NASM 'org' directive with '-fobj'

Question

Why doesn't NASM compile to an object file with the org directive?

org 0x7C00
nop
mov ax, ax
nop

If I compile this with:

nasm -O0 -fobj temp.asm

NASM gives me an error for whatever reason:

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

Answer 1

In this case instead of org you should use resb:

; file: nasmOrg.asm
; assemble: nasm -f obj nasmOrg.asm -o nasmOrg.obj

SEGMENT _TEXT PUBLIC CLASS=CODE USE16

;        resb    0x0100 ; reserve 0x0100 bytes for a .COM program
        resb    0x7C00 ; reserve 0x7C00 bytes for a boot sector
..start:
nop
mov ax, ax
nop

This is how you can compile different parts of a .COM program into separate object files. If you use TLINK as the linker, the next step would be:

tlink.exe /t nasmOrg.obj [any other object files], nasmOrg.bin

Note that if you omit the comma and the following name of the resultant binary (nasmOrg.bin) or if you specify a name with the .COM extension (e.g. nasmOrg.com), TLINK will refuse to link, it will say something like:

Error: Cannot generate COM file : invalid initial entry point address

And you'll have to change 0x7C00 to 0x0100 to make a .COM program.