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Integer Compliment Routine

Submitted by Steve Fewell

Routine: icomp
Name: Integer Compliment
Starting Address: &ACDE
Entry criteria: The IWA contains the integer to compliment.
Exit: IWA contains the compliment integer [i.e. IWA = - IWA]

Description:
Subtracts each byte of the IWA from zero. The least significant byte is processed first. The carry flag is set at the beginning because its reset state (0) indicates that a borrow has occurred. This routine makes clever usage of the Y register to hold the value of zero, and to transfer the value to A before each byte is subtracted. This is a lot faster than if A was loaded with #&00 each time.

On exit, A is loaded with #&40, which indicates that an integer value is being processed.

Disassembly for the integer compliment routine

ACDE 8 056 38 SEC
ACDF   169 000 A9 00 LDA#&00
ACE1   168 A8 TAY
ACE2 * 229 042 E5 2A SBC &2A
ACE4 * 133 042 85 2A STA &2A
ACE6   152 98 TYA
ACE7 + 229 043 E5 2B SBC &2B
ACE9 + 133 043 85 2B STA &2B
ACEB   152 98 TYA
ACEC , 229 044 E5 2C SBC &2C
ACEE , 133 044 85 2C STA &2C
ACF0   152 98 TYA
ACF1 - 229 045 E5 2D SBC &2D
ACF3 - 133 045 85 2D STA &2D
ACF5 @ 169 064 A9 40 LDA#&40
ACF7 ` 096 60 RTS


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