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From: mitchalsup@aol.com (MitchAlsup1)
Newsgroups: comp.arch
Subject: Re: address architecture, not interactive use, The Design of Design
Date: Sun, 12 May 2024 23:02:21 +0000
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Thomas Koenig wrote:

> MitchAlsup1 <mitchalsup@aol.com> schrieb:
>> Thomas Koenig wrote:
>>
>>> John Levine <johnl@taugh.com> schrieb:
>>
>>>> Brooks said it was ugly that some instructions (RX) had both base and
>>>> index registers while others (SS) only had base registers, which I
>>>> expect made it even harder to do what you suggested.
>>
>>> Depending on base registers for both data and branches was one
>>> of the ideas that did not age well, I think.  We have since
>>> seen in the RISC machines that having a stack implemented via
>>> a register, with possibly a frame pointer, a global offset and
>>> larger offsets (16 bits) works well, and we know how to generate
>>> position-independent code.
>>
>> Position independent data is still difficult, though.

> Touché.  Data is not the problem, but pointers to date (such as
> addresses of arguments) is...

Position independent external data requires a load of the base of the
region the data sits, and a second memory access to do something with
the data. You CAN use this base register multiple times--IFF you can
figure out matching regions external data resides and share use of the
base register.

Modern systems put these bases in GOT.

This is a hangover from the days where one built fully resolved object
modules. ASLR is a driving force for a "better solution" {whatever it
ends up being.}

> So, having a base register added to all addresses of user code
> would definitely have been a better choice.