1. Boot Sequence (DOS) for the IBM PC
Charles Abzug, Ph.D.
Department of Computer Science
James Madison University
Harrisonburg, VA 22807
Voice Phone: ,
Home Page:
© 1998 Charles Abzug

2. Stage 1: Power-up 1. Switch on electrical power.
2. ~ msec until “Power Good”.
3. Timer chip stops giving “Reset” signal to CPU.
4. Set microprocessor to “Real-Address” mode.

3. Stage 2: Test and Initial Set-up
1. Load PC with the value FFFF0.
2. Execute ROM-BIOS starting at location FFFF0.
NOTE: This is only 16 Bytes before the end of the original PC’s memory address space.
3. One of the first instructions consists of a JMP to the actual ROM BIOS starting address.
4. Test central hardware (error reports signaled via audio beeps).
5. Look for video ROM in memory locations C0000 through C7800, and execute video ROM BIOS checksum test.
6. Initialize video adaptor and display cursor.
7. Scan memory locations C8000 through DF800 looking for adaptor-card ROMs, test their checksums and execute.

4. Stage 3: POST 1. Check word value at memory location 00472.
2. A value of 1234h indicates a warm start (skip POST); if any other value, do POST.
3. A single beep indicates successful POST.

5. Stage 4: Disk Volume Boot Sector
1. Look for DOS volume Boot Sector on A: drive at cylinder 0, head 0, sector 1.
2. Load it into memory location 07C00 and test.
3. If there is no DOS boot sector on drive A:, then look for Partition Boot Sector on first fixed disk at cylinder 0, head 0, sector 1.
NOTE: Master Boot Sector has to have previously been written to the hard disk by either the DOS FDISK utility or by some other equivalent program.
4. Load it into memory location 07C00 and test.
5. The master Partition Boot Sector program searches the Partition Table for a boot indication byte, indicating an active partition.
6. Load the Volume Boot Sector from the active partition, and test it for a signature of 55AAh in the last two bytes

6. Stage 5: Load, Initialize, and Start the Operating System.
1. Execute the Volume Boot Sector.
2. Check root directory for the presence of IBMBIO.COM or IO.SYS, and of IBMDOS.COM or MSDOS.SYS.
3. Load IBMBIO.COM or IO.SYS, and IBMDOS.COM or MSDOS.SYS, into a low part of conventional memory area.
4. Execute IBMBIO.COM or IO.SYS.
5. Program copies itself into highest region of region of contiguous DOS memory.
6. Transfer execution to the higher copy of the program.

7. Stage 5 (CONTINUED)
7. Relocate IBMDOS.COM or MSDOS.SYS in memory in place of the lower copy of IBMBIO.Com or IO.SYS, which is no longer needed there because this program is now operating from the upper site.
8. Execute IBMDOS.COM or MSDOS.SYS.
9. Initialize device drivers, determine equipment status, reset and initialize attached devices and disk system, and set default parameters.
10. Restore control to IBMBIO.COM or IO.SYS.
11. Read the file CONFIG.SYS from beginning to end, looking for “DEVICE” statements. Load and execute in order each device driver file specified in such a statement.
12. Read the file CONFIG.SYS again from beginning to end, looking this time for “INSTALL” statements. Load and execute in order the TSR programs named in these statements.

8. Stage 5 (CONTINUED)
13. Load the Shell specified in the “SHELL” statement, with specified parameters, into memory in place of the IBMBIO.COM or IO.SYS initialization code, which is no longer needed. Default shell is \COMMAND.COM.
14. Read and process all remaining statements in CONFIG.SYS.
BREAK, NUMLOCK
BUFFERS, FCBS, FILES, STACKS, DRIVPARM
COUNTRY
15. Load and execute COMMAND.COM.
16. Load AUTOEXEC.BAT into memory and execute its commands.
17. Execute internal DATE and TIME commands.
18. Display copyright message, and then the DOS prompt.

9. The END