RE: [PATCH] x86/Kconfig: decrease maximum of X86_RESERVE_LOW to 512K

[David Laight]

From: H. Peter Anvin
> Sent: 28 May 2021 03:13
....
> BIOSes have been known to clobber more than 64K. They aren't supposed to
> clobber any.

They probably shouldn't need anything above the base of the DOS
transient program area preserved.
Can't remember where that is though :-(

It is hard enough finding a safe memory area for the MBR
code to relocate itself to before loading the PBR.
Both the MBR and PBR load at the same address - 0xc00.

> 640K is the limit because that is the address of the EGA/VGA frame
> buffer. In the words of Bill Gates "640K ought to be enough for anyone."

I thought the original memory map allocated 512K for memory
and 512k for memory mapped I/O.
No one could afford more then 512K DRAM :-)

The 640K limit appears because nothing was actually mapped
as the bottom of the 'I/O area' so memory could expand up
that far.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)