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https://github.com/nathanriggs/AppleIIAsm-Collection.git
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145 lines
3.8 KiB
Plaintext
145 lines
3.8 KiB
Plaintext
*
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*``````````````````````````````*
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* CMP16 :: 16-BIT COMPARE *
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*- -*
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* COMPARE TWO 16BIT SIGNED OR *
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* UNSIGNED WORDS AND RETURN *
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* THE C,Z,N FLAGS AS SET OR *
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* CLEARED. *
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*- -*
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* CLOBBERS: *
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* *
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* FLAGS: CZN----- 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 #>123 *
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* PHA *
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* LDA #<123 *
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* PHA ; WORD 1 *
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* LDA #>1023 *
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* PHA *
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* LDA #<1023 *
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* PHA ; WORD 2 *
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* JSR CMP16 *
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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 WORD 2 (SUBTRA) *
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* HIGH BYTE OF WORD 2 *
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* LOW BYTE OF WORD 1 (MINU) *
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* HIGH BYTE OF WORD 1 *
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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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* FLAGS RETURNED BASED ON WORD *
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* 1 - WORD 2. *
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* *
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* IF W1 & W2 ARE 2S COMPLEMENT *
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* IF W1 = W2 Z=1,N=0 *
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* IF W1 > W2 Z=0,N=0 *
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* IF W1 < W2 Z=0,N=1 *
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* ELSE *
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* IF W1 = W2 Z=1,C=1 *
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* IF W1 > W2 Z=0,C=1 *
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* IF W1 < W2 Z=0,C=0 *
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* *
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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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CMP16
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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 ; SUBTRAHEND
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PLA
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STA :SUBT+1
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PLA
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STA :MINU ; MINUEND
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PLA
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STA :MINU+1
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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 :MINU
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CMP :SUBT ; COMPARE LOW BYTES
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BEQ :EQUAL
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*
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** LOW BYTES ARE NOTE EQUAL
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** COMPARE HIGH BYTES
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*
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LDA :MINU+1
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SBC :SUBT+1 ; COMPARE HIGH BYTES
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ORA #1 ; MAKE Z=0, SINCE LOW
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; BYTES ARE NOT EQUAL
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BVS :OVFLOW ; MUST HANDLE OVERFLOW
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; FOR SIGNED MATH
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RTS ; EXIT
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*
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** LOW BYTES ARE UQAL -- COMPARE HIGH
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*
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:EQUAL
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LDA :MINU+1
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SBC :SUBT+1 ; UPPER BYTES
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BVS :OVFLOW
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RTS ; RETURN W FLAGS SET
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*
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**
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** OVERFLOW WITH SIGNED ARITHMETIC SO
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** COMPLEMENT THE NEGATIVE FLAG.
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** DO NO CHANGE THE CARRY FLAG AND
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** MAKE THE ZERO FLAG EQUAL TO 0.
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** COMPLEMENT NEG FLAG BY EORING
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** #$80 AND ACCUMULATOR.
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**
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*
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:OVFLOW
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EOR #$80 ; COMPLEMENT N FLAG
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ORA #1 ; IF OVERFLOW THEN THE
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; WORDS ARE !=. Z= 0
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; CARRY UNCHANGED
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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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:RETADR DS 2
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