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https://github.com/nathanriggs/AppleIIAsm-Collection.git
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60 lines
1.5 KiB
Plaintext
60 lines
1.5 KiB
Plaintext
MULT16
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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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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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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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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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CLC ; NEXT BIT=1 SO ADD MCAND
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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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: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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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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LDA RETADR+1
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PHA
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LDA RETADR
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PHA
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LDA :HPROD ;24BIT
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RTS
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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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