34 lines
860 B
ArmAsm
34 lines
860 B
ArmAsm
.define And
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! This subroutine performs the logical and on two groups of
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! atmost 254 bytes. The number of bytes is in register Y.
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! The two groups are on the stack.
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! First the value of the stackpointer is saved in zeropage
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! locations ADDR, ADDR+1. Then an offset of Y is added
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! and stored in ADDR+2, ADDR+3.
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! The result is pushed back on the stack.
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And:
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lda SP+1
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sta ADDR+1 ! address of first group (lowbyte)
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lda SP+2
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sta ADDR ! address of first group (highbyte)
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clc
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tya
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adc SP+2
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sta SP+2 ! new stackpointer (lowbyte)
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sta ADDR+2 ! stackpointer + Y (lowbyte)
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lda #0
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adc SP+1
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sta SP+1 ! new stackpointer (highbyte)
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sta ADDR+3 ! stackpointer + Y (highbyte)
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1: dey
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lda (ADDR),y ! get byte first group
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and (ADDR+2),y ! perform logical and with second group
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sta (ADDR+2),y ! push result on real_stack
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tya
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bne 1b ! do it n times
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rts
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