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I know i have been asking alot of stupid questions lately, but this just blow my mind.
I descided to start reading through the manuals again. As you may have guessed i'm very much interested in the a20 as i still have some trouble understanding what all the fuss is about. "Surely the intel manuals mus hold the answer" i thought, but i have been unable to find anything usefull on the subject in the manuals. Of course it's alot of pages, so i might have missed it, but if thats the case could someone please the page number of which of the manuals?

Thanks

As far as I know it's not in the processor manuals, probably because those sneaky hardware designers put the control of the thing in the keyboard controller (you'd think they'd make it a flags bit, eh?).

Still, they have, in the manual, a step by step on how to enter pmode, which involves the a20 gate, but it isn't even mentioned...

it is an external device. I guess in their minds it goes along the same line as mentioning any other external device (hdd,net,etc...).

Also, even though they don't mention the 'A20' gate specifically, they do talk about the control register values and what the bits do.

Since the A20 line is completely PC specific, it makes no sense to mention it in the processor manuals. You could make your own computer with an intel cpu and not have an a20 line and the intel manual should still apply.

So basicly you have to detect the keyboard controler to know which methode to use, which is probably impossible.

When you enable the processor to run in protected mode, it's supposed to be able to access the whole 4gb range of addresses. That it can't is an artificial limitation of the PC platform, that the Apple intel doenst have. I think, ( but is probably emulated by the emulation bios to boot windows on the mac).
What you need, is the docs of the keyboard. Normally the PC platform computer cant boot without the keyboard attached. But some pc can, like some business servers, which, I suppose, have the keyboard logic into the motherboard. But now all the keyboard logic are inthe chipset and not in the keyboard proper anymore, but the computer still request the keyboard to boot, just like some computyer still requst the mouse to be present to boot.
You need to seek the IBM manual for the keyboard.

the a20 is mentioned in the intel manuals -- the term a20 simply refers to the address line #20

the a20 gate on the other hand (as others before me said), is part of the PC not the CPU -- and the intel manuals wont mention that any more than they would mention the CD-RW drive -- its not made or supported by intel in any way

enabling a20 gate is not part of switching to PMode, its part of initializing the other parts of the computer

( but is probably emulated by the emulation bios to boot windows on the mac)

there is no reason to need it... the a20 gate i mean, but its probably present, simply because its part of the standard intel chipset which (iirc) apple uses

Normally the PC platform computer cant boot without the keyboard attached.

not true... all PCs can boot just fine without a keyboard (older computers will usually give an error and halt booting if it doesnt detect one, but this can easily be disabled in bios settings)

But some pc can, like some business servers, which, I suppose, have the keyboard logic into the motherboard.

by the time the a20 gate was invented all keyboard controllers were located on the motherboard (in the original PC and XT it was in the keyboard, but with the AT it was moved to the motherboard, which is why older keyboards often have a switch to switch between XT and AT modes)

But now all the keyboard logic are inthe chipset and not in the keyboard proper anymore, but the computer still request the keyboard to boot, just like some computyer still requst the mouse to be present to boot.

as i said, all computers can boot without keyboard (in fact by default in most systems BIOS wont even recognize USB keyboards)

You need to seek the IBM manual for the keyboard.

wont tell you much... because the a20 gate isnt in the keyboard...

the keyboard controller is a standard (or emulated these days) 8242 intel multi-purpose programmable controller -- basically a CPU designed for interfacing with components, the one in the PC is programmed to interface with the keyboard, but it is a very powerful chip, and the keyboard only uses a fraction of its function (iirc only 1 data pin is used... tied directly to the serial keyboard port) when you use port 60, you are talking to this controller, not the keyboard, this controller just happens to interface with the keyboard

when systems using the 286 were designed, certain things were needed for compatability, including a reset switch for re-entering RMode, and a way to disable the 21st address line... these were tied to the 8242 because it was already there, and wasnt being used

so the part thats actually controlling the a20 address line, is either the 8242 programmable controller chip, or the north-bridge controller (emulating an 8242 programmable controller chip)


for official documentation on this, rather than the IBM keyboard manual, what you need is the intel chipset documentation (freely availible for download from http://developer.intel.com

Just ignore enabling the A20 gate and only use even megabytes of memory. Simple