I'm using MS DOS debug (weird, i know, but I'm going for REALLY small) to write a little XOR encryption program, but I'm confused about one thing.

I have moved an address into AX and I am trying to load the first byte from that address into BH, but with no success.
Code:
mov bh, [ax]
doesnt work, although

Code:
mov bh, [150]
does, but I need the first one to work.

how can i resolve this? Thanks in advance.
Brian, of cprogramming.com