Power ISA

Power ISA is a reduced instruction set computer (RISC) instruction set architecture (ISA) currently developed by the OpenPOWER Foundation, led by IBM. It was originally developed by IBM and the 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.

Prior to version 3.0, the ISA is divided into several categories. 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.

Power ISA is a RISC load/store architecture. It has multiple sets of registers:

• 32 × 32-bit or 64-bit general-purpose registers (GPRs) for integer operations.
• 64 × 128-bit vector scalar registers (VSRs) for vector operations and floating-point operations.
  • 32 × 64-bit floating-point registers (FPRs) as part of the VSRs for floating-point operations.
  • 32 × 128-bit vector registers (VRs) as part of the VSRs for vector operations.
• 8 × 4-bit condition register fields (CRs) for comparison and control flow.
• 11 special registers of various sizes: Counter Register (CTR), link register (LR), time base (TBU, TBL), alternate time base (ATBU, ATBL), accumulator (ACC), status registers (XER, FPSCR, VSCR, SPEFSCR).

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 single instruction, multiple data (SIMD) operations on integer and floating-point data on up to 16 elements in one instruction.

Power ISA has support for Harvard cache, i.e. split data and instruction caches, and 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, and support for both 32-bit and 64-bit addressing.

Different modes of operation include user, supervisor and hypervisor.