Re: [Tinycc-devel] Very sorry - newbie questions

[Michael Matz]

Hi,

On Sun, 12 Mar 2017, Tony Robinson wrote:

> Very sorry for newbie questions but I'm looking for C compiler for a 
> processor that I've been very slowly building as a hobby.  The 
> instruction set is super simple (Rz = Rx op Ry for op in ADD, SUB, AND, 
> OR, XOR, ASR plus basic LOAD/STORE/PUSH/POP instructions) and as the 
> clock rate is only about 50kHz I'm not too worried about optimization. 
> Portability is key - I will port FORTH but would love C.

Writing a TCC backend is relatively straight forward if you don't need to 
care for compatibility with existing target practice (like e.g. calling 
conventions).  The few registers you have should be enough.  The 
limitations in the instruction set (e.g. no multiplication, no float) can 
be ignored at first (you'd have to write some routines to do that for you 
and call these instead of the missing insns).

But that's only the code generator itself, there might be some 
difficulties because you have a bare-metal target: no executable file 
format, no standard libraries.  And there might be hidden hardcoded 
assumptions about e.g. int being at least 32bit, or the width of 
relocations.

I would suggest to start with emitting some ELF files (that's the native 
exe format of TCC) and converting them after the fact to some binary blob 
containing only the data and instructions (you'll have to think about how 
to actually layout your executables and load them only your board anyway).

(Well, the actual start will of course be with the code generator :) )

> a) tcc is the simplest C compiler for me to retarget and

TCC probably has the simplest target interface with the advantage that you 
don't need to learn any domain specific new language to write it, just 
normal C code emitting the right bytes.  The compiler itself (also because 
it contains preprocessor and linker (and assembler) in one place and 
implements C fairly completely) is very small but not the simplest 
possible.

> b) assuming that it is, I did the standard:
>
> $ tar xvf tcc-0.9.26.tar.bz2
> $ cd tcc-0.9.26

That's already too old, don't even start with that.  We're currently in 
the process of working towards a new release, but it's simpler to just 
start with the mob branch from git:

% git clone git://repo.or.cz/tinycc.git

> P.S. more CPU info than you probably wanted to know at nanocpu.org

A daunting plan you have there :)  (but a very nice thing!)


Ciao,
Michael.