Path: news.eternal-september.org!eternal-september.org!news.eternal-september.org!.POSTED!not-for-mail
From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.arch
Subject: Re: Keeping other stuff with addresses
Date: Mon, 2 Dec 2024 16:54:15 -0800
Organization: A noiseless patient Spider
Lines: 34
Message-ID: <vilkrn$3k21l$15@dont-email.me>
References: <memo.20241128153105.12904U@jgd.cix.co.uk>
 <20241128185548.000031c9@yahoo.com> <vidtpt$pon$1@gal.iecc.com>
 <2024Nov30.072829@mips.complang.tuwien.ac.at> <viir36$2qq41$9@dont-email.me>
 <f837d724b113e20e38aa71dd14cdf500@www.novabbs.org>
 <vijc3q$33feh$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Date: Tue, 03 Dec 2024 01:54:15 +0100 (CET)
Injection-Info: dont-email.me; posting-host="f777e730b2e8168ebcdb2932c05660c1";
	logging-data="3803189"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/MSG0FjTriLcePFY0tO2OF9WncPHnOIeE="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:SW0pNWGqpg2nlvO/aQYf/wmOxZw=
In-Reply-To: <vijc3q$33feh$1@dont-email.me>
Content-Language: en-US

On 12/1/2024 8:12 PM, Chris M. Thomasson wrote:
> On 12/1/2024 5:26 PM, MitchAlsup1 wrote:
>> On Sun, 1 Dec 2024 23:22:14 +0000, Chris M. Thomasson wrote:
>>
>>> On 11/29/2024 10:28 PM, Anton Ertl wrote:
>>>> John Levine <johnl@taugh.com> writes:
>>>>> S/360 had 24 bit addresses and 32 bit registers. When doing address
>>>>> arithmetic
>>>>> the high 8 bits of the register were ignored. That turned out to be a
>>>>> really bad
>>>>> decision since a few instructions and a lot of programming conventions
>>>>> stored
>>>>> other stuff in that high byte, causing severe pain a few years later
>>>>> when
>>>>> memories got bigger than 16 meg.
>>>>
>>>> The technique of putting stuff in unused bits of an address has its
>>>> drawbacks, but it also has benefits, in particular type information is
>>>> often stored there (even on architectures that do not ignore any
>>>> bits).  Of course AMD and Intel have the bad examples of S/360 and
>>>> 68000 in mind, and did not want to have anything to do with that
>>>> during the first two decades of AMD64.
>>>
>>> Fwiw, stealing bits from pointers is a common practice in exotic
>>> lock-free algorithms. It's not portable at all, but comes in handy!
>>
>> How are you going to write these algorithms when a pointer consumes
>> all 64-bits of the register/memory container ?? VAS = 64-bits.
> 
> You don't for they are not portable at all. However, on systems were we 
> can steal some bits of a pointer value in C/C++, well, we can do it and 
> create some interesting algorithms.

another way to steal bits is over alignment.