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https://github.com/nathanriggs/AppleIIAsm-Collection.git
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109 lines
2.7 KiB
Plaintext
109 lines
2.7 KiB
Plaintext
*
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*``````````````````````````````*
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* ADD16 :: ADD 16-BIT VALUES *
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*- -*
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* ADDS TWO SIXTEEN BIT VALUES *
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* AND RETURNS A 16 BIT VALUE *
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* IN Y (LOW) AND X (HIGH). *
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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 *
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* PHA *
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* LDA #>1200 ; ADD2 *
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* PHA *
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* LDA #<1200 *
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* PHA *
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* JSR ADD16 *
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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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* ADDEND 2 LOW BYTE *
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* ADDEND 2 HIGH BYTE *
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* ADDEND 1 LOW BYTE *
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* ADDEND 1 HIGH BYTE *
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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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* *
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* Y = LOW BYTE OF RESULT *
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* X = HIGH BYTE OF RESULT *
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* A = LOW BYTE OF RET ADDR *
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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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* AS SUCH, THIS MAY NOT FALL *
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* UNDER THE APACHE 2.0 LICENSE *
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* AGREEMENT, SINCE THE BOOK *
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* WAS WRITTEN BEFORE THE *
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* LICENSE! *
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* *
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* THAT SAID, LEVENTHAL WROTE *
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* THAT THE PURPOSE OF THE BOOK *
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* WAS TO COPY THE SOURCE CODE, *
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* AS REINVENTING THE WHEEL IS *
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* TEDIOUS, AND HE PLACED NO *
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* RESTRICTIONS ON ITS USE. *
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*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
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*
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ADD16
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*
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** GET RETURN ADDR
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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 :ADD2
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PLA
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STA :ADD2+1
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PLA
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STA :ADD1
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PLA
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STA :ADD1+1
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*
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LDA :ADD1
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CLC
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ADC :ADD2
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TAY ; LOW BYTE
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LDA :ADD1+1
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ADC :ADD2+1
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TAX ; HIGH BYTE
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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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RTS
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*
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** DATA
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*
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:RETADR DS 2
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:ADD1 DS 2
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:ADD2 DS 2
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