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DEX
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BNE :MULLP ; CONTINUE UNTIL DONE
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LDY :HPROD ; LOW BYTE OF HIGH WORD
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LDX :HPROD+1 ; HIGH BYTE OF HIGH WORD
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*
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** PUSH LOW WORD OF PRODUCT ONTO THE STACK
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*
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LDA :MLIER+1
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TAX
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STX RETURN+1
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LDA :MLIER
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TAY
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STY RETURN
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LDA :HPROD
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STA RETURN+2
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LDA #2
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STA RETLEN
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*
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** RESTORE RETURN ADDRESS
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*
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LDA RETADR+1
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PHA
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LDA RETADR
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PHA
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*
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LDA :HPROD ;24BIT
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RTS
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*
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** DATA
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*
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:MCAND DS 3
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:MLIER DS 3
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:HPROD DS 2
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*
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