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{{tag>Assembly}}

====== Assembly Programming ======

Assembly is about as close as you can get to machine code. 
There are many Assembly compilers out there. ''nasm'' is one compiler that is available across platforms and can be used to program x86 processors.

===== x86 on Linux =====

Use interrupt ''0x80'' to make Linux system calls like reading and writing to file descriptors and sockets.
For a list of system calls in Linux, run ''man syscalls'' or refer to an online man page like [[https://man7.org/linux/man-pages/man2/syscalls.2.html]]. To get the actual number, try to find the ''unistd.h'' file. For example, in Debian, ''/usr/include/asm-generic/unistd.h'' has all of the statements that define the syscall numbers (''NR''). Or, refer to the source code of Linux (pick the right branch) [[https://github.com/torvalds/linux/blob/v4.17/arch/x86/entry/syscalls/syscall_64.tbl]] and [[https://github.com/torvalds/linux/blob/v4.17/arch/x86/entry/syscalls/syscall_32.tbl]].

Here is an example of compiling with ''nasm'':

<code bash>
nasm -f elf64 file.asm 
# or for 32-bit
nasm -f elf file.asm
</code>

If you need to link against something you can use:

<code bash>
ln -d -o outfile file.o
</code>


Here is a Hello World example:

<code asm hello.asm>
SECTION .DATA
	hello:     db 'Hello world!',10
	helloLen:  equ $-hello

SECTION .TEXT
	GLOBAL _start 

_start:
	mov eax,4            ; 'write' system call = 4
	mov ebx,2            ; file descriptor 1 = STDOUT
	mov ecx,hello        ; string to write
	mov edx,helloLen     ; length of string to write
	int 80h              ; call the kernel

	; Terminate program
	mov eax,1            ; 'exit' system call
	mov ebx,0            ; exit with error code 0
	int 80h              ; call the kernel

</code>

Compile and run with:

<code bash>
# Compile
nasm -f elf64 hello.asm -o hello.o

# Link
ld hello.o -o hello

# Run
./hello
</code>
===== x86 on DOS =====

Use interrupt ''0x21'' to make DOS system calls like reading and writing files.
For a list of DOS system calls, refer to [[https://en.wikipedia.org/wiki/DOS_API]]

===== x86 on BIOS =====

When interacting with the BIOS, you use a different system call for each function. BIOS will have less system calls available than an kernel like DOS or Linux, but it gives you the tools you need to build an operating system. For example, BIOS will let you change the video mode, get input from keyboard, write text to screen, and draw pixels on the screen.

For a list of BIOS system calls, refer to [[https://en.wikipedia.org/wiki/BIOS_interrupt_call]].

===== Create a library =====

You can write and compile libraries that can be linked against by other programs. 

This example shows how to create a function called ''print_hello()'' that can be used from other Assembly or C programs.

<code asm static_lib.asm>
; Compile this program using
; nasm -f elf64 static_lib.asm 
; gcc myprogram.c static_lib.o
; ./a.out

SECTION .DATA
	hello:     db 'Hello world!',10
	helloLen:  equ $-hello

SECTION .TEXT
	GLOBAL print_hello

print_hello:
	mov eax,4            ; 'write' system call = 4
	mov ebx,2            ; file descriptor 1 = STDOUT
	mov ecx,hello        ; string to write
	mov edx,helloLen     ; length of string to write
	int 80h              ; call the kernel

	; Terminate program
	mov eax,1            ; 'exit' system call
	mov ebx,0            ; exit with error code 0
	int 80h              ; call the kernel
</code>


===== Create an executable =====

By creating a ''main()'' function, we can make an executable instead of a library.

<code asm c_main.asm>
; Because we have a reference to 'main'
; we can compile with nasm to create the
; .o object file, and then compile that with
; gcc. Example
; nasm -f elf64 c_main.asm 
; gcc c_main.o
; ./a.out

SECTION .DATA
	hello:     db 'Hello world!',10
	helloLen:  equ $-hello

SECTION .TEXT
	GLOBAL main

main:
	mov eax,4            ; 'write' system call = 4
	mov ebx,2            ; file descriptor 1 = STDOUT
	mov ecx,hello        ; string to write
	mov edx,helloLen     ; length of string to write
	int 80h              ; call the kernel

	; Terminate program
	mov eax,1            ; 'exit' system call
	mov ebx,0            ; exit with error code 0
	int 80h              ; call the kernel
</code>

And compile and run with:

<code bash>
nasm -f elf64 c_main.asm 
gcc c_main.o
./a.out
</code>

===== Call Assembly functions from C =====

<code c example.c>
/* example.c */
/* Compile and run with `gcc example.c say_hi.o -o hello` */
#include <stdio.h>

int main(int argc, char *argv[]) {
	extern say_hi();
	say_hi();
}
</code>

Next, compile and link the C program with gcc. Do that with:

<code bash>
gcc example.c say_hi.o -o hello
</code>

You can then run the ''hello'' program that was just created.

<code bash>
./hello
</code>

===== Call C function from Assembly =====


You can call C functions from Assembly as well. This example calls printf(). In the main function, we push and pop the stack and put our operations in between. We move all the parameters in to the appropriate registers, and then we call the function.

<code asm printf.asm>
; printf.asm
; Define printf as an external function
extern	printf

SECTION .DATA
    msg:	db "Hello world", 0 ; Zero is Null terminator 
    fmt:    db "%s", 10, 0 ; printf format string follow by a newline(10) and a null terminator(0), "\n",'0'

SECTION .TEXT
    global main
    
main:
    push rbp ; Push stack

    ; Set up parameters and call the C function
    mov	rdi,fmt
    mov	rsi,msg
    mov	rax,0
    call printf

    pop	rbp		; Pop stack

    mov	rax,0	; Exit code 0
    ret			; Return
</code>

Compile and run that with:

<code bash>
nasm printf.asm -f elf64 -o printf.o
gcc printf.o
./a.out
</code>
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