Simple program to count to 0xff and fill memory.

count.s

mov %a, 0
mov %b, %a
mov %c, %a
mov %d, %a
mov %e, %a
mov %f, %a

loop:

; Write output
mov %c, %e
mov %d, %f
mov %mem, %a
mov %out, %a

; Increment %e:f
mov %c, %a ; stash a
mov %a, %f
inc
mov %f, %a
mov %a, %e
mov %b, 0
addc
mov %e, %b
mov %a, %c ; restore a

; Increment counter
inc

; Terminate at overflow
mov %b, 0x00
cmp
mov %c:d, $end
je

; Otherwise keep counting
mov %c:d, $loop
jmp

; Halt
end:
jmp