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38 lines
951 B
Plaintext
38 lines
951 B
Plaintext
COMP16
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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 :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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LDA RETADR+1
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PHA
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LDA RETADR
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PHA
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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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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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RTS ; EXIT
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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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:OVFLOW
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EOR #$80 ; COMPLEMENT N FLAG
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ORA #1 ; IF OVERFLOW THEN THE
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RTS
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:MINU DS 2
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:SUBT DS 2
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