linux-kernel - Re: somebody dropped a (warning) bomb

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Message-ID: <Pine.LNX.4.61.0702091842450.31955@yvahk01.tjqt.qr>
Date:	Fri, 9 Feb 2007 18:45:54 +0100 (MET)
From:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
To:	"linux-os (Dick Johnson)" <linux-os@...logic.com>
cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	David Rientjes <rientjes@...gle.com>,
	Jeff Garzik <jeff@...zik.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: somebody dropped a (warning) bomb


On Feb 9 2007 08:16, linux-os (Dick Johnson) wrote:
>On Fri, 9 Feb 2007, Jan Engelhardt wrote:
>> On Feb 8 2007 16:42, Linus Torvalds wrote:
>>>
>> Further, giving again answer to the question whether they generate
>> signed or unsigned comparisons: Have you ever seen a computer which
>> addresses memory with negative numbers? Since the answer is most likely
>> no, signed comparisons would
>
>Yes, most all do. Indexed memory operands are signed displacements. See
>page 2-16, Intel486 Microprocessor Programmer's Reference Manual. This

I was referring to "absolute memory", not the offset magic that assembler
allows. After all, (reg+relativeOffset) will yield an absolute address.
What I was out at: for machines that have more than 2 GB of memory, you
don't call the address that is given by 0x80000000U actually "byte
-2147483648", but "byte 2147483648".

>gets hidden by higher-level languages such as 'C'. It is also non-obvious
>when using the AT&T (GNU) assembly language. However, the Intel documentation
>shows it clearly:
> 			MOV	AL,  [EBX+0xFFFFFFFF]	; -1

It can be debated what exactly this is... negative offset or "using
overflow to get where we want".



Jan
-- 
ft: http://freshmeat.net/p/chaostables/
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