Assembly, detecting overflow register

Question

I have this very simple assembly code:

start:
add ax, 100    
; if ax overflow add to bx 1
jmp start

but i don't know how to detect ax register overflow, can anyone help me?

Answer 1

ADD instruction sets appropriate FLAGS reflecting the overflow condition. There are two kinds of overflow: signed and unsigned. You have to decide what's appropriate and use jc/jnc for unsigned and jo/jno for signed.

add ax, 100
jnc noUnsignedOverflow
...
noUnsignedOverflow:

add ax, 100
jno noSignedOverflow
...
noSignedOverflow:

Answer 2

Use jo (jump if overflow) or jno (jump if no overflow)

Check out the Intel x86 JUMP quick reference