From: Luke Kenneth Casson Leighton Date: Tue, 4 Dec 2018 18:51:56 +0000 (+0000) Subject: reword X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=472a44f454edb8b8a4a67065175273c260c2bc2d;p=crowdsupply.git reword --- diff --git a/updates/003_2018dec04_microarchitecture.mdwn b/updates/003_2018dec04_microarchitecture.mdwn index 64487a7..88093b9 100644 --- a/updates/003_2018dec04_microarchitecture.mdwn +++ b/updates/003_2018dec04_microarchitecture.mdwn @@ -106,6 +106,19 @@ to Reorder Buffers: * There's no clear way to handle branch prediction, where the Reorder Buffer of Tomasulo handles it really cleanly. +However there are downsides to Reorder Buffers: + +* The Common Data Bus may become a serious bottleneck, as it delivers + data from multiple ALUs which may be generating results simultaneously. + To keep up with result generation, *multiple* CDBs may be needed, which + results in each receiver having multiple ports +* The Destination field in the ROB has to act as a key in a CAM (Content + Addressble Memory). As a result, power consumption of the ROB may be + quite high. It may or may not be possible to reduce power consumption + by testing an "active" bitfield (separate from but augmenting the ROB) + to indicate whether Destination Registers are in use. If inactive, + the CAM lookup need not take place. + Whilst nothing's firmly set in stone, here, as we have a Charter that requires unanimous decision-making from contributors, so far it's leaning towards Reorder Buffers and Tomasulo as a good, clean fit. In part that