Working with 3 byte numbers

[Bruce]

You don't even need to mess with assembler. Here's a really simple example,
and it's all in BASIC;

  A VAR WORD
  B VAR WORD
  X VAR BYTE
  NumToAdd VAR BYTE
  NumToAdd = $2E
    
MAIN:
    A = $0001
    B = $FFFE
    FOR X = 1 TO NumToAdd
      B = B + 1
      IF B = 0 THEN A = A + 1
    NEXT X

You can extend this to pretty much any size variable you need.

[Melanie]

Why so complex? Again all in PBP... assuming positive numbers only...

A, B and C must all be the same type (ie all WORDS or all BYTES).

C=A+B
If C < A then Goto SumOverflow

Actually, B can be a BYTE if C & A are WORDS and it still will hold water.


Extending this to the original post...

AHighWord var Word
ALowWord var Word
BByte var Byte
CHighWord var Word
CLowWord var Word

AHighWord=$0001
ALowWord=$FFFE
BByte=$2E

CHighWord=AHighWord
CLowWord=ALowWord+BByte
If CLowWord < ALowWord then CHighWord=CHighWord+1

or without using the variable C...

ALowWord=ALowWord+BByte
If ALowWord < BByte then AHighWord=AHighWord+1

Actually in both above examples, BByte can be replaced with BWord and it'll still work.