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Possible fix for the incorrect behavior of the pseudo LRU replacement policy for 4 ways set associative caches.

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Ross Thompson 2021-10-29 11:03:37 -05:00
parent 35fcadbe7f
commit 41dbb59e24
1 changed files with 28 additions and 1 deletions
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29
wally-pipelined/src/cache/cachereplacementpolicy.sv vendored
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@ -82,7 +82,31 @@ module cachereplacementpolicy
assign VictimWay[1] = ~BlockReplacementBits[0]; assign VictimWay[1] = ~BlockReplacementBits[0];
assign VictimWay[0] = BlockReplacementBits[0]; assign VictimWay[0] = BlockReplacementBits[0];
end else if (NUMWAYS == 4) begin : FourWay end else if (NUMWAYS == 4) begin : FourWay
// VictimWay is a function only of the current value of the LRU.
// binary encoding
//assign VictimWay[0] = BlockReplacementBits[2] ? BlockReplacementBits[1] : BlockReplacementBits[0];
//assign VictimWay[1] = BlockReplacementBits[2];
// 1 hot encoding
assign VictimWay[0] = ~BlockReplacementBits[2] & ~BlockReplacementBits[0];
assign VictimWay[1] = ~BlockReplacementBits[2] & BlockReplacementBits[0];
assign VictimWay[2] = BlockReplacementBits[2] & ~BlockReplacementBits[1];
assign VictimWay[3] = BlockReplacementBits[2] & BlockReplacementBits[1];
// New LRU bits which are updated is function only of the WayHit.
// However the not updated bits come from the old LRU.
assign LRUEn[2] = |WayHit;
assign LRUEn[1] = WayHit[3] | WayHit[2];
assign LRUEn[0] = WayHit[1] | WayHit[0];
assign LRUMask[2] = WayHit[1] | WayHit[0];
assign LRUMask[1] = WayHit[2];
assign LRUMask[0] = WayHit[0];
/* -----\/----- EXCLUDED -----\/-----
// selects // selects
assign LRUEn[2] = 1'b1; assign LRUEn[2] = 1'b1;
@ -93,16 +117,19 @@ module cachereplacementpolicy
assign LRUMask[0] = WayHit[1]; assign LRUMask[0] = WayHit[1];
assign LRUMask[1] = WayHit[3]; assign LRUMask[1] = WayHit[3];
assign LRUMask[2] = WayHit[3] | WayHit[2]; assign LRUMask[2] = WayHit[3] | WayHit[2];
-----/\----- EXCLUDED -----/\----- */
for(index = 0; index < NUMWAYS-1; index++) for(index = 0; index < NUMWAYS-1; index++)
assign NewReplacement[index] = LRUEn[index] ? LRUMask[index] : BlockReplacementBits[index]; assign NewReplacement[index] = LRUEn[index] ? LRUMask[index] : BlockReplacementBits[index];
/* -----\/----- EXCLUDED -----\/-----
assign EncVicWay[1] = BlockReplacementBits[2]; assign EncVicWay[1] = BlockReplacementBits[2];
assign EncVicWay[0] = BlockReplacementBits[2] ? BlockReplacementBits[0] : BlockReplacementBits[1]; assign EncVicWay[0] = BlockReplacementBits[2] ? BlockReplacementBits[0] : BlockReplacementBits[1];
onehotdecoder #(2) onehotdecoder #(2)
waydec(.bin(EncVicWay), waydec(.bin(EncVicWay),
.decoded({VictimWay[0], VictimWay[1], VictimWay[2], VictimWay[3]})); .decoded({VictimWay[0], VictimWay[1], VictimWay[2], VictimWay[3]}));
-----/\----- EXCLUDED -----/\----- */
end else if (NUMWAYS == 8) begin : EightWay end else if (NUMWAYS == 8) begin : EightWay