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Re: [f-cpu] use of 1r1w regfile for our 3r2w regfile



hello,

Anup Gangwar wrote:

Hi,

I have been reading posts on this list for long but have never
posted any message myself. In my opinion going for simpler (less R+W
ports) RF's will be advantageous from two viewpoints:

1. If we stick to at most 2R+2W ports then the CPU could easily work with FPGA technology a big+ while prototyping.

this creates a problem : F-CPU is not meant to use *only* FPGA.
so if we develop with FPGA in mind (call this a "least common feature problem")
the design will not exploit all the potential of ASICs.

and after all : even if a 3R2W RF is 3x slower than 1R1W, you're still overlooking something :
it will *work* ! (look at Don's post).

the architecture of the RF can be locally modified to maybe allow more cycles
or better behaviour. If we make it work now, with a full-featured set of features
(and no compiler constraints) then people will use it happily, even if it is 1 million times
slower than the latest Intel/AMD horse. Not only will they use it but they will get used
to the fact that there is few coding constraints.

2. The larger the RF the slower it will be.

The RF has 63 registers that are *at least* 64-bit wide.
so it's already slow from the start, right ?

Everyone is breaking up the RF into smaller chunks (clusters of FUs).

can you point us to references about "everyone" ?

As per one heuristic estimate the power, delay and max freq grow

as atleast N^1.5 where N is the no. of FUs per RF.

6. The compiler complexity will not grow too much

there is a problem here with the "too much" :
it's not quantifiable if it is not programmed.
and as you have probably remarked, few people here
are eager to write code.

if each of the individual clusters is connected to the other in a homogeneous fashion. Though a variety of interconnect strategies are possible.

here you propose an architecture where the core is split into "clusters"
with each a small register set and some "Execution Units".
right ? (if yes, then it has already been discussed ....)

Regards,

read you soon,

Anup

YG

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