How to write and executable Windows .exe manually (machine code with Hex editor)?

Question

I\'d like to know how is it possible to write something as simple as an Hello World program just by using an Hex Editor. I know that I could use an assembler and assembly langua

Answer 1

you can do a disassembly and try figure out the machine code for the opcodes you use in your assembler

for example

org 0x100
mov dx,msg
mov ah,0x09
int 0x21
ret
msg db 'hello$'

compiled with nasm -fbin ./a.asm -o ./a.com has ndisasm a.com deliver the following disassembly:

00000000  BA0801            mov dx,0x108
00000003  B409              mov ah,0x9
00000005  CD21              int 0x21
00000007  C3                ret
00000008  68656C            push word 0x6c65
0000000B  6C                insb
0000000C  6F                outsw
0000000D  24                db 0x24

00000000 to 00000007 are the instructions

so you can play with the ba0801 machine code, using some hex editor, try changing it to ba0901, and only 'ello' will be printed, you can play around with your hex editor and pad stuff out with NOP, which is 0x90 in machine code, for example:

00000000:  ba 50 01 90 90 90 90 90  90 90 90 90 90 90 90 90  .@..............
00000010:  b4 09 90 90 90 90 90 90  90 90 90 90 90 90 90 90  ................
00000020:  cd 21 90 90 90 90 90 90  90 90 90 90 90 90 90 90  .!..............
00000030:  c3 90 90 90 90 90 90 90  90 90 90 90 90 90 90 90  ................
00000040:  71 77 65 72 74 79 75 69  61 73 64 66 67 68 6a 24  qwertyuiasdfghj$
00000050:  61 73 64 66 67 68 6a 6b  61 73 64 66 67 68 6a 24  asdfghjkasdfghj$
00000060:  -- -- -- -- -- -- -- --  -- -- -- -- -- -- -- --  ----------------

if you save this with the extension .com you can run it in DosBox