Input

Checking for ESCAPE

This is fairly easy: Monitor the keyboard scancode port for ESC which is #1, then deal with it:

        in      al,60h          ;read whatever is at keyboard port; looking for ESC which is #1
        dec     ax              ;if ESC, AX now 0
        jnz     mainloop        ;fall through if 0, do jump somewhere else if otherwise

Sometimes your AH register does not contain 00h at any given time. In that case dec ax will never trigger the zero flag, so you would have to spend the extra byte and use dec al. Alternatively you can redesign your codeflow as to have 00h in AH at least sometimes ;)

Checking for any keypress

If you need to shave a byte, it might be possible in some environments to check for any keypress instead of just escape using less code:

        in      al,60h          ;read whatever is at keyboard port
        das                     ;If non-zero, parity will be set
        jp      mainloop        ;Jump if parity

An alternative if not running DOSBox:

        in      al,60h          ;read whatever is at keyboard port
        aaa
        jz      mainloop        ;Jump if zero

The above are somewhat flaky due to different hardware, VMs, and emulators not honoring flags correctly.

The DOS alternative

In very small productions, it's sometimes impossible to use one of the above, since AL contains intermediate results or pixel values all the time. Luckily there is int 16h which checks for any key with the following code :

        mov      ah,1           ; subfunction AH = 1, check for key
        int      16h
        jz       mainloop       ; ZF set if no keystroke available

See also : Ralf Browns Interrupt List : Int 16/AH=01h