Help porting newlib to a new CPU architecture (sorta)

[Brian Inglis]

On 2021-07-06 08:35, Orlando Arias wrote:
> Keep in mind that C assumes you are working on a von Neumann
> architecture. This means that the language expects both program and data
> to be in the same address space. If your CPU [which I can not really
> check since I do not work on Logisim, I use Verilog/SystemVerilog/VHDL
> myself] uses a Harvard or modified Harvard architecture, you need to
> figure out a way for your C runtime to emulate a von Neumann machine.
C has no assumptions or requirements for outdated von Neumann 
architectures, but it also may not assume it is not running on such 
architectures.
Early C and Unix took advantage of the Harvard architecture of larger 
PDP-11s which supported split I/D space with mapping registers to 
support double the program size (64KB each 16b I and D) as well as 
splitting kernel/supervisor/runtime/user spaces [and similar features on 
other systems], and shared text (the original meaning of the 't' "sticky 
text" perm on executables), which is why there is wording in the 
standards about unmodifiable constant values.

One of the big issues with systems around the time C was developed was 
the unreliability resulting from systems, runtimes, and programs using 
self-modifying code "tricks", and programs ability to change constant 
data values used in programs (PI did not always stay π).
That was one of the benefits touted for higher level system 
implementation languages such as Bliss, C, PL/S.
In the face of failures frequently caused by arbitrary code and data 
modifications, branches and jumps, worries about trivial issues such as 
delimited strings, unsized APIs, dynamic memory management were decades 
in the future!

Various implementations, ABIs, and systems require modifiable code and 
constant data, but it is *NOT* a C language assumption or requirement; 
in fact I would consider it to be a C anti-pattern, that has long 
limited the abilities of compilers and optimizers to do better jobs.
Copying sections tends to be due to limitations of CPUs, systems, 
runtimes, or compilers to DTRT to make programs more reliable. ;^>

-- 
Take care. Thanks, Brian Inglis, Calgary, Alberta, Canada

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