How do I use CMP in assembly to test a segment and offset address?

Question

I want to leave a loop if the current address I'm looking at is at least 0xFFFF0. Here is the portion of code I wrote, but obviously does not work:

CMP DS:[BX], FFFF0H
JGE LeaveLoop

I'm very new to assembly and have not used CMP for anything more than simple number comparisons.

Answer 1

This sounds like an XY problem, you should have specified what you wanted to achieve ultimately.

Anyway, FFFF0H is a 20 bit address, you can't compare with that directly if you are limited to 16 bits. You can use two 16 bit registers to calculate the physical address, and do a 32 bit comparison using those.

    MOV AX, DS
    MOV DX, DS
    SHL AX, 4
    SHR DX, 12    ; DX:AX has segment base now
    ADD AX, BX    ; add offset
    ADC DX, 0     ; DX:AX has full address now
    CMP DX, 0Fh   ; high word has to be at least F
    JB  false
    JA  true      ; if it's more we are ok
    CMP AX, FFF0h ; low word has to be at least FFF0h
    JAE true
false:
    ...
true:
    ...

Answer 2

    MOV AX, BX
    MOV DX, DS
    SHR AX, 4    ; doesn't need lowest nibble
    ADD AX,DX    ; add offset
    JC LeaveLoop ; See note!!!
    CMP AX, FFFFh
    JE LeaveLoop
    ...
LeaveLoop:

Note: if overflow does NOT count, then JC LeaveLoop should be removed.