POST card

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POST card

A POST card will save your life: it's the only output device (beside beeper) you have during the boot process. The term POST means Power On Self Test and comes from the original IBM specifications for the BIOS. Port 80 is a pre-defined I/O port to which programs can output a byte. The POST card displays the byte in hex on its 2 digit display. We use a lot of POST codes in coreboot, so if you can tell us the POST code you see, we will have some idea of what happened.

If your coreboot machine is working properly, you will see it count up from 0xd0 to 0xd9 (while it is gunzipping the kernel) and then display 0x98 (Linux idle loop). There are POST cards with ISA bus, PCI bus, USB und parallel port connectors (the latter for laptops).

Often they carry status LEDs for ISA/PCI signals such as: IRDY, BIOS-access, FRAME, OSC, PCI-CLK, RESET, 12V, -12V, 5V, -5V, 3.3V. Some cards were known to not function because the mainboard switches off the CLK on their slot after non-standard registration on PCI.

PCI POST cards can be found in various places.

See also How can I write to port 0x80 from userspace.