The BBC and Master Computer Public Domain Library

BBFA Push FWA to Stack

Submitted by Steve Fewell

Routine:pushFWA
Name: Push FWA to Stack
Starting Address: &BBFA
Entry criteria: The FWA contains a floating-point value.

Description:

Check whether the Stack has room for 5 bytes. If no space, then generate 'No Room' error;
otherwise, update the Stack Pointer to point to the next free location on the Stack.

Store the FWA Exponent byte on the Stack (1st Byte).
Load FWA Sign, EOR with FWA Mantissa byte 1, AND with &80 (to restore the top bit)
and EOR with the FWA Mantissa byte 1 again to set the Mantissa byte 1 value. Store result on
stack (2nd byte). This is done to pack the FWA Mantissa, as the FWA is pushed to the Stack
in its 5-byte packed variable format to save on Stack space.
Store the FWA Mantissa Byte 2 on the stack (3rd byte of Stack Value).
Store the FWA Mantissa Byte 3 on the stack (4th byte of Stack Value).
Store the FWA Mantissa Byte 4 on the stack (5th byte of Stack Value).
all done, so return.

Disassembly for the Push FWA to Stack routine