The BBC and Master Computer Public Domain Library

Integer Reverse Order Complement (compliment Integer in FWA)

Submitted by Steve Fewell

Routine: icompFWA
Name: Test and Complement Integer in FWA
Starting Address: &82C4
Entry criteria: The FWA contains an Integer number.
Exit: The Integer Number in the FWA has been complemented (FWA = zero - FWA)

Description:
Complements an Integer  stored in the Floating-Point A Mantissa (which is stored the opposite way to Integers, that is: most significant byte first and least significant byte last; whereas Integers are stored least significant byte first, most significant byte last [as twos complement is used]).

If the FWA Sign Bit is positive, then no compliment is required (as we already have a positive Integer), so exit.

[82C8] As the FWA Sign Bit is negative (i.e. &FF), we need to complement the Integer, by subtracting each of the bytes (?&34 -> ?&31) from zero, using the carry flag to keep track of any borrows.

Disassembly for the Test and Compliment Integer in FWA routine