The BBC and Master Computer Public Domain Library

A4E0 Unpack Floating-Point Variable to FWB

Submitted by Steve Fewell

Routine:bunp
Name: Unpack Floating-Point Variable to FWB
Starting Address: &A4E0
Entry criteria: &4A and &4B (the argp) point to a 5-byte Floating-Point variable to unpack.
Exit: The FWB contains the value of the variable.

Description:

Set FWB Rounding byte = 0.
Set FWB Overflow byte = 0.
FWB Mantissa 4 = 5th byte of the packed variable
FWB Mantissa 3 = 4th byte of the packed variable
FWB Mantissa 2 = 3rd byte of the packed variable
FWB Mantissa Sign = 2nd byte of the packed variable
FWB 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 FWB value = 0.0 so store zero in
FWB 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 FWB routine