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MR BIOS POST Procedures




See if a warm boot (Ctrl+Alt+Del) or a cold boot (Reset) is needed.

Chipset Initialisation

Reset the support chips (8259) DMAs and timers to defaults before proceeding.

Disable Chips

Disable NMI/DMA and Video (6845) to get accurate results later. Failure here is normally a NMI generated by one of the disabled chips.

ROM BIOS Checksum

Perform checksum test, add a preset value stored in BIOS to create value of 00.

DMA Test

Perform a test of the page registers in the DMA controller.

Keyboard Controller Test

Send a command to the 8042 keyboard controller to perform a selftest. The keyboard controller will return a buffer and error buffer address.

Chipset Initialisation

Initialise the DMA (8237)/PIC (8259)/PIT (8254) and RTC chips.

DMA Test

Test the registers of the master 16-bit and slave 8-bit DMA controllers by writing bit patterns and reading the results.

Cache/Shadow Disable

Disable cache and shadow RAM before processing with POST.


Test interval in which PIT (8254) chip sends a refresh signal to the DMA chips.

Base 64K Memory Test

Test the first 64K of system memory with a walking-bit pattern.

PIC Test

Test the mask registers of the master and slave interrupt controllers by setting the mask-bit in the registers and generating an interrupt to see if the interrupt is trapped. Then test the additional registers in the PICs with a walking-bit pattern.

PIT Test

Test the interrupt timer channels 0-2 and initialise if no failures occur.


Perform read/write test of RTC portion of CMOS and initialise if no failures occur.


Test and initialise the video adapter, which will perform an internal diagnostic and sign on before returning an OK status.

CMOS Checksum

Perform a checksum on the system RAM.

Keybd Initialisation

Initialise the keyboard and read the buffer address for errors.

OEM Specific



Base Memory Test

Test memory addresses between 64-640K with a walking-bit pattern. There may be a hex display of the failing it.

Keyboard 2nd Init

Tries again if the first failed.

Protected Mode Test

Test the ability of the keyboard controller address line 20 to respond to commands that switch the CPU in and out of protected mode.

Extended Memory Test

Test addresses above 1 Mb in 64K blocks and perform pattern tests.

OEM Memory

Normally test the cache controller and shadow RAM.

RTC Time Test

Test the write active line of the RTC/CMOS chip. Check bad CMOS/battery

Serial Port

Generate an interrupt of the CPU through I/O ports reserved for RS232 devices. Failure to see a device could be the device itself or more than one set to the same port. Checks are only made for two devices.


Check for parallel devices. Failure to see a device could be the device itself or more than one using the same port. Checks are only made for three.

NPU Test

Perform a register test on the NPU then initialise if passed.

Floppy Test

Test floppy controller and drive.

Fixed Disk

Test fixed disk controller and drive and compare the results against the CMOS setting. This is skipped if no drive is installed.

CMOS Update

Update information in CMOS RAM based on the previous results.

Non-Fatal Errors



Lock Check

Check if a system lock-byte is set and wait for user response if an error is generated. Check the panel lock or circuitry.


Set NumLock on (if set) and ask for password (if set) and display setup message.

Typematic Rate

Set the typematic rate.

Floppy Disk

Perform any further initialisation needed.

Hard Disk

Perform any further initialisation needed.

Video Mode

Set primary video mode and display any errors found during initialisation routines.

Shadow/Cache Enable

Adapter ROM

Initialise adapters with a ROM signature of 55AA. Self tests will be performed by the equipment concerned before handing back control to the POST.

Video Monitor Mode

Set the video mode based on the information in the CMOS and update the time variables from the RTC.

Parity/NMI Enable

Enable NMI by setting bit 7 of CMOS address 41 and enable parity. There should be no failures during this.

Set Stack

Set the last significant byte of the stack pointer and install the shadow RAM at E000 if set by CMOS.


Acknowledge errors and set primary video mode before calling Int 19 boot loader. Errors reported will await a keyboard response before proceeding. Errors beyond this point are normally



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