The BBC and Master Computer Public Domain Library

9EBD '-' operator - Subtraction

Submitted by Steve Fewell

Description:

Check the value type (in A).
If the value is a string then issue a Type Mismatch error, as you cannot subtract
Strings in BASIC.

If variable type is negative then the first value is a Float, so jump to &9EE7, which
does the following:
* Push FWA to Stack
* Get the second value from the expression handler (level 5)
* If second value is a String then generate a Type Mismatch error.
* If second value is an Integer then convert it to a Float
* Pop the first Float value from Stack to argp (&4A, &4B)
* Call the Floating-Point Subtraction routine to subtract the 2nd value from the 1st (&A68A)
* Jump to 9E4F (Expression handler level 4) to check for further + or - operators.

Otherwise, the value is Integer, so push the first value to the Stack and call the
Expression Handler level 5 routine (&9FC4) to get the second value to add.
If the second value is a string then generate a Type Mismatch error.

If the second value is a Float then jump to &9EFF to do the following:
* Pop the first value (the Integer) from the Stack
* Push the second value (the Float) to the Stack
* Convert the Integer to a Float (and put it in the FWA).
* Pop the second Float value from Stack to argp (&4A, &4B)
* Call the Floating-Point Subtraction routine to subtract the 1st value from the 2nd (&ACC7)
* If the FWA is not zero, then reverse the sign of the FWA (as the subtraction was performed
  the opposite way than it should have been.
* Jump to 9E4F (Expression handler level 4) to check for further + or - operators.

Otherwise, both values are Integers, so carry out the Integer Addition routine. This
routine automatically jumps back to 9E4F on completion, to check for further + or - operators.

Disassembly for the '-' Operator - Subtraction routine

Disassembly for the Floating Point '-' Operator

Disassembly for the Floating Point Subtraction (with reverse sign of result) routine