The BBC and Master Computer Public Domain Library

A695 Round FWA Mantissa to 4 bytes (loose rounding byte)

Submitted by Steve Fewell

Description:

If FWA Mantissa rounding byte is less than #&80 then jump to A6AE.

Otherwise we need to round-up the current 4-byte Mantissa value.
If the Mantissa rounding byte is equal to #&80 then, the LSB of the FWA Mantissa
byte 4 will be set to 1 (if it was zero). No other rounding is done.
[Note: the ROL A instruction in this code will change value 1000 0000 to 0000 0001].
Otherwise (rounding > #&80) to round up the value we increment the FWA Mantissa byte 4 by 1 (if
necessary, &A4D3 will be called to follow through the increment (to the rest of the Mantissa)
if the increment caused the value of Mantissa byte 4 to overflow (from &FF to &00).
Continue with &A6AE (described below).

&A6AE checks the FWA Exponent overflow byte. If it is zero then the number
is fine, so clear FWA Mantissa rounding byte and exit.
Otherwise, FWA exponent overflow byte is positive, so generate a "Too Big" error.

Disassembly for the Round FWA Mantissa to 4 bytes routine