NASM with GCC or Visual C++

[golfexodus]

I'm new to OS development, how do i compile and execute nasm using gcc. Whenever i try to compile nasm using visual C++ I always get - LINK : fatal error LNK1561: entry point must be defined.

This is my source code:

%include "asm_io.inc"

segment .data

prompt1 db "Enter first number: ", 0
prompt2 db "Enter second number: ", 0
outmsg1 db "You entered ", 0
outmsg2 db "and ", 0
outmsg3 db "The sum is ", 0

segment .bss

input1 resd 1
input2 resd 1

segment .text

global _asm_main
_asm_main:
enter 0,0 ; set up routine
pusha

mov eax, prompt1 ; print out prompt
call print_string

call read_int ;read integer
mov [input1], eax ;store into input1

mov eax, prompt2 ; print out prompt
call print_string

call read_int ;read integer
mov [input2], eax ;store into input2

mov eax, [input1] ;eax = dword at input1
add eax, [input2] ;eax += dword at input2

mov ebx, eax ;ebx = eax

dump_regs 1 ;print out register values
dump_mem 2, outmsg1, 1 ;print out memory

;print out result messages as series of step

mov eax, outmsg1
call print_string
mov eax, [input1]
call print_int
mov eax, outmsg2
call print_int
mov eax, outmsg3
call print_string
mov eax, ebx
call print_int ;print sum(ebx)
call print_nl ;print new line

popa
mov eax, 0 ;return to C
leave
ret

[Nable]

I don't get how exactly you are trying to compile it, what cmdline do you use?

Also, if you are using MSVS projects, then you must specify entry point for you application.

[gedd]

What you are doing is not clear.
Please take a while to give some details.

[LindusSystem]

You may use GNU C Compiler and NASM.
And use the extern keyword to tell the compiler/assembler that it is defined elsewhere in other language and define a variable/function in assembly as global if you want to call it in C with extern.

Build Process:
Assemble
Compile
Link

Advantages of using GCC:
1]You can compile your C Scripts in Linux,Windows(AS GCC is available for both platforms),etc too whereas VC Scripts compile only on Windows(AS VC Compiler is for Windows)

2]Cross-Compiler

3]VC++ Errors are sometimes rubbish

If you have no idea of linking,assembling,etc ,try the Brans Kernel Tutorial there you will get to know to linker script and how to build scripts

[Casm]

If you are linking from the command line with Microsoft's linker, use the switch /ENTRY:main. Replace "main" with whatever your entry point is (and remember to make it "public" in the asm source file).