Re: Problems installing Debian on a 486
On Wed, 24 Mar 1999, G. Crimp wrote:
> The A20 line has something to do with working around a bug in memory
> addressing that first showed up in 286's I think. I don't really know much
> about it, but on my 486, their is an option in the BIOS to set the line.
> You might try changing this option in the Bios at boot and see if it helps.
The original 8088 had 20 address lines called A0 through A19. This
allowed it to directly access 1024k of memory. (It was split 640k/384k as
RAM/ROM. It could have been worse, my understanding is that the original
plan was to split it half and half. Oh, and the 8086 had 19 address lines
called A1 through A19. I digress.) Anyway, on the 8086's and 8088's you
could access the bottom 65520 bytes of RAM by setting the segment register
to the very top of RAM and using an offset larger than the amount of RAM
above the start of the segment register.
Anyway, along comes the 80286. It has, in effect, 24 address lines (A0
through A23) for a total allowed memory of 16,384k. That broke those
programs that relied upon the memory wrapping around. Since they all ran
under DOS and since DOS was limited to 1024k, PC manufacturers put a
control in which would not pass the A20 line through to the RAM, which had
the effect of simulating the behavior of the 8086/8. That's what the A20
gate is about. You could run some DOS programs with the A20 gate disabled
that you couldn't run with it enabled.
You, of course, want to run a protected-mode operating system, where
relying on tricks like that simply cannot work. Since many people also
wanted to run protected mode software, (to do things like "loadhi" and
with extended memory and suchlike) the mboard manufacturers made that gate
configurable.
That's the whole story, to the best of my knowledge.
--
Jonathan Guthrie (jguthrie@brokersys.com)
Brokersys +281-895-8101 http://www.brokersys.com/
12703 Veterans Memorial #106, Houston, TX 77014, USA
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