On Wed, 7 Jul 1999, Greg Haerr wrote:
> On Wednesday, July 07, 1999 3:43 PM, Blaz Antonic [SMTP:[EMAIL PROTECTED]] wrote:
> : > i had an idea a few weeks ago to toggle the a20 gate on a 286, and use the
> : > memory from ffff:0000 to ffff:ffff for a ramdisk. this way we could use
> : > 64k-1byte of memory, which would otherwise be wasted.
> :
> : 8086 doesn't have A20 line ?! It might actually be more useful to find
> : out what would be required to switch processor into protected mode
> : (286). This way we'd have 16 MB of addressable memory. If my
> : understanding is correct bcc wouldn't need any modifications in order to
> : produce pmode binaries, segment limits are still there, task switching
> : mechanism would still work (because of same CS and DS limitations).
>
> The pmode ideas are good, but I think you missed the point.
> A normal, old-fashioned 8086 can be tricked into getting 65535 more bytes
> of memory than the normal 1 megabyte. This is accomplished
> by programming the keyboard controller on PC's to hold the A20 line
> high and then use the 8086's real mode address wrap "feature".
I thought that address mode wrap only worked on the 80286; by default I
believe that it emulates an 8086 and reflects requests for ffff:0000 through
ffff:ffff to 0000:0000 through 0000:ffff. The whole deal is that this
behaviour on a 286 can be switched off.
Besides, usually the 8086 only has 640k of RAM and nothing there at
ffff:0000 through ffff:ffff.
pat
