"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

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Micro Decision

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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."


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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... "

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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.... "

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"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

Olivetti POST Codes

For EISA and PS/2, the code is issued after the test has passed, so a stuck code indicates the next test failed. Codes are sent to printer ports 3BC (the mono adapter's parallel port), 278, or 378; they will not be printed because no strobe data is sent. AT&Ts using the Olivetti motherboard and BIOS (e.g. the AT&T 6300) do the same.

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

1076/AT&T 6312 Codes

The first checkpoint, 40, resets and initializes a test monitoring device on the parallel port. When an error occurs, the most recent checkpoint code sent to port 378 is exclusive-ored with 3F to complement the lower 6 bits, and then sent to 378, so if the refresh test fails (45), the POST card will show 7B because the most recent code sent before the failure was 44.

If an error occurs, the POST tries to run through activities that display a message on the monitor, showing tttt Error: xx, where tttt is the name of the failing routine, and xx is a suberror number. If the error is fatal, the display will show Unrecoverable power-up error, wait for you to press F1, and return to the failing test. If video has failed, the POST will output beep codes.

41 7F CPU flag and register test
42 7E Check and verify CMOS shutdown code
43 7D BIOS ROM checksum test
44 7C Test the 8253 timer
45 7B Start memory refresh
46 7A Test the 8041 keyboard controller
47 79 Test the first 8KB of RAM
48 78 Test protected mode operation
49 77 Test CMOS RAM shutdown byte
4A 76 Test protected mod operation
4B 75 Test RAM from 8KB to 640KB
4C 74 Test all RAM above 1MB
4D 73 Test NMI
4E 72 Test RAM parity system
50 71 Test 8259 PIC 1
51 6F test 8259 PIC 2
52 6E Test DMA page register
53 6D Test 8237 DMA controller 1
54 6C Test 8237 DMA controller 2
55 6B Test PIO port 61h
56 6A Test the keyboard controller
57 69 Test the CMOS clock/calendar IC
59 68 Test the CPU protected mode
5A 66 Test CMOS RAM battery
5B 65 Test CMOS RAM
5C 64 Verify CMOS RAM checksum
5D 63 Test parallel port configuration
5E 62 Test serial port configuration
5F 61 Test memory configuration below 640KB
60 60 Test memory configuration above 1MB
61 5F Detect and test math coprocessor
62 5E Test configuration of game port adapter
62 5D Test key lock switch
63 5D Test hard drive configuration
64 5C Configure floppy drives
66 5B Test option ROM's
- - Call interrupt 19 boot loader

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M20 Codes

Not a true IBM clone, as it had a Zilog Z8001 CPU. Also, a typical POST card will not fit in a slot, so you can only monitor codes from the parallel port. The POST shows a triangle, diamond, or 4 lines on the screen to indicate early POST failure, as shown in the table.

Triangle Test CPU registers and instructions
Triangle Test system RAM
4 vertical lines Test CPU call and trap instructions
Diamond Initialize screen and printer drivers
EC0 8255 parallel interface IC test failed
EC1 6845 CRT controller IC test failed
EC2 1797 floppy disk controller chip failed
EC3 8253 timer IC failed
EC4 8251 keyboard interface failed
EC5 8251 keyboard test failed
EC6 8259 PIC IC test failed
EK0 Keyboard did not respond
Ek1 Keyboard responds, but self test failed
ED1 Disk drive 1 test failed
ED0 Disk drive 0 test failed
E10 Non-vectored interrupt error
E11 Vectored interrupt error

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M21 & M24/AT&T 6300 Codes

The M24 went to the US as the AT&T 6300. It had an 8086, so was faster than the PC, albeit difficult to work on. codes are sent to 378 (LPT1). If a fatal error occurs, it performs more initialization of DMA and interrupt controller circuits, tries to display an error message, complements the lower 6 bits of the POST code, sends the result to port 378, and halts the CPU, so numbers will flicker on the POST display with bit 6 on and the lower bits running from 0 upward. The codes start at 40 because a black box was used to monitor POST status at the parallel port. Bit 6 was set true (1) to alert the box the POST was starting.

40 CPU flags and register test failed
41 BIOS ROM checksum test failed
42 Disable 8253 timer channel 1
43 8237 DMA controller test failed
44 8259 PIC test failed
45 Install the real interrupt vectors
48 Send beep and initialize all basic hardware

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PS/2 Codes


01 Test CPU
02 Check CMOS shutdown byte
03 Initialize the PIC
04 Test refresh
05 Test CMOS/RTC periodic interrupt
06 Test timer ratio
07 Test first 64KB of RAM
08 Test 8042 keyboard controller
09 Test NMI
0A Test 8254 PIT
0B Test port 94h
0C Test port 103h
0D Test port 102h
0E Test port 96h
0F Test port 107h
10 Blank the display
11 Check the keyboard
12 Test CMOS RAM battery
13 Verify CMOS RAM checksum
14 Verify extended CMOS RAM checksum
15 Initialize system board and adapter
16 Initialize and test RAM
17 Test protected mode registers
18 Test CMOS RAM shutdown byte
19 Test CMOS protected mode
1A Initialize video adapter ROM scan
1B Test BIOS ROM checksum
1C Test PIC #1
1D Test PIC #2
1E Initialize interrupt vectors
20 Test extended CMOS RAM
21 Test CMOS real-time clock
22 Test clock calendar
23 Dummy checkpoint
24 Test watchdog timer
25 Test 64KB to 640KB RAM
26 Configure lower 640KB RAM
27 Test extended memory
28 Initialize extended BIOS data segment and log POST error
29 Configure memory above 1MB
2A Dummy checkpoint
2B Test RAM parity
2C Test DMA page registers
2D Test DMA controller registers
2E Test DMA transfer-count register
2F Initialize DMA controller
30 Test PIO 61
31 Test the keyboard
32 Initialize keyboard typematic rate and delay
33 Test auxiliary device
34 Test advanced protected mode
35 Configure parallel ports
36 Configure 8250 serial ports
37 Test and configure math co-processor
38 Test and configure game-port adapter
39 Configure and initialize hard disk
3A Floppy-disk configuration
3B Initialize ROM drivers
3C Display total memory and hard drives
3D Final initialization
3E Detect and initialize parallel ports
3F Initialize hard drive and controller
40 Initialize math co-processor
42 Initialize adapter ROM scan
CC Unexpected processor excerption occurred
DD Save DDNIL status
EE NMI handler shutdown
FF Call interrupt 19 boot loader

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