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From: Staf Verhaegen <staf@fibraservi.eu>
To: libre-riscv-dev@lists.libre-riscv.org
Date: Wed, 25 Mar 2020 14:46:38 +0100
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Subject: Re: [libre-riscv-dev] cache SRAM organisation
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Luke Kenneth Casson Leighton schreef op wo 25-03-2020 om 12:33 [+0000]:
> On Wednesday, March 25, 2020, Staf Verhaegen <staf@fibraservi.eu> wrote:
> > A single port SRAM has one port where you can do a read or a write each=
clock cycle. The 2-port one has one read port and one write port so you can=
do a read and write each clock cycle. The dual port one now has two portsth=
at each can do a read or write each clock cycle. So you can do two reads,tw=
o write or a read+write each clock cycle.For each of them you can have a sy=
nchronous or an asynchronous version. Asynchronous RAM has a clock input an=
d the address and data inputs arelatched on that clock signal. It thus mean=
s that the FFs are integrated inthe SRAM, e.g. thus very close :) . The RAM=
 currently being developed in myNLNet project is a synchronous SRAM as this=
 is easier from timing point ofview because all the timing can be related t=
o the clock. A synchronous RAMactually functions as an addressable bunch of=
 FFs and the synthesis and P&Rtools know how to handle them.
>=20
> ok.  later, we will definitely need an aysynchronous version.

Keep the discussion on development of other types til after the prototype t=
ape-out in October. I know there is other development ongoing for dual port=
 RAM in the OpenRAM community so likely it is more clear what extra still h=
as to happen.
BTW, I think there also is possibility to have register files block with 2R=
1W RAM blocks.

> this because it turns out that asynchronous SRAM can act, when used in aR=
egister File, as if it was a (separate) Register Bypass / ForwardingPort.  =
with the Out-of-Order Engine being a huge cyclic feedback loopbetween ALUs =
and RegFile, clock delays are an impediment, and havingcompletely separate =
(extra) Regfile Bypass ports dramatically increases thenumber of wires and =
Multiplexers.

Could detail more on how the adress, data and output signals of this asynch=
ronous block would be used and switched between synchronous and asynchronou=
s functioning. To me it seems that it would just place of the multiplexers,=
 not the amount.

greets,
Staf.

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