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From: bart <bc@freeuk.com>
Newsgroups: comp.lang.c
Subject: Re: Baby X is bor nagain
Date: Fri, 28 Jun 2024 11:41:58 +0100
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On 28/06/2024 04:30, Kaz Kylheku wrote:
> On 2024-06-27, bart <bc@freeuk.com> wrote:
>> For most, PIC isn't a necessity.
> 
> Only because they use a virtual memory operating system which allows
> every executable to be mapped to the same fixed address in its own
> address space.

PIC never seemed to be a requirement during the 1980s and half the 90s. 
But then OSes only ran one program a time.

And when virtual addressing came along, and multiple programs could 
co-exist at the same address, PIC wasn't really needed either.

> 
> If you designed your personal OS, which would be the case?
> 
> [ ] Programs must be PIC
> 
> [ ] Programs needn't be PIC

With or without virtual addressing?

On x64, there are several aspects to this:

* Code that runs within the low 2GB (4GB is troublesome)

* Code that runs above 2GB, so that 32-bit fields (often signed so that 
only 31 bits are usable) that refer to absolute addresses of code or 
data will need 33 bits or more

* Dynamically linked /shared/ libraries, which need to have 
base-relocation tables (since you can't have multiple libraries all at 
the same address within the host process's virtual space), but which are 
not necessarily relocated above 2GB

* True PIC that requires special compiler support (to avoid generating 
address modes that involve registers plus absolute 32-bit address 
offsets). RIP-relative address modes, when there are no registers, can 
be done with instruction encoding outside of the compiler

It's all rather messy.

I can generate true PIC if necessary (for OBJ files that might be linked 
with ld, or DLLs that /might/ be relocated above 2GB).

But usually I generate code to load within the first 2GB.