The BBC and Master Computer Public Domain Library

Integer Subtraction Routine

Submitted by Steve Fewell

Routine: iminus
Name: Integer Subtraction
Starting Address: &9ECA
Entry criteria: ?&4 and ?&5 [The BASIC stack pointer] points to the first Integer variable. The IWA contains the integer to subtract. X = The next operator code (or 4, if there is no further operation).
Exit: IWA contains the result of [Integer variable - IWA]

Description:
Subtracts the IWA from the 32-bit Integer pointed to by the BASIC Stack Pointer (&4 (lo),&5 (hi)) storing the result in the IWA.  The least significant byte [&2A] is subtracted first, followed by &2B, &2C and &2D (The most significant byte). The carry flag allows any overflow to be borrowed forward and updated to the next significant byte of the number. The carry flag is set at the beginning because its reset state (0) indicates that a borrow has occurred.

The routine &9E80 in iplus is jumped to. This routine stores the result in A back to &2D and updates the Stack pointer, so that it now points to the address after the added number. To do this, 4 bytes are added to the address pointed to by (&4, &5) and then jumps to &9E4F to test the value of X [the next operator]. If X contains the operator code for '+' or '-' then send the result to the appropriate routine, for further calculation, otherwise exit the routine.

Disassembly for the integer subtraction routine