Re: [f-cpu] new cjump instruction

[nico <nico@seul.org>]

On Sat, 12 Apr 2003 02:54:07 +0200
Yann Guidon <whygee@f-cpu.org> wrote:

> hi all,
> 
> nico wrote:
> 
> >On Fri, 11 Apr 2003 11:16:04 -0600
> >ben franchuk <bfranchuk@jetnet.ab.ca> wrote:
> >
> >>nico wrote:
> >>    
> >>
> >>>I propose a new cjump to enable 0 cost jump.
> >>>
> >>>cjump (R1,#imm12, cond)
> >>>{
> >>>if (cond(R1))
> >>>{
> >>> PC = (PC & 0x0FFF) | #imm12; 
> >>>}
> >>>else
> >>>{
> >>> PC++;
> >>>}
> >>>
> >>>}
> >>>
> >>>PC lsb is just replaced by the 12 bit constant, that's lightning
> >>>fast. No cycle could be lost by reading register bank.
> >>>
> >>>What do you think of it ?
> >>>      
> >>>
> >>Try using it with real code and you will see the problem.
> >>hint: page boundry is random.:(
> >>    
> >>
> >
> >?? 
> >That's a compiler problem. So ?
> >  
> >
> huh, i don't think that it's a good answer ....
> 
> on top of that, this technique poses new problems in FC0's pipeline.
> sure, addresses are computed fast, but what about their validation,
> their fetch, their lookup in the buffers ......

Validation are usefull because you are inside a pages.

fetch are ligthening fast. You have an entire clock cycle to do it (no
adder or register read before accessing L1 caches).

But the real problem is for the compiler. What is the opinion of the
compiler writter ?

nicO

> 
> >>Ben.
> >>    
> >>
> YG
> 
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