The BBC and Master Computer Public Domain Library

9D02 Compare String values

Submitted by Steve Fewell

Starting Address: &9D02
Entry criteria: SWA contains the value to compare.
BASIC Text pointer points to the expression to compare SWA with.
Exit: Zero Flag = 1 if values are equal. Carry is set if value 1 > value 2; clear otherwise.
Y = 00.

Description:

Push the SWA to the 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].
If the second value is not a string, then issue a Type Mismatch error.

Set &37 to the number of bytes to compare (the length of the shortest string).
If the number of bytes to compare is 0 then stop comparing [&9D23].
Keep comparing each character in both strings until we either encounter a difference
between the strings, in which case we have our result (C = 1 if first value > second value) [goto &9D27],
or until we have compared the number of bytes that we needed to compare (&37) [goto &9D23].

[9D23:]
Now we have compared the number of bytes we needed to compare, and the values we have
compared so far are equal.
Compare the length of the first value with the length of the second value.
Whichever string is the longer will be the greater. The Zero flag will be set if
the lengths are equal, and thus the strings are equal. (C = 1 if the first value length
is greater than the second value length).

[9D27:]
We have our result, so store result (PHP), restore stack space used by the string, then
restore the result value (PLP) and exit.

Disassembly for the Compare String values routine