How can I feed bytes to video memory from another location in 16 bit assembly?

Question

I have stored specific data on memory location 0x4000. I wanted to pass a byte from there to memory location 0xb800 so that the data is printed in the screen. The problem arises when I try to store in 0xb800:0 what would be my first byte that I assume is in 0x4000:0. The code I used as example is below:

mov ax, 0xb800
mov es, ax
mov byte [es:0], 'A'

This compiles fine and runs perfectly, but what I'm trying to adapt throws an "invalid segment override" error on NASM. Here is my non working code:

mov ax, 0xb800
mov es, ax
mov byte [es:0], byte [0x4000:0]

Is it possible to get a single byte from 0x4000:[offset] and feed it to 0xb800:[offset] by using ES segment? If so, how?

Answer 1

You can do this, but you can't do it in a single instruction like you have. You'll need to use another segment register, such as ds, plus an intermediate register (al here):

mov ax, 0x4000
mov ds, ax
mov ax, 0xb800
mov es, ax
mov al, [ds:0]
mov [es:0], al

Note that if you've got ds pointed somewhere else, then you'll have consider that too (such as by saving and restoring it).