DOS Reboot Program

The program REBOOT.COM was developed on MS-DOS Version 6.22 using the DOS program named DEBUG.EXE. It can be used to reboot MS-DOS.

Assemble

Here is the complete DEBUG.EXE session that shows how this program was written:

C:\>DEBUG
-A
1165:0100 JMP FFFF:0000
1165:0105
-N REBOOT.COM
-R CX
CX 0000
:5
-W
Writing 00005 bytes
-Q

C:\>

Note that the N (name) command specifies the name of the file where we write the binary machine code to. Also, note that the W (write) command expects the registers BX and CX to contain the number of bytes to be written to the file. When DEBUG.EXE starts, it already initializes BX to 0 automatically, so we only set the register CX to 5 with the R CX command above.

The debugger session inputs are archived in the file named REBOOT.TXT, so the binary file named REBOOT.COM can also be created by running the following DOS command:

DEBUG < REBOOT.TXT

The binary executable file can be created on a Unix or Linux system using the printf command as follows:

echo EA 00 00 FF FF | xxd -r -p > REBOOT.COM

Unassemble

Here is a disassembly of REBOOT.COM to confirm that it has been written correctly:

C:\>DEBUG
-N REBOOT.COM
-L
-U 100 104
117C:0100 EA0000FFFF    JMP     FFFF:0000

Run

To run this program on MS-DOS, simply enter the following command at the command prompt:

REBOOT

How Does It Work?

When the computer boots, the x86 microprocessor starts in real mode and executes the instruction at FFFF:0000. This is an address in the BIOS ROM that contains a far jump instruction to go to another address, typically F000:E05B.

C:\>DEBUG
-U FFFF:0000 4
FFFF:0000 EA5BE000F0    JMP     F000:E05B

The address F000:E05B contains the BIOS start-up program which performs a power-on self-test (POST), initializes the peripheral devices, loads the boot sector code, and executes it. These operations complete the booting sequence.

The important point worth noting here is that the very first instruction the microprocessor executes after booting is the instruction at FFFF:0000. The REBOOT.COM program available in this project too jumps to this address thereby triggering the boot sequence.

License

This is free and open source software. You can use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of it, under the terms of the MIT License. See LICENSE.md for details.

This software is provided "AS IS", WITHOUT WARRANTY OF ANY KIND, express or implied. See LICENSE.md for details.