The BBC and Master Computer Public Domain Library

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