From 871644d93ebddcaa87e2e22186882ac8bb48c440 Mon Sep 17 00:00:00 2001 From: addw Date: Sat, 28 Mar 2020 15:04:14 +0000 Subject: [PATCH] --- openpower/isans_letter.mdwn | 50 +++++++++++++++++++++++++++++++++++++ 1 file changed, 50 insertions(+) diff --git a/openpower/isans_letter.mdwn b/openpower/isans_letter.mdwn index 22563a8e0..52a59f535 100644 --- a/openpower/isans_letter.mdwn +++ b/openpower/isans_letter.mdwn @@ -1,3 +1,53 @@ +# Letter regarding ISAMUX / NS + +This is a quick overview of the changes that we are proposing to the PowerPC +instruction set. + +## Overview + +The PowerPC Instruction Set Architecture (ISA) is an abstract model of a +computer. This is what programmers use when they write programs for the machine, +even if indirectly via a compiler for a high level language. We must be +conservative in how we add to the ISA to: + +* not break existing programs + +* be mindful as to how others may wish to add to the ISA in the future + +This document describes our strategy. + + +## ISA modes and escape sequences + +New chips usually need to be able to run older (legacy) software that is +incompatible with the latest and greatest ISA. Eg: 64 bit chip must be able to +run older 16 bit and 32 bit software. + +To enable backwards compatability the CPU will be set into 'legacy' mode. This +is done with an ISA Mode switch, also known as ISA Muxing or ISA Namespaces. + +The operating system is able to quickly change between 'modern' ISA mode and +various legacy modes. + +Another technique is an ISA escape-sequence. This is a type of mode that is +only operational for a short time, unlike 32 or 64 bit which would be for the +entire run of a program. + + +## What are we adding to the ISA + +When high quality graphical display were developed the CPUs at the time were +shown to not be able to run the display fast enough. The solution was the use of +Graphics cards, these are specialised computers that are good at rendering +pixels; often by doing the same thing in different parts of the screen at the +same time (in parallel). These specialised computers are called Graphical +Processing Units (GPUs). + +The parallelism of some GPUs is thousands. This has led to GPUs being used to +solve non graphical problems where high parallelism is useful. + +**break** + # Letter regarding ISAMUX / NS Hardware-level dynamic ISA Muxing (also known as ISA Namespaces and ISA -- 2.30.2