Help porting newlib to a new CPU architecture (sorta)

[Joel Sherrill]

On Tue, Jul 6, 2021, 4:10 PM ElijaxApps <elijaxapps@gmail.com> wrote:

> Hi, thank you a lot four your great responses, them really got me on some
> direction.
>
> >even DOS (iiuc) requires at least a 16-bit CPU.  not sure FreeDOS supports
> >16-bit though, or if it too requires 32-bit.
> >-mike
>
> Well, Libre 8 has:
> 4 GP 8 Bit registers for math operations. 16 Bit integers are not a big
> problem,  if:
>   - I could use 2 cycles for batching the operation,
>   - I could use registers 2 on 2 (and update ALU width).
> Or even I could rewrite the microcode of my CPU to make it work
> independently, as in
>   - Sum lower 8 bits, push to stack,
>   - Sum higher 8 bits, push to stack,
>   - On return (int) pop twice.
>
> In memory terms it has 24 bit address RAM, all of them addressable and
> directionable in 3 cycle jumps. No segmentation, no paragraphs, just plain
> 16MB of RAM in a single component. That's why I am confident about using a
> DOS-like OS.
>

I think you might get some ideas from the old m6809 port of gcc

http://vectrexc.malban.de/documentation/gcc-6809-documentation

That was an 8 bit CPU with only 64k memory space if I remember everything
correctly.

I think given an add with carry, gcc can be taught to use 8 bit operations
in sequence on larger types. I recall seeing this on the h8 or avr.


> >Consider the AVR toolchain, used in Arduino.  Isn't that an 8-bit
> machine?  It certainly is small.
> >paul
>
> Thanks for the tip! I have checked, I think this is the source code:
>
> https://github.com/esp8266/Arduino/tree/master/cores/esp8266
>
> Could you point out what files I need to rewrite? Thanks in advance.
> Correct me if that is not the code, and in case I finally decide this
> approach I'll move to its mailing list.
>

The gcc source would be in a directory like gcc/config/avr


> >The full toolchain includes libc, libbfd, binutils, gcc, which would
> >have to be bootstrapped by cross-compiling from a host system.
>
> >There is an AVR target which may support 8b and avr-libc which should
> >support the toolchain.
>
> >There is also an Arduino toolchain based off the AVR toolchain which
> >supports developing and loading sketches which run on pretty bare 8b MCUs.
>
> >You may be better off targeting the latter approach for simplicity.
> >A lot may depend on how close a match you can find between your target
> >architecture and some existing architecture.
>
> Yeah, I've been checking GCC repository and there is already an AVR target,
> that I could use as a base to make Libre 8 target. Another candidate is z80
> you mentioned, with the only caveat of having to implement 16b integer
> operations on my CPU firmware (not hard at all).
>
> I know I will have to cross-compile at least once libc for using it, then
> crosscompiling gcc itself, and it should be done then the compilation of
> the full toolchain in the Libre 8 CPU itself. But the latter part is for
> the moment out of scope, as I have very limited resources on the CPU and no
> disk nor filesystem, just plain RAM (I was thinking on defining programs
> and files as memory offsets, and made them reside in memory, in a future,
> or . . . designing a component in logisim suitable as a hard disk with its
> filesystem and such...)
>
> Or, compiling every monolithic program I want to run as STATIC, so it will
> include dependant libraries binary code on the executable.
>
> For this, I need the libc library to be embedded, and that is why I reached
> newlib.
>
> @Orlando: You're pointing the very right direction there, I'll be quiet on
> the quotes.
>
> - I got already an assembler, written in Java. Is more like a parser, it
> just transcribes a bunch of implemented opcodes into its binary format for
> Libre 8. But I want to use any preexisting toolchain, as gcc ones. In other
> case I would end rewriting the full toolchain. However, your indication
> about porting GAS is actually a thing I really considered before.
>
> So as far as I understand now, it is not only porting newlib, *I have to
> port also to port GAS (GNU Assembler) and GCC (GNU compiler)? Take in mind
> I just want to CROSS-COMPILE binaries.*
>
> - How its an ABI formally defined on code? How is defined AVR - ABI? Could
> you point some sources? I googled without finding much about ABI
> definitions - "as is".
>
> - Architecture is Von-Neumann (Instructions and Data ar mixed in a single
> memory compartiment). Decoder will find instructions and data in the same
> input.
>
> - I found this project (
> https://docs.platformio.org/en/latest/platforms/creating_platform.html )
> which allows you to define toolchain components and right, I would need
> only to change gas to libre 8 compiler. BUT, I would need some kind of
> workaround to make C compiler to output mine pseudo assy, which I no doubt
> it is possible but would be crappy. - OR, I just have to port GAS to Libre
> 8, and use "Standard ASSY".
>
> - Thanks a lot for suggesting other 8 bit DOS OS's.
>
>
>
> *So, what would be the first step? - Defining ABI (bring examples of
> existing 8 bit ABI's) for entire toolchain. LINK NEEDED - Porting GAS, and
> use a custom GAS-libre8 in platformio project. Could anyone point out out
> which files need to be edited exactly?*
>
> Thank you all.
> Elijax Apps.
>