What is A20?
Posted: Mon May 21, 2001 11:00 pm
I know it's a dumb newbie question, but that's what
I am. What _is_ the A20 (line)? Thanks for your
patience
I am. What _is_ the A20 (line)? Thanks for your
patience
>I know it's a dumb newbie question, but that's what
>I am. What _is_ the A20 (line)? Thanks for your
>patience
I'm going off the assumption that you understand binary
numbering here. Back in the olden days x86 cpu's could
only access the first meg of memory. When the cpu's where
modified to support more memory a problem was found. Some
old application relied on wrap around. If they access memory
right past the 1st meg then they'd get the memory at the
bottom of the 1st meg. So the 20th bit in a cpu's memory address
location was designed to be setable and is off by default.
I think it was IBM that desided to use the new keyboard controller
in AT style systems to control the cpu's behavior of the A20 line.