The BBC and Master Computer Public Domain Library

9C82 Compare Float values

Submitted by Steve Fewell

Starting Address: &9C82 (or &9C65 to convert Integer first)
Entry criteria: FWA contains the value to compare.
BASIC Text pointer points to the expression to compare FWA with.
Exit: Zero flag = 1 if values are equal. Carry is set if value 1 > value 2; clear otherwise.
Y = 00.

Description:

9C65 Convert Integer to Float and initialise Float values:
This routine is called during Compare Integer values if the second value in the
compare is a Float value (not an Integer value, as expected).
Pop the IWA value from the processor stack (the first value to compare).
Push the FWA value (second value to compare) to the BASIC Stack.
Convert the IWA (first value to compare) to a Float value.
Copy this result from the FWA to the FWB.
Pop the Float from the BASIC Stack (second value to compare) and unpack the returned
variable (pointed to by &4A, &4B) to the FWA.
Now, FWA = second value to compare & FWB = first value to compare. Go to &9C92 to perform
the compare.

9C82 Initialise and get the Float values:
Push FWA to stack (first value to compare).
Gosub &9E4C to get the result of the expression (in the BASIC program text line),
[we need to process expression level (+,-) upwards as any operators below this level should
only be evaluated once this compare has been evaluated].
Copy A to Y [to update processor flags] and call &96DD to check that the result obtained
is a Float value - if it is an Integer it will be converted to Float. If it is a String then a
Type Mismatch error will be generated.
Pop the Float variable from the stack and unpack this variable to the FWB.
Now, the FWB = the first value to compare, and the FWA = second value to compare.

9C92 Compare the Float values:
Y = #&00
Reset bottom 7 bits of the FWB sign byte.
Load the FWA sign bit & reset the bottom 7 bits of the loaded value, then compare
this sign with the FWB sign byte. If the signs are not equal then exit (with C=1 if FWA>FWB).

Compare the FWB exponent with the FWA exponent (if not equal then stop as we have result).
Compare the FWB Mantissa byte 1 with the FWA Mantissa byte 1 (if not equal then stop as we have result).
Compare the FWB Mantissa byte 2 with the FWA Mantissa byte 2 (if not equal then stop as we have result).
Compare the FWB Mantissa byte 3 with the FWA Mantissa byte 3 (if not equal then stop as we have result).
Compare the FWB Mantissa byte 4 with the FWA Mantissa byte 4 (if not equal then stop as we have result).
If the Mantissa and exponents are equal, then the values are equal, so exit with A = 0.
Otherwise, C = 1 (if FWA > FWB). However, if both values are negative (Sign byte is 1), then
we need to reverse the carry, so that C = 0 (if it was 1), or C = 1 (if it was zero)
this is because the value that was higher is actually lower (as it is negative).
Exit with A = &01 (as values not equal).

Disassembly for the Compare Float values routine