Wikipedia says:
By design, the 286 could not revert from protected mode to the basic 8086-compatible real address mode ("real mode") without a hardware-initiated reset. In the PC/AT introduced in 1984, IBM added external circuitry, as well as specialized code in the ROM BIOS and the 8042 peripheral microcontroller to enable software to cause the reset, allowing real-mode reentry while retaining active memory and returning control to the program that initiated the reset. (The BIOS is necessarily involved because it obtains control directly whenever the CPU resets.) Though it worked correctly, the method imposed a huge performance penalty.
So naturally, I'm having a look at the reset lines now. Reset on the 286 never goes high again after the initial start. So if it does enter protected mode, it never leaves it.
By design, the 286 could not revert from protected mode to the basic 8086-compatible real address mode ("real mode") without a hardware-initiated reset. In the PC/AT introduced in 1984, IBM added external circuitry, as well as specialized code in the ROM BIOS and the 8042 peripheral microcontroller to enable software to cause the reset, allowing real-mode reentry while retaining active memory and returning control to the program that initiated the reset. (The BIOS is necessarily involved because it obtains control directly whenever the CPU resets.) Though it worked correctly, the method imposed a huge performance penalty.
So naturally, I'm having a look at the reset lines now. Reset on the 286 never goes high again after the initial start. So if it does enter protected mode, it never leaves it.
With the original Phoenix BIOS (copyright 1989) the POST codes are, in reverse order:
38 3a 00 35 34 2e 2c 29 28 27 25 23 22 21 20 0d 09 08 06 05 04 03 02 01 00
It beeps once, then twice. Does initialize the video card, same garbage though.
38 3a 00 35 34 2e 2c 29 28 27 25 23 22 21 20 0d 09 08 06 05 04 03 02 01 00
It beeps once, then twice. Does initialize the video card, same garbage though.
Forwarded from Keyjn Tombowölf
I suspect something fails to write correctly to the graphics card (Adress error or Data error).
Because it's now a fault that the POST can't detect.
Means it must be a checkpoint were the POST doesn't read back the current state of the card rather it's just writing and assuming that it went through successfully.
Because it's now a fault that the POST can't detect.
Means it must be a checkpoint were the POST doesn't read back the current state of the card rather it's just writing and assuming that it went through successfully.
Interestingly enough, I/O ports work fine. The POST code card works, the floppy controller and of course setting video modes with the VGA card. However, writing into memory on the ISA bus does not work.
There are control lines on the ISA bus to differentiate between I/O port reads / writes and memory reads / writes. There was activity on the I/O pins, they lead to a 244 I already replaced. The memory write line however never sees any activity. And since we know this board gets very far in the boot process, the video BIOS gets passed control to clear the video memory. Which never happens however. So I followed that MEMW control line to ..
And who would have thought. This gate is dead.
Yellow is out, blue in, red output enable (active low).
Yellow is out, blue in, red output enable (active low).
What I need now is a small compact flash card with all the DOS hardware testing tools. If you have suggestions beyond the usual CheckIt and Norton Sysinfo, please post them in the comments. I will share the image file once I'm done
Tbh I can't quite believe I managed to bring it back to life. I thought the chipset was toast. Luckily it has all those buffers on there. I think I will pull the other suspect buffers too, lest they put a strain on the chipset due to bad input gates.