|Bits||32-bit/64-bit (32 → 64)|
|Extensions||AltiVec, APU, DSP, CBEA|
|Open||Yes, and royalty free|
The Power ISA is an instruction set architecture (ISA) currently developed by the OpenPOWER Foundation, led by IBM. It was originally developed by IBM and then now-defunct Power.org industry group. Power ISA is an evolution of the PowerPC ISA, created by the mergers of the core PowerPC ISA and the optional Book E for embedded applications. The merger of these two components in 2006 was led by Power.org founders IBM and Freescale Semiconductor.
The ISA is divided into several categories which are described in a certain Book. Processors implement a set of these categories as required for their task. Different classes of processors are required to implement certain categories, for example a server class processor includes the categories Base, Server, Floating-Point, 64-Bit, etc. All processors implement the Base category.
Instructions up to version 3.0 have a length of 32 bits, with the exception of the VLE (variable-length encoding) subset that provides for higher code density for low-end embedded applications, and version 3.1 which introduced prefixing to create 64-bit instructions. Most instructions are triadic, i.e. have two source operands and one destination. Single and double precision IEEE-754 compliant floating point operations are supported, including additional fused multiply–add (FMA) and decimal floating-point instructions. There are provisions for SIMD operations on integer and floating point data on up to 16 elements in a single instruction.
Power ISA has support for Harvard cache, i.e. split data and instruction caches, as well as support for unified caches. Memory operations are strictly load/store, but allow for out-of-order execution. There is also support for both big and little-endian addressing with separate categories for moded and per-page endianness, as well as support for both 32-bit and 64-bit addressing.
Different modes of operation include user, supervisor and hypervisor.
The Power ISA specification is divided into five parts, called "books":
New in version 3 of the Power ISA is that you don't have to implement the entire specification to be compliant. The sprawl of instructions and technologies has made the complete specification unwieldy, so the OpenPOWER Foundation have decided to enabled tiered compliancy.
These levels include optional as well as mandatory requirements, however one common misunderstanding is that there is nothing stopping an implementation from being compliant at a lower level but having additional selected functionality from higher levels as well as custom extensions. It is however recommended that an option be provided to disable any additional functionality beyond the design's declared subset level.
A design must be compliant at its declared subset level to make use of the Foundation's protection regarding use of intellectual property, be it patents and trademarks. This is explained in the OpenPOWER EULA.
A compliant design must…
If the extension is general purpose enough, the OpenPOWER Foundation asks that implementors submit it as a Request for Comments (RFC) to the OpenPOWER ISA Workgroup. Note that it is not strictly necessary to join the OpenPOWER Foundation in order to submit RFCs.
The EABI specifications predate the announcement and creation of the Compliancy subsets.
Regarding the Linux Compliancy subset having VSX (SIMD) optional: in 2003-4, 64-bit EABI v1.9 made SIMD optional, but in July 2015, in order to improve performance for IBM POWER9 systems, SIMD was made mandatory in EABI v2.0. This discrepancy between SIMD being optional in the Linux Compliancy level but mandatory in EABI v2.0 cannot be rectified without considerable effort: backwards incompatibility for Linux distributions is not a viable option. At present this leaves new OpenPOWER implementors wishing to run standard Linux distributions having to implement a massive 962 instructions. By contrast, RISC-V RV64GC, the minimum to run Linux, requires only 165.
The specification for Power ISA v.2.03 is based on the former PowerPC ISA v.2.02 in POWER5+ and the Book E extension of the PowerPC specification. The Book I included five new chapters regarding auxiliary processing units like DSPs and the AltiVec extension.
The specification for Power ISA v.2.04 was finalized in June 2007. It is based on Power ISA v.2.03 and includes changes primarily to the Book III-S part regarding virtualization, hypervisor functionality, logical partitioning and virtual page handling.
The specification for Power ISA v.2.05 was released in December 2007. It is based on Power ISA v.2.04 and includes changes primarily to Book I and Book III-S, including significant enhancements such as decimal arithmetic (Category: Decimal Floating-Point in Book I) and server hypervisor improvements.
The specification for Power ISA v.2.06 was released in February 2009, and revised in July 2010. It is based on Power ISA v.2.05 and includes extensions for the POWER7 processor and e500-mc core. One significant new feature is vector-scalar floating-point instructions (VSX). Book III-E also includes significant enhancement for the embedded specification regarding hypervisor and virtualisation on single and multi core implementations.
The specification for Power ISA v.2.07 was released in May 2013. It is based on Power ISA v.2.06 and includes major enhancements to logical partition functionality, transactional memory, expanded performance monitoring, new storage control features, additions to the VMX and VSX vector facilities (VSX-2), along with AES: 257  and Galois Counter Mode (GCM), SHA-224, SHA-256,: 258 SHA-384 and SHA-512: 258 (SHA-2) cryptographic extensions and cyclic redundancy check (CRC) algorithms.
The spec was revised in April 2015 to the Power ISA v.2.07 B spec.
The specification for Power ISA v.3.0 was released in November 2015. It is the first to come out after the founding of the OpenPOWER Foundation and includes enhancements for a broad spectrum of workloads and removes the server and embedded categories while retaining backwards compatibility and adds support for VSX-3 instructions. New functions include 128-bit quad-precision floating-point operations, a random number generator, hardware-assisted garbage collection and hardware-enforced trusted computing.
The spec was revised in March 2017 to the Power ISA v.3.0 B spec. and revised again to v3.0C on May 2020. The key difference between v3.0B and v3.0C is that the Compliancy Levels listed in v3.1 were also added to v3.0C.
The specification for Power ISA v.3.1 was released in May 2020. Mainly giving support for new functionality introduced in Power10, but also includes the notion of optionality to the PowerISA specification. Instructions can now be eight bytes long, "prefixed instructions", compared to the usual four byte "word instructions". A lot of new functionality to SIMD and VSX instructions are also added.
One key benefit of the new 64 bit prefixed instructions is the extension of immediates in branches to 34 bit.