Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!warwick!bsrdp
From: bsrdp@warwick.ac.uk (Hylton Boothroyd)
Newsgroups: comp.sys.ibm.pc
Subject: Re: System boot and INT 19
Keywords: system boot
Message-ID: <369@clover.warwick.ac.uk>
Date: 17 Jan 90 12:21:46 GMT
References: <361@clover.warwick.ac.uk>
Organization: Warwick Business School
Lines: 24

A sequel.

In article <361@clover.warwick.ac.uk> bsrdp@warwick.ac.uk (Hylton
Boothroyd) I wrote:
>   b) setting 0040:0072 to 1234hex and jumping to ffff:0000 is the
>      standard IBM PC way to achieve a warm boot, and I can reasonably
>      expect it to work,
and commented that it didn't work on my XT clone and looked like a
mistake in the BIOS ROM.

A few minutes ago I finally connected with the man who wrote the BIOS
back in 1985. The absence of a warm boot via ffff:0000 wasn't a mistake
- it was deliberate.  The aim of clone makers was to have XT
functionality with BIOS code that was manifestly different from IBM's
and therefore manifestly not in breach of copyright.  That required a
great deal of ingenuity, and ffff:0000 was left to give a cold boot.
There was a different point of entry, via the keyboard driver, for a
warm boot conditioned by 0040:0072.

Miracle. He had the BIOS printout still on his shelves. He quoted me the
jump address in my version of the ROM, and two minutes later I had a
warm reboot programme.

Hylton