Re: Design a microcontroller for gcc

[Sylvain Munaut <tnt@246tNt.com>]

Hans-Peter Nilsson wrote:
> On Wed, 15 Feb 2006, Sylvain Munaut wrote:
> 
>>	* 2 flags Carry & Zero for testing.
> 
> 
> I think most of your questions have been answered, so let me
> just add that if nothing else, the port will be much simplified
> if you make sure that only specific compare instructions set
> condition codes, i.e. not as a nice side-effect of move, add and
> sub - or at least make such condition-code side-effects
> optional.  It depends on too many undisclosed details like
> pipeline restrictions to say whether performance is generally
> better or worse, but I can tell for sure that the GCC port will
> be simpler with a specific set of condition-code setting insns.

Making it optionnal is not hard nor expensive in hardware, the problem
is that my opcodes need to be 18 bits and I won't have space to stuff
another option bit ...

What I was thinking for the moment was to have :
 - sign is always the msb of the last ALU output
 - add/sub to modify all flags
 - move/xor/and/not/or only affect zero (and sign)
 - shift operations always affect carry and zero
 - Have some specific instructions like compare and test, but theses
   would only operate on registers (and not on immediate)

What's so bad about have the flag as side-effects ?

Here it's a simple MCU, it doesn't have a very long pipeline and that
pipeline is 'almost' invisible to the end-user exception for memory
fetch and io/access ...


> BTW, it depends on the compare (and branch) instructions whether
> just two flags are sufficient.
g
Adding Sign and overflow is pretty easy. And the compare
instruction/logic path shouldn't be a problem either.

MIPS has no flag ??? how does branching work ?



Finally, about immediates, I'm thinking of having instruction like add
could have 4 different forms :
add rD, rA, rB
add rA, rA, imm
add rA, rA, imm<<8
add rA, rA, signextend(imm)

Is that kind of manipulation on the immediate well understood by gcc
internals ?

Or maybe just allow immediates in the mov but that seems like a big
penalty ...


Sylvain