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flat assembler > OS Construction > when DOS is not DOS

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Mike Gonta



Joined: 26 Dec 2010
Posts: 213
In the beginning, int 0x21 was two MS-DOS system files - io.sys and msdos.sys. Originally io.sys was an OEM
customizable BIOS similar to the earlier CPM's BDOS. When IBM introduced the "PC" io.sys was and is an
interface to the classic PC BIOS. msdos.sys is the low level file system interface. In the latest actual DOS version
(the one that came with Windows98) io.sys contains the functionality of msdos.sys (msdos.sys is actually a text
file for configuration). io.sys is a high level BIOS and while it is the essence of DOS - it's not actually DOS, nor is
it an operating system - that's the providence of command.com.

Simply rename your own real mode operating system (standalone, game, etc) "command.com" and create a
bootable DOS USB flash drive with the latest version of io.sys (freely downloadable on the net). io.sys will load
and initialize and then load and run your own real mode code - giving you full access to the MS-DOS "BIOS"
without actually running DOS.

You've always been able to replace command.com with your own version, however the user will see a screen asking
for the name and location of the replacement (even if it is named command.com and is located in the root directory.
To get around this and have io.sys load and run your own code directly it must be a DOS "MZ" exe file even though
it is named command.com and the first byte of the code must be 0x7A which is is checked by io.sys.

0x7A is the "jp" opcode.
Code:
use16
format mz
  jp .1 ; first byte must be 0x7A
.1:

; io.sys displays a Windows98 splash screen in graphics mode
  mov ax, 3
  int 0x10 ; change to text mode
  
  push cs
  pop ds
  mov ah, 9
  mov dx, hello
  int 0x21
  jmp $
hello: db 'hello IOsys world$'
    
In this version of io.sys - io.sys does not have to be the first file in the root directory, msdos.sys is not required
(not even an empty file) and long file names are supported. Another cool thing is that you can have io.sys display
your own splash screen instead of the Windows98. Simply create a 320X400 256 color bmp, rename it "logo.sys"
and put it in the root directory.

_________________
Mike Gonta
look and see - many look but few see

http://mikegonta.com
Post 14 Jun 2015, 12:21
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0ffer



Joined: 06 Sep 2014
Posts: 2
Quote:

the first byte of the code must be 0x7A which is is checked by io.sys


How to change io.sys from MS-DOS 7.10 to remove this check?
Post 16 Jun 2015, 15:06
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Trinitek



Joined: 06 Nov 2011
Posts: 257
0ffer wrote:
How to change io.sys from MS-DOS 7.10 to remove this check?

Run it through a disassembler and look for something that looks like
Code:
cmp byte [???], 0x7A    
and replace the following JE or JNE instruction as you see fit to make it work.
Post 16 Jun 2015, 19:29
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