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During a discussion, I was told that the POST was part of the BIOS, and so, it is the responsibility of the BIOS to set up everything (self tests and all)

I was reading the intel manual... http://bitsavers.org/components/intel/80386/230985-003_386DX_Microprocessor_Programmers_Reference_Ma... In section 10.1, it describes the microprocessor state after reset...

A self-test may be requested at power-up. The self-test is requested by asserting the signal on the BUSY # pin during the falling edge of the RESET# signal. It is the re-sponsibility of the hardware designer to provide the request for self-test, if desired. Reset initialization takes 350 to 450 CLK2 clock periods. If the self-test is selected, it takes about 220 clock periods (Intel reserves the right to change the exact number of periods without notification). For a 16 MHz processor, this takes about 33 milliseconds.

And then the control is transferred to the BIOS firmware, ie, IP is set to reset vector (flash memory mapped to 0xfffffff0 in 32 bit x86 processors and later)...

So, there is some kind of hardware test even before BIOS? If POST is part of BIOS, what's this self test?

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It seems that the CPU has a built in test which the motherboard can configure the CPU to perform at reset. But BIOS does more than test the CPU during POST. Besides testing the CPU registers and operation just by executing a software that tests it, it will test other things as well such as the chipset test for timer, DMA controller, memory refresh, memory read/write test, interrupt controller, etc. So almost any peripheral it knows of that is vital to the computer operation is tested and initialized to known state.

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  • This is a test that is different from the POST? So POST is just an abstraction created by programmers, and doesn't really exist 'hardcoded' in hardware?
    – Suraaj K S
    Commented Feb 3, 2020 at 6:48
  • Power On Self Test is just executable code in BIOS ROM that just does a self test to know if the hardware is good or malfuctioning. The CPU test ia completely separate and specific feature of that CPU, which the earlier CPUs did not have.
    – Justme
    Commented Feb 3, 2020 at 7:35
  • What does it mean that the motherboard can configure the CPU to perform this test?
    – Suraaj K S
    Commented Feb 3, 2020 at 7:37
  • You quoted how to request the CPU to go into test mode and execute it. Either the motherboard supports setting the signals to execute the test, or it doesn't. And then the BIOS code would need to know if the test was performed or not, so it can check for test result to know if there was a failure or not.
    – Justme
    Commented Feb 3, 2020 at 7:50

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