What about take any byte of an int in a char variable?

16 Dec 2009 here is

;;; file dwordtobyte.asm
segment .text
  global lwordhbyte, lwordlbyte, hwordhbyte, hwordlbyte, bytestodword
lwordhbyte:
;;;
;;; unsigned char lwordhbyte (unsigned int dword)
;;; return a byte containing the 0x0000FF00 bits of dword
;;; 
  enter  0,0
  push  ebx

  mov  eax, [ebp + 8] ; first argument
  rol  ax, 8
  and  eax, 0x000000FF

  pop  ebx
  leave
  ret

segment .text
lwordlbyte:
;;;
;;; unsigned char lwordlbyte (unsigned int dword)
;;; return a byte containing the 0x000000XX bits of dword
;;; 
  enter 0,0
  push ebx

  mov  eax, [ebp + 8] ; first argument
  and  eax, 0x000000FF

  pop  ebx
  leave
  ret

hwordlbyte:
;;; 
;;; unsigned char hwordlbyte (unsigned int dword)
;;; return a byte contaning the 0x00XX0000 bits of dword
;;; 
  enter  0, 0
  push ebx
  
  mov  eax, [ebp + 8]
  rol  eax, 16   ; 0x00XX0000 -> 0x000000XX
  and  eax, 0x000000FF ; let only the lower 8 bits on

  pop  ebx
  leave
  ret

hwordhbyte:
;;;
;;; unsgined char hwordhbyte (unsigned int dword)
;;; return a byte containing the 0xXX000000 bits of dword
;;; 
  enter 0,0
  push ebx

  mov  eax, [ebp + 8]
  rol  eax, 16   ; 0xXX000000 -> 0x0000XX00
  rol  ax,  8   ; 0x0000XX00 -> 0x000000XX
  and  eax, 0x000000FF

  pop  ebx
  leave
  ret

bytestodword:
;;;
;;; unsgined int bytestodword (unsigned char HH, unsigned char HL
;;;                  unsigned char LH, unsigned char LL)
;;; return an dword made by its four arguments
;;; in this fashion 0xHHHLLHLL
;;; 
  enter  0,0
  push ebx
        ; High word  
  mov  ah, [ebp + 8] ; 0x0000XX00 
  mov  al, [ebp + 12] ; 0x000000XX
  rol  eax, 16   ; 0x0000XXXX -> 0xXXXX0000

        ; Low word 
  mov  ah, [ebp + 16] ; 0x0000XX00
  mov  al, [ebp + 20] ; 0x000000XX

  pop  ebx
  leave
  ret

and a little test

/* file driver.c */
#include <stdio.h>
#include <stdlib.h>

/*
 * functions from dword2byte.o file
 */
unsigned char lwordhbyte  (unsigned int);
unsigned char lwordlbyte  (unsigned int);
unsigned char hwordlbyte  (unsigned int);
unsigned char hwordhbyte  (unsigned int);
unsigned int  bytestoword (unsigned char, unsigned char, 
         unsigned char, unsigned char);

int
main (void)
{
  unsigned int num = 0xFF0a5040, /* 255 10 80 64 */
 newnum;
  unsigned char HH, HL, LH, LL;

  HH = hwordhbyte (num);
  HL = hwordlbyte (num);
  LH = lwordhbyte (num);
  LL = lwordlbyte (num);
  newnum = bytestodword (HH, HL, LH, LL);
  printf ("bytes values of num %d %d %d %d (in decimal)\n", 
    HH, HL, LH, LL);
  printf ("dword value of newnum %x (in hex)\n", newnum);

  return 0;
}

Compiling:
nasm -f elf dwordtobyte.asm
gcc -o bytesfun driver.c dwordtobyte.o