Re: [Tinycc-devel] Question about tcc (C67) assembler code.

[Kim Lux]

Comments below.

On Thu, 2005-12-15 at 18:54 -0500, Dave Dodge wrote:
> On Thu, Dec 15, 2005 at 02:35:54PM -0700, Kim Lux wrote:
> > What are the "<<23"s about ?
> [...]
> >     C67_g((0 << 29) |       //creg
> >           (0 << 28) |       //inv
> >           (C67_map_regn(c) << 23) | //dst
> >           (C67_map_regn(b) << 18) | //src2 (possible x path)
> >           (C67_map_regn(a) << 13) | //src1 
> >           (xpath << 12) |   //x cross path if opposite sides
> >           (0x7b << 5) |     //opcode
> >           (0x6 << 2) |      //opcode fixed
> >           (C67_map_regs(c) << 1) |  //side of dest
> >           (0 << 0));        //parallel
> 
> I don't know anything about C67, but this appears to be a simple
> bitfield packing function.  I assume C67_map_regn(c) returns a 5-bit
> value in the range [0,31], which is then shifted 23 bits so that it
> ends up in bits 24-28 of the resulting machine word.

I need to look at this closer before I ask more questions. 

> > If one is writing a new assembler, should it be done in the same way ?
> 
> That probably depends on the machine architecture.  Each system will
> have its own special issues and requirements.  C67 looks like a simple
> design with 32-bit instructions and not a lot of variation in the
> instruction formats.  Other architectures are not so simple, for
> example:

<snip>
We are writing for Freescale 9S12, 9S12X and Xgate.  Small instruction
sets, fairly simple, etc. 

xgate has about 150 instruction/addressing combos, for example. 

We already have assemblers for all of our targets.

I *really* like the fact that the tcc assembler will do C pre
processing, even if it is non standard. 


-- 
Kim Lux,  Diesel Research Inc.