Code

Meaning

01

Processor test part 1. Processor status verification. Tests the following processor-status flags: set/clear carry; zero; sign and overflow (fatal). BIOS sets each flag; verifies they are set and turns each flag off verifying its state. Failure of a flag means a fatal error. Output: infinite loop if failed; continue test if OK. Registers: AX/BP.

02

Determine POST type; whether normal (boot when POST finished) or manufacturing (run 01-05 in loop) which is often set by a jumper on some motherboards. Fails if keyboard interface buffer filled with data. Output: infinite loop if failed; continue test if OK. Registers: AX/BX/BP.

03

Clear 8042 keyboard interface. Send verify TEST_KBRD command (AAh). Output: infinite loop if failed; continue test if OK. Registers: AX/BX/BP.

04

Reset 8042 keyboard controller. Verify AAh return from 03. Infinite loop if test fails. Registers: AX/BX/BP.

05

Get 8042 keyboard controller manufacturing status; read input port via keyboard controller to determine manufacturing or normal mode operation. Reset system if manufacturing; i.e. if 02 found the status to be Manufacturing triggers a reset and 01-05 are repeated continuously. Output: infinite loop if failed; continue test if OK. Registers: AX/BX/BP.

06

Initialise chips on board LSI chips. Disables colour and mono video/parity circuits/DMA (8237) chips; resets maths coprocessor; initialises timer 1 (8255); clears DMA chip and all page registers and the CMOS shutdown byte.

07

Processor Test 2. Reads writes and verifies all CPU registers except SS/SP/BP with data pattern FF and 00.

08

Initialises CMOS timer/RTC and updates timer cycle; normally CMOS (8254) timer; (8237A) DMA; (8259) interrupt and EPROM.

09

EPROM Checksum; test fails if not equal to 0. Also checksums sign-on message.

0A

Initialise Video Interface; specifically register 6845 to 80 characters per row and 25 rows per screen and 8/14 scan lines per row for mono/colour; first scan line of cursor 6/11; last scan line of cursor 7/12; reset display offset to 0.

0B

Test Timer (8254) Channel 0. See also below.

0C

Test Timer (8254) Channel 1.

0D

Test Timer (8254) Channel 2.

0E

Test CMOS Shutdown Byte using a walking-bit algorithm.

0F

Test Extended CMOS. On motherboards supporting extended CMOS configuration such as C & T the BIOS tables of CMOS information configure the chipset which has an extended storage facility enabling you to keep the configuration with the power off. A checksum is used for verification.

10

Test DMA Channel 0. This and the next two tests initialise the DMA chip and test it with an AA/55/FF/00 pattern. Port addresses are used to check the address circuit to DMA page circuit registers.

11

DMA Channel 1

12

DMA Page Registers

13

Test keyboard controller interface.

14

Test memory refresh toggle circuits.

15

First 64K of system memory which is used by the BIOS; an extensive parity test.

16

Interrupt Vector Table. Sets up and loads interrupt vector tables in memory for the 8259 PIC.

17

Video I/O operations. Initialises the video; EGA and VGA ROMs are used if present.

18

Video memory test for CGA and mono cards (EGA and VGA have their own procedures).

19

Test 8259 mask bits?aChannel 1.Interrupt lines turned alternately off and on. Failure is fatal.

1A

8259 Mask Bits?aChannel 2

1B

CMOS battery level; verifies battery status bit set to 1. 0 could indicate bad battery at CMOS.

1C

Tests the CMOS checksum data at 2E and 2Fh and extended CMOS checksum if present.

1D

Configuration of the system from CMOS values if the checksum is good.

1E

System memory size is determined by writing to addresses from 0-640K continuing till there is no response. The size is then compared to the CMOS and a flag set if they do not compare. An error message will then be displayed.

1F

Tests memory from the top of 64K to the top of memory found by writing patterns FFAA and 5500 and reading them back byte by byte for verification

20

Stuck 8259 Interrupt Bits.

21

Stuck NMI bits (parity or I/O channel check).

22

8259 function.

23

Verifies protected mode; 8086 virtual and page mode.

24

As for 1E but for extended memory from 1-16Mb on 286/386SX systems and 64 Mb on 386s and above. The value found is compared to the CMOS settings.

25

Tests extended memory found above using virtual 8086 paging mode and writing an FFFF/AA55/0000 pattern.

26

Protected Mode Exceptions; tests other aspects of protected mode operations.

27

Tests cache control (386/486) or Shadow RAM. Systems with CGA and MDA indicate that video shadow RAM is enabled even though there is no BIOS ROM to shadow.

28

Set up cache controller or 8242 keyboard controller. Optional Intel 8242/8248 keyboard controller detection and support.

29

Reserved.

2A

Initialise keyboard and controller.

2B

Initialise floppy drive(s) and controller.

2C

Detect and initialise serial ports.

2D

Detect and initialise parallel ports.

2E

Initialise hard drive and controller.

2F

Detect and initialise maths coprocessor.

30

Reserved.

31

Detect and initialise option ROMs. Initialises any between C800-EFFF.

3B

Initialise secondary cache with OPTi chipset (486 only).

CC

NMI Handler Shutdown. Detects untrapped NMIs during boot.

EE

Unexpected Processor Exception.

FF

Boot Attempt; if POST is complete and all components are initialised with no errors.