The BBC and Master Computer Public Domain Library

Integer Addition Routine

Submitted by Steve Fewell

Routine: iplus
Name: Integer Addition
Starting Address: &9E65
Entry criteria: IWA contains the first Integer number. ?&4 and ?&5 [The BASIC stack pointer] points to the second Integer variable. X = The next operator code (or 4, if there is no further operation).
Exit: IWA contains the result of [Integer variable + IWA]

Description:
Adds the 32-bit Integer pointed to by the BASIC Stack Pointer (&4 (lo),&5 (hi)) to the number in the IWA.  The least significant byte [&2A] is added first, followed by &2B, &2C and &2D (The most significant byte). The carry flag allows any overflow to be carried forward and updated to the next significant byte of the number.

The code from &9E82 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).

Jump 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 addition routine.

Disassembly for the integer addition routine