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PhoenixBIOS 4.0 Release 6.0
POST Tasks and Beep Codes
When you turn on or reset an IBM-compatible PC, the BIOS first performs a
number of tasks, called the Power-On-Self-Test (POST). These tasks test
and initialize the hardware and then boot the Operating System from the hard
disk.
At the beginning of each POST task, the BIOS outputs the test-point error
code to I/O port 80h. Programmers and technicians use this code during
trouble shooting to establish at what point the system failed and what routine
was being performed. Some motherboards are equipped with a seven-
segment LED display that displays the current value of port 80h. For
production boards which do not contain the LED display, you can purchase
an installable "Port 80h" card that performs the same function. If the BIOS
detects a terminal error condition, it issues a terminal-error beep code (See
following), attempts to display the error code on upper left corner of the
screen and on the port 80h LED display, and halts POST. It attempts
repeatedly to write the error to the screen. This attempt may "hash" some
CGA displays.
If the system hangs before the BIOS can process the error, the value
displayed at the port 80h is the last test performed. In this case, the screen
does not display the error code.
Copyright © 1997 by Phoenix Technologies Ltd.
All Rights Reserved
Phoenix Technologies Ltd.
Desktop Division
135 Technology Drive
Irvine, CA 92618
http://www.phoenix.com
Terminal POST Errors
There are several POST routines that require success to finish POST. If they
fail, they issue a POST Terminal Error and shut down the system. Before
shutting down the system, the error handler issues a beep code signifying the
test point error, writes the error to port 80h, attempts to initialize the video,
and writes the error in the upper left corner of the screen (using both mono
and color adapters).
The routine derives the beep code from the test point error as follows:
1. The 8-bit error code is broken down to four 2-bit groups.
2. Each group is made one-based (1 through 4) by adding 1.
3. Short beeps are generated for the number in each group.
Example:
Testpoint 16h = 00 01 01 10 = 1-2-2-3 beeps
POST Task Routines
The following is a list of the Test Point codes written to port 80h at the start
of each routine, the beep codes issued for terminal errors, and a description
of the POST routine. Unless otherwise noted, these codes are valid for
PhoenixBIOS 4.0 Release 6.0.
NOTE: The following routines are sorted by their test point numbers
assigned in the BIOS code. Their actual order as executed during POST can
be quite different.
Code Beeps POST Routine Description
02h Verify Real Mode
03h Disable Non-Maskable Interrupt (NMI)
04h Get CPU type
06h Initialize system hardware
08h Initialize chipset with initial POST values
09h Set IN POST flag
0Ah Initialize CPU registers
0Bh Enable CPU cache
0Ch Initialize caches to initial POST values
0Eh Initialize I/O component
