"The computer book of the month is The Bios Companion by Phil Croucher. Long-time readers of this column will recall I have recommended his book before. This tells you everything you ought to know about the BIOS in your system. Post codes, options, upgrades, you name it.

Years ago, I called an earlier edition of this invaluable and I see no reason to change my view. Recommended.

Jerry Pournelle
Byte Magazine

"Croucher's book was invaluable 20 years ago and remains so today...... Every one of the 468 pages holds useful information, the author having zero tolerance for padding."

Davey Winder
PC Plus Magazine

"You've completerly impressed me with the quality and scope of your guides - extreme care has obviously gone into their research and preparation."

Jeremy Fleming

"His style is lovely: clear, jargon-free where possible, chatty and friendly with beautiful short paragraphs. It [Communications & Networks] joins my list of 'I wish I'd written it' books. I haven't come across such a good subject book in a long time at this price."

Lorna Kyle
Personal Computer World

"To any micro owner who wants to know what this comms business is all about, [Communications & Networks] is certainly one of the best around. ... Croucher is not only knowledgable about his subject, he's interested in it and communicates that interest to you. "

Ron Peck
New Computer Express

"There are few books about which explain as accurately and comprehensively as [Communications & Networks] does what communications are all about. The glossary is excellent."

Micro Decision

"...any PC user with a genuine interest in using his machine to its full potential will recognise [Communications & Networks] for the completely invaluable tool and often the inspiration that it is."

Yvonne Taylor
Which PC

"[Communications & Networks] is the type of book you can read on a bus or train without getting a headache from trying to understand what it means."


"As a recently reformed computer illiterate myself, I appreciated the author's straighforward yet humourous and light-hearted style of writing. He manages to strike the balance between too much and too little information. ... [Computing Under Protest] is never in danger of either boring or confusing the reader."

Emma Tyrrall
Practical PC

"Good documentation is important for any software, but even more so for an operating system - the documentation [for DOS 386 Professional] has some exceptionally clear explanations. Altogether, the documentation is an example of the right way of doing things, and other software manufacturers would do well to take note."

Matthew Holbrook
Computer Shopper

"The DOS 386 Professional manual sets arrived at 2130 local time today. We have had a brief look at them and wish to congratulate IMS on the excellent, clear and concise manner in which they have been presented."

Fred Parker
ETL Soft

"I remain impressed by Mr. Croucher's fluid writing style, practical orientation, enthusiasm, and strong technical knowledge."

Lance A Leventhal
Slawson Communications, Inc

JAR Professional Pilot Studies

"Its real value lies in taking the plethora of booklets and ring-bound photocopies associated with commercial examinations and condensing them into an attractive and portable form. The guy really does deserve a medal ~ he has made a genuine contribution to the body of professional aviation literature assembled during the course of the last century.

I heartily recommend it to students of commercial flight, if only as a souvenir of their efforts... "

Colin Hilton
Pro Pilot's Rumour Network

"(The) Book is quite a good compliment to our notes for students. We will be recommending all our students buy the books.... "

Naples Air Centre

"I got your book last month and am in the process of ploughing through it! Actually, I really like it and find it very interesting and easy to read."

Simon Rouse

"So far I have found your book an extremely helpful reference manual to carry around in my flight bag."

Blair Clubley

"I wish to commend you on a "work of art". I wish the CAA would produce a quick reference easy to read manual such as yours. Truly wonderful!"

Ebrahim Parkar

Post Codes

 Chips & Technologies
 Faraday A-Tease

Chips & Technologies POST Codes

Some codes appear on the display in decimal as well as to port address 80h.  Micro Channel uses ports 680 and 3BC.

Note: Not all POST codes are included below - the latest information is in the The BIOS Companion, which contains over 100 pages of them.

POST Procedures


Power on tests CPU synchronizes with clock.  Check the CPU or clock if a failure occurs
System ROM Check The BIOS runs a checksum on itself.   Check the BIOS chips if a failure occurs
DMA controller Fail DMA controllers are initialized and tested.  Check the DMA chips if a failure occurs
System Timer Failed Channels 0, 1, and 2 are tested in sequence.  If a failure occurs, check the PIT chips
Base 64K Memory Testing Walking bit test performed on first 64K of RAM which is critical for the BIOS vector area to be initialized.  Check for bad RAM chips or a bad data or address line
Interrupt Controller Failed Test the 8259 chip
CPU Still In Protected Mode Attempts are made to read the configuration of the system through the 8042 keyboard controller
Refresh Not Occurring Memory refresh is tested; standard refresh is 120-140ns.  Check the PIT if a failure occurs
Keyboard Controller Not Responding Tests are run on the keyboard controller.  the the KBC 8042 if a failure occurs
Could Not Enter Protected Mode BIOS attempts to enter protected mode to test extended memory.  Check the 8042 KBC chip or the A20 gate address line if a failure occurs
Initialize Timer Attempts are made to initialize the PIT
Initialize DMA Controller Attempts are made to initialize the DMA controllers
Entering/Exiting Protected Mode The translation is handled by the keyboard controller and the A20 gate address line.  Check the keyboard controller if a failure occurs
Relocate Shadow RAM BIOS attempts to shadow itself into extended memory.  Check for memory problems if a failure occurs
Test For EMS Check for EMS adapter or an improper CMOS setting/Jumper setting if a failure occurs
Test Video Capabilities Normally includes a memory test on the adapter memory up to 256K
Test Memory Extensive testing of Base, Extended,and Expanded memory.  Check for defective memory modules, 8042 KBC, A20 gate address line or an improper CMOS and/or jumper setting
Check System Options The hardware in the system is compared with the values stored in CMOS.  The PIT, PIC, 8042 KBC, RTC and other system board components are tested again
Peripheral Check and test Checks are made for the peripherals at standard I/O ports including serial and parallel ports, keyboards, and math coprocessor.   You should see an error message on screen at the point if a failure occurs
Floppy Test Floppy devices set in CMOS are checked and initialized.  If a bootable floppy is found the fixed disks are tested and the BIOS will boot to the floppy disk.  Check for defective controllers or an improper CMOS Setup if a failure occurs
Fixed Disk Test Checks for fixed disks in CMOS.  If no bootable floppy in the A: drive, then the BIOS loads the first sector off the first fixed disk and jumps to the area of memory where the sector was loaded.  You may just see a flashing cursor or an error message from the potential operating system.  Check for improper CMOS Setup, defective controller, or fixed disk failure.  Also, check for a corrupt bootloader on the fixed disk
Advanced Options These include the mouse, cache, etc.   You should see an error message on the screen at this point, except that a defective cache may hang the system.  In most cases, the cache will be disabled by the BIOS

Back to Top

POST Codes


00 Error in POS register
01 CPU flag register failed
02 CPU register test failed
03 BIOS ROM checksum failed
04 DMA controller test failed
05 System timer IC failed
06 Base 64K RAM failed address test: not installed, misconfigured, or bad addressing
07 Base 64K RAM failed data test
08 Interrupt controller failed
09 Hot (unexpected) interrupt occurred
0A System timer does not interrupt
0B CPU still in protected mode
0C DMA page registers failed
0D Refresh not occurring
0E Keyboard controller not responding
0F Could not enter protected mode
10 GDT or IDT failed, Keyboard controller failed
11 LDT register failed, Keyboard controller failed
12 Task register failed, Keyboard controller failed
13 LSL instruction failed, keyboard controller failed
14 LAR register failed, Keyboard controller failed
15 VERR/VERW failed, Keyboard controller failed
16 Keyboard controller gate A20 failed
17 Exception failed/unexpected exception
18 Shutdown during memory test
19 Last used error code
1A Copyright checksum error
1B Shutdown during memory sizing
1C Chipset initialization
50 Initialize hardware
51 Initialize timer
52 Initialize DMA controller
53 Initialize 8259 programmable interrupt controller
54 Initialize system chipset
55 Setup EMS configuration
56 Entering protected mode for first time
57 Size memory chips
58 Configure memory chip interleave
59 Exiting protected mode for first time
5A Determine system board memory size
5B Relocate shadow RAM
5C Configure EMS
5D Set up wait state configuration
5E Re-test 64K RAM
5F Test shadow RAM
60 Test CMOS RAM
61 Test video
62 Test and initialize DDNIL bits
63 Test protected mode interrupt
64 Test address line A20
65 Test memory address lines
66 Test memory
67 Test extended memory
68 Test timer interrupt
69 Test real time clock (RTC)
6A Test keyboard
6B Test 80x87 math chip
6C Test RS232 serial ports
6D Test parallel ports
6E Test dual card
6F Test floppy drive controller
70 Test hard drive controller
71 Test keylock
72 Test pointing device
90 Setup RAM
91 Calculate CPU speed
92 Check configuration
93 Initialize BIOS
94 POST Bootstrap
95 Reset IC's
96 PEAK: System board POS.   NEAT/OC8291 ELEAT: Test /initialize cache RAM and controller.
97 VGA power on diagnostics and setup
98 Adapter BIOS
99 Re-initialize DDNIL bits
A0 Exception 0
A1 Exception 1
A2 Exception 2
A3 Exception 3
A4 Exception 4
A5 Exception 5
A6 Exception 6
A7 Exception 7
A8 Exception 8
A9 Exception 9
AA Exception A
AB Exception B
AC Exception C
AD Exception D
C0 System board memory failure
C1 I/O channel check activated
C2 Watchdog timer timeout
C3 Bus timer timeout



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