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IBM POST ports and procedures
     Tests are performed by PC/XT/AT and PS/2 machines. There will be POST Codes (below), beep codes and screen displays if possible, but the XT does not give POST codes. ATs emit codes to 80h, while PS/2 models 25 and 30 emit to 90h, and 35 and above to 680. The BIOS will test main system components first, then non-critical ones. If there is an error, the BIOS will look for a reference diskette in drive A: so diagnostics can be performed.
IBM POST I/O Ports

Architecture

Typical Computer

Port

PC

PC

none

ISA

XT

60

AT

80

PS/2 25,30

90, 190

MCA

PS/2 50 up

680, 3BC

EISA

none

none

IBM POST Procedures

Procedure

Meaning

CPU

Perform register test on the CPU by writing data patterns to the registers and reading the results of the write.

ROM BIOS Checksum

The value of the bits inside the BIOS chip(s) is added to a preset value that should create a total of 00.

CMOS RAM

RAM within the CMOS chip is tested by writing data patterns to the area and verifying that the data was stored correctly.

DMA

Test DMA chips by forcing control inputs to the CPU to an active state and verifying that the proper reactions occur.

8042/8742 Keyboard Controller

Test including Gate A20 and the reset command. The buffer space is prepared and data is sent to the determined area via the keyboard controller to see if commands are received and executed correctly.

Base 64K System RAM.

Perform a walking-bit test of the first 64K of RAM so the BIOS vector area can be initialised. Check for bad RAM chips or a data/address line.

8259A PIC

Determine if commands to interrupt CPU processes are carried out correctly. Check the PIC/PIT/RTC/CMOS or Clock chip(s).

8254 PIT

Check that proper setup and hold times are given to the PIC for interrupts of the CPU processes. Check the PIT or Clock chip.

82385 Cache Controller

This is normally responsible for cache and shadow memory.

CMOS RAM Configuration Data

Check information in CMOS RAM before further testing so any failures after this could also be down to the CMOS chip.

CRT controllers

Test any video adapters listed in the CMOS.

RAM above 64K

Perform a walking-bit test on memory above 64K listed in the CMOS.

Keyboard

Test interface to the keyboard including scan code stuck keys etc.

Pointing Device (mouse etc)

Test and initialise vector for any pointing devices found. Failure to see a device may be down to the device itself but there may be a problem with the CMOS or 8042/8742.

Diskette Drive A:

Test and initialise the A: drive.

Serial Interface Circuitry

Test any RS232 devices found at the proper I/O address.

Diskette Controllers

If an A: drive has been found further testing is performed before proceeding to the bootloader. This test includes reading the first sector of any diskette in the drive to see if a valid boot code is there.

Fixed Disk Controllers

Test and initialise any hard drives set in the CMOS including reading the first sector of the hard drive to see if a valid boot code exists.

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