The BBC and Master Computer Public Domain Library

A541 Unpack Floating-Point Variable to FWA

Submitted by Steve Fewell

Routine:aunp
Name: Unpack Floating-Point Variable to FWA
Starting Address: &A541 (or &A539 to set argp to &046C first)
Entry criteria: &4A and &4B (the argp) point to a 5-byte Floating-Point variable to unpack.
Exit: The FWA contains the value of the variable.

Description:

If called from &A539 then set argp to point to variable location &046C first.
The FWA value will be extracted from the packed floating-point value at the address pointed to by argp.

Set FWA Rounding byte = 0.
Set FWA Overflow byte = 0.
FWA Mantissa 4 = 5th byte of the packed variable
FWA Mantissa 3 = 4th byte of the packed variable
FWA Mantissa 2 = 3rd byte of the packed variable
FWA Mantissa Sign = 2nd byte of the packed variable
FWA Exponent = 1st byte of the packed variable

If the Exponent is zero, then check whether the Mantissa byte 2, Mantissa byte 3
and Mantissa byte 4 are also zero. If so, then the FWA value = 0.0 so store zero in
FWA Mantissa 1 and exit.
Mantissa 1 is formed by ORing the 2nd byte of the packed variable with #&80 to ensure that
the top bit is set - as the top bit is replaced by the sign bit in the packed form.

Disassembly for the Unpack FP Variable to FWA routine