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| 1. | User turns on power supply. |
| 2. | Motherboard receives +5 volts from the power supply. |
| 3. | Also received from the power supply is the POWER GOOD signal, a digital signal of TTL levels, which at this time, is low. <----- Important: At this time, the signal is LOW |
| 4. | Because the POWER GOOD signal is low, the motherboard's 8284A chip takes the motherboard's RESET line high. Per here, various chips on the motherboard (including the 8088 CPU) are held in a reset state. |
| 5. | Refer to here. On the motherboard, the 8284A chip generates a 4.77 MHz clock signal for the 8088 CPU (on pin 19) and the 8288A bus controller chip (on pin 2). (And optional 8087 NPU, if fitted.) |
| 6. | Later, when all voltages from the power supply are within specification, the power supply takes the POWER GOOD line high. Pictured at here. |
| 7. | When the 8284A chip sees the power supply's POWER GOOD signal go high, it takes the motherboard's RESET line from high to low. As a result, chips are taken out of the reset state. |
| 8. | When the 8088 CPU leaves the reset state (see note 1 below), it starts execution at address FFFF0. In the IBM 5160, that address corresponds to an address near the end of motherboard ROM chip U18, one of the motherboard's two BIOS ROM chips (per here). |
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Basically (repeat: basically), the read of address FFFF0 goes something like: - 8088 starts a 'Code Access' bus transaction. - 8088 sets its three status pins (see here) to indicate to the 8288 chip that a 'Code Access' bus transaction is in progress. - The 8288 chip, seeing the 8088 indicate a status of 'Bus Access', asserts its /MRDC output pin. - - Per here and here, the 8088, the 8288, and certain circuitry, puts address FFFF0 onto the address bus. - The address is then put onto the external address bus by driver chips U14/U16/U17. - The 15 address pins of ROM chip U18, which are on the external address bus, see address 7FF0 (the low 15 bits of address FFFF0). - - Certain circuitry establishes that motherboard address FFFF0 corresponds to ROM chip U18, and accordingly, drops the /CS pin (/CE pin) on ROM chip U18. See here and here. - As a result, ROM chip U18 outputs the data it has at its 7FF0 address onto the external data bus. - Certain circuitry instructs U15 to put that data from the external data bus onto the data bus. - Per here, the 8088 reads the data bus. BTW. If you have a logic probe, you can see that for yourself via the procedure at here. |
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| 9. | Starting at ROM chip U18's address of 7FF0, are five bytes, which together form a jump instruction to the start of the BIOS' power-on self test (POST). At here is a capture of that via a logic analyser. |
| 10. | The POST executes. Details are at here. |
| Note 1 | The data sheet for the Intel 8088 indicates that there is minimum period in which the 8088 needs to be held in a reset state for. That minimum period is 4 clock cycles, which in the IBM 5160, equates to 840 ns. At here is an oscilloscope measurement showing the power-on reset in one of my IBM 5160's being approximately 300 ms long; that is about 350 times longer than the minimum requirement. Therefore, perhaps the 840 ns requirement will only be violated if someone disconnects the POWER GOOD signal, or simply ties POWER GOOD directly to +5V. |
