Help porting newlib to a new CPU architecture (sorta)

Joel Sherrill
Tue Jul 6 22:00:20 GMT 2021

On Tue, Jul 6, 2021, 4:10 PM ElijaxApps <> 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

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

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:
> 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 (
> )
> 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.

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