Question

Beginner to assembly programming for x86. I have a simple asm file which I assemble using nasm version – NASM version 2.10rc6 compiled on Jun 6 2011, for windows(My windows is Windowa-7 64 bit). NASM is downloaded from here ( nasm-2.10rc6-win32.zip).

ORG 100
USE16

    mov ah, 09
    mov dx, msg
    int 21h

    mov ah, 01
    int 21h
    mov ah, 4ch
    int 21h

    msg db 'Hello assembly', 0Ah, '$'

Then I assemble using command –

nasm -f bin hello.asm -o hello.com

Then I run the generated executable hello.com using Dosbox(Dos emulator for 64 bit OS Windows-7).
When it runs the output output on the console has my string ‘Hello assembly’ plus some junk characters/control characters printed before it, as shown below:

What is the reason for this. Anything wrong in the code?

What do I need to do to fix this?

EDIT: When i tried to give a option -f to nasm to generate a specific type of executable output , e.g. Win32 or Win64 output i keep getting error saying:

nasm -f win64 hello.asm -o hello.com

hello.asm:1: error: parser: instruction expected

What is it expecting? How can i generate a win32/win64 executable using nasm? or for that matter any other executable like elf32/coff, which nasm says it supports?

Answer 1

The problem is:

ORG 100

which should be:

ORG 100h

The binary is a .COM so will load and run at 100h regardless; the error means that the address calculated by the assembler for msg will be 156 bytes earlier than it should be, hence the extra junk.

The ORG directive is for the bin format only. Other executable formats have sections (or segments). (See the “Output Formats” section of the NASM manual.)