mirror of
https://github.com/nathanriggs/AppleIIAsm-Collection.git
synced 2025-02-17 09:30:26 +00:00
875dd80998
- fixed a bunch of bugs - separated common and stdio libraries - commenting upgrades - created min versions of each library
111 lines
2.7 KiB
Plaintext
111 lines
2.7 KiB
Plaintext
*
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*``````````````````````````````*
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* SUB16 :: SUBTRACT WORDS *
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*- -*
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* SUBTRACT ONE 16BIT NUMBER *
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* FROM ANOTHER, RETURNING THE *
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* RESULT IN X (HI) AND Y (LOW) *
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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 ; MINU *
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* PHA *
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* LDA #<1000 *
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* PHA *
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* LDA #>500 ; SUBT *
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* PHA *
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* LDA #<500 *
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* PHA *
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* JSR SUB16 *
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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 SUBTRAHEND *
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* HIGH BYTE OF SUBTRAHEND *
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* LOW BYTE OF MINUEND *
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* HIGH BYTE OF MINUEND *
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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 = RESULT LOW BYTE *
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* X = RESULT HIGH BYTE *
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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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SUB16
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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 :SUBT
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PLA
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STA :SUBT+1 ; SUBTRAHEND
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PLA
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STA :MINU
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PLA
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STA :MINU+1 ; MINUEND
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*
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** MINUEND - SUBTRAHEND
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*
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LDA :MINU
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SEC
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SBC :SUBT
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TAY ; LOW BYTE
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LDA :MINU+1
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SBC :SUBT+1
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TAX ; HIGH BYTE
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*
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** RESTORE REUTNR 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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:MINU DS 2
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:SUBT DS 2
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