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Date:   Mon, 31 May 2021 05:59:47 -0700
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Subject: RE: [PATCH] x86/Kconfig: decrease maximum of X86_RESERVE_LOW to 512K
To:     David Laight <David.Laight@ACULAB.COM>,
        Borislav Petkov <bp@alien8.de>, Mike Rapoport <rppt@kernel.org>
CC:     Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Mike Rapoport <rppt@linux.ibm.com>,
        "untaintableangel@hotmail.co.uk" <untaintableangel@hotmail.co.uk>,
        "x86@kernel.org" <x86@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From:   "H. Peter Anvin" <hpa@zytor.com>
Message-ID: <E49AE58D-3C42-410B-8D6B-0768A11BA858@zytor.com>
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Sorry=2E=2E=2E very few correct answers here=2E

The load address is 0x7c00=2E The MBR is relocated to 0x600 by DOS=2E

MS-DOS doesn't *have* a TPR base, you are thinking of CP/M-80=2E

The IBM PC memory map reserved 640-768K for video, but the first generatio=
n adapters (CGA and MDA) didn't use the bottom 96K/64K (respectively) so it=
 was possible to add a little more RAM=2E The ROM BIOS wouldn't enumerate i=
t, though, so you had to hack around that=2E EGA took that away, though, bu=
t it seems that virtually no machines including clones had taken advantage =
of it anyway=2E

As far as affording it: the first IBM PC 5150 I personally used had 512K R=
AM=2E Not cheap, but not unheard of either; this was still before the 64K D=
RAM market crashed in early 1985=2E


On May 31, 2021 2:32:22 AM PDT, David Laight <David=2ELaight@ACULAB=2ECOM>=
 wrote:
>From: H=2E Peter Anvin
>> Sent: 28 May 2021 03:13
>=2E=2E=2E=2E
>> BIOSes have been known to clobber more than 64K=2E They aren't supposed
>to
>> clobber any=2E
>
>They probably shouldn't need anything above the base of the DOS
>transient program area preserved=2E
>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=2E
>Both the MBR and PBR load at the same address - 0xc00=2E
>
>> 640K is the limit because that is the address of the EGA/VGA frame
>> buffer=2E In the words of Bill Gates "640K ought to be enough for
>anyone=2E"
>
>I thought the original memory map allocated 512K for memory
>and 512k for memory mapped I/O=2E
>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=2E
>
>	David
>
>-
>Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes,
>MK1 1PT, UK
>Registration No: 1397386 (Wales)

--=20
Sent from my Android device with K-9 Mail=2E Please excuse my brevity=2E