You should support symbols... For example, the monitor on my 6800 / 6809 Exorciser simulator has this (the "Offset at..." is printed when it fixes forward references):

    ~/exorsim$ exor -d
    % a 1000
    1000: start bra skip
    1002:  nop
    1003:  nop
    1004: skip ldx #10
    Offset at 1001 set to 02
    1007: loop dex
    1008:  nop
    1009:  bne loop
    100b:  rts
    100c: 
    % sy
    1007 loop
    1004 skip
    1000 start
    % u 1000
    1000: 20 02               BRA $1004
    1002: 01                  NOP
    1003: 01                  NOP
    1004: CE 00 0A            LDX #$000A
    1007: 09                  DEX
    1008: 01                  NOP
    1009: 26 FC               BNE $1007
    100B: 39                  RTS

What would be even better: "a" brings up a built-in mini-editor. When you exit, the code is assembled.

https://github.com/jhallen/exorsim