From 2d64ca8465d9291ea78c2febbe0010f72898ae14 Mon Sep 17 00:00:00 2001 From: Luke Kenneth Casson Leighton Date: Thu, 10 Jan 2019 12:24:06 +0000 Subject: [PATCH] add exception section --- 3d_gpu/requirements_specification.mdwn | 38 ++++++++++++++++++++++++++ 1 file changed, 38 insertions(+) diff --git a/3d_gpu/requirements_specification.mdwn b/3d_gpu/requirements_specification.mdwn index 391ce11ac..ab0a5088f 100644 --- a/3d_gpu/requirements_specification.mdwn +++ b/3d_gpu/requirements_specification.mdwn @@ -109,6 +109,7 @@ cycle, such that the register file may effectively be used as an # Function Units +## Commit Phase (instruction order preservation) # 6600 Scoreboards @@ -164,5 +165,42 @@ read and write hazards - dependencies - between Function Units. ## Branch Speculation +Branch speculation is done by preventing instructions from becoming +"writeable" until the Branch Unit knows if it has resolved or not. +This is done with the addition of "Shadow" lines, as shown below: + +This image reproduced with kind permission, Copyright (C) Mitch Alsup [[!img shadow_issue_flipflops.png]] +Note that there are multiple "Shadow" signals, coming not just from Branch +Speculation but also from predication and exception shadows. + +On a "Failed" signal, the instruction is told to "Go Die". This is +passed to the Computation Unit as well. When all "Success" signals +are raised the instruction is permitted to enter "Writeable". + +## Exceptions + +Exceptions shall be handled by each instruction that *may* throw an +exception having and holding a "Shadow" wire over all dependent +Function Units, in exactly the same way as Branch Speculation. +Likewise, dependent instructions are prevented and prohibited from +entering the "Writeable" state. + +Dependent downstream instructions, if the exception is thrown, +shall have the "Failed" bit ASSERTED (by the Function Unit throwing +the exception) such that the down-stream dependent instruction is told +to "Go Die". + +If the point is reached at which the instruction knows that the +Exception cannot possibly occur, the "Success" signal is raised +instead, thus cancelling the "hold" over dependent downstream +instructions - again in exactly the same way as Branch Speculation +"Success". + +Exceptions may **only** be actually raised if they are at the front of +the instruction queue, i.e. if they are free of write hazards. +See section on "Function Unit Commit" phase, as the Function Units +have a "link bit" that preserves the instruction issue order, which +must also be respected. + -- 2.30.2