The BBC and Master Computer Public Domain Library

Normalise FWA#2 [only bit-by-bit method]

Submitted by Steve Fewell

Routine: anorm[2]
Name: Normalise Floating-Point Accumulator (FWA)#2 [only bit-by-bit method]
Starting Address: &81E2
Entry criteria: The FWA contains a floating-point number. Y = Exponent (?&30). A = MSB of FWA's Mantissa (?&31).
Exit: FWA has been normalised

Description:
'OR' the Accumulator with 0 to set the status flags according to the value.

If A = Negative then the top bit is set and the FWA is already normalised, so store A & Y back into &31 and &30 (respectively) and exit.

If A is zero then the number in the FWA is considered to be zero, so the FWA's sign, Exponent and Mantissa byte 1 are set to zero to indicate this, and the routine is exited. This uniquely identifies the number zero.

Otherwise, the top bit of the Mantissa is not 1, so we need to normalise the FWA as follows:
            ==> Keep on decrementing the exponent (Y) and multiplying the mantissa by 2 (moving its bits left a position), until the top bit is set (A becomes negative).

When the number is normalised, store A & Y back into &31 and &30 (respectively) and exit.

Disassembly for the Normalise FWA#2 [only bit-by-bit method] routine

Complete Zero Number: