From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dave Eckhardt <davide+p9@cs.cmu.edu>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
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Content-ID: <19495.1452037046.1@LUNACY.UGRAD.CS.CMU.EDU>
Date: Tue,  5 Jan 2016 18:37:26 -0500
Message-ID: <19496.1452037046@LUNACY.UGRAD.CS.CMU.EDU>
Subject: Re: [9fans] subtracting pointers on amd64 6c
Topicbox-Message-UUID: 7eacd3c4-ead9-11e9-9d60-3106f5b1d025

> for instance, although the range of subtraction is theoretically
> -2^64+1 to 2^64-1, amd64 can only address 48 bits of memory
> (currently) despite using 64 bits to represent addresses.  as long
> as virtual addresses in the system aren't exabytes apart, this
> shouldn't result in undefined behavior in practice.

Unfortunately AMD64 VM is a hack.  The 2^48 addressable bytes aren't
contiguous!  2^47 of them go up from 0 and the other 2^47 of them go
down from (64-bit) -1.

See: https://en.wikipedia.org/wiki/X86-64#Canonical_form_addresses

Dave Eckhardt