+# 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
projects / libreriscv.git / commitdiff