Invalid effective address when trying to copy a byte elsewhere

Asked May 02 '18 at 21:56

Active May 02 '18 at 22:25

Viewed 31 times

I'm writing some NASM code, running on an x86 architecture.

As part of an exercise to write a procedure to print a number as hexedecimal (the exact details of my implementation of this are irrelevant, as far as I can tell), I need to copy a byte from one string to another.

Here are my strings:

hex_digits:
    db '0123456789abcdef'
hex_out:
    db '0000'

Let's say I the byte in hex_digits at the axth index, to the third index in hex_out.

In C, this would look like this:

hex_out[3] = hex_digits[ax]

And here's my assembly:

mov cx, [hex_digits+ax]
mov [hex_out+3], cx

Shouldn't this work? Well -- clearly not, but I've read about how effective addresses work, here. After reading that, I was convinced that I understood effective addresses, but it seems that's not the case.

I'm sure it's something glaringly obvious, but I'm fairly new to nasm.

Thanks!

edited May 02 '18 at 22:25

Michael Petch

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asked May 02 '18 at 21:56

Jacob Garby

If you are generating 16-bit code the registers you can use for addressing are limited. If this is for 32-bit code you should be using the 32-bit registers for addresses. – Michael Petch May 02 '18 at 22:05
I'm generating 16-bit code, AFAIK – Jacob Garby May 02 '18 at 22:18
@MichaelPetch so, which registers can I use for addressing in 16-bit? – Jacob Garby May 02 '18 at 22:22
If you are creating 16-bit code I believe you'll find the answer you are looking for in the question/answer I have marked as a duplicate. – Michael Petch May 02 '18 at 22:24
@MichaelPetch that looks perfect, thanks! I'll test it tomorrow – Jacob Garby May 02 '18 at 22:27
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Since you’re working with one byte at a time, you should be using a one-byte register, such as cl instead of cx. – prl May 03 '18 at 06:05

Invalid effective address when trying to copy a byte elsewhere

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