mirror of
https://github.com/nathanriggs/AppleIIAsm-Collection.git
synced 2024-11-24 07:31:09 +00:00
101 lines
2.4 KiB
Plaintext
101 lines
2.4 KiB
Plaintext
*``````````````````````````````*
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* SUBT16 :: 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 .Y (LOW), .X (HI). *
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* SIGNED OR UNSIGNED. *
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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 ; MINUEND *
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* PHA *
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* LDA #<1000 *
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* PHA *
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* LDA #>500 ; SUBTRAHEND *
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* PHA *
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* LDA #<500 *
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* PHA *
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* JSR SUBT16 *
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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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* [RETURN] = RESULT (2 BYTES) *
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* [RETLEN] = 2 (RESULT LENGTH) *
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*,,,,,,,,,,,,,,,,,,,,,,,,,,,,,,*
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*
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SUBT16
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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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STY RETURN
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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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STX RETURN+1
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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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RTS
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*
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** DATA
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*
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:MINU DS 2
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:SUBT DS 2
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