|Introduced||1986 (1996 PA-RISC 2.0)|
|Branching||Compare and branch|
|Extensions||Multimedia Acceleration eXtensions (MAX), MAX-2|
|Floating point||32 64-bit (16 64-bit in PA-RISC 1.0)|
PA-RISC is an instruction set architecture (ISA) developed by Hewlett-Packard. As the name implies, it is a reduced instruction set computer (RISC) architecture, where the PA stands for Precision Architecture. The design is also referred to as HP/PA for Hewlett Packard Precision Architecture.
PA-RISC has been succeeded by the Itanium (originally IA-64) ISA jointly developed by HP and Intel. HP stopped selling PA-RISC-based HP 9000 systems at the end of 2008 but will support servers running PA-RISC chips until 2013.
In the late 1980s HP was building four series of computers, all based on CISC CPUs. One line was the IBM PC compatible Intel i286 based Vectra Series started 1986. All others were non-Intel systems. One of them was the HP Series 300 of Motorola 68000-based workstations, another Series 200 line of technical workstations based on a custom silicon on sapphire (SOS) chip design, the SOS based 16-bit HP 3000 classic series and finally the HP 9000 Series 500 minicomputers, based on their own (16 and 32-bit) FOCUS microprocessor. HP planned to use PA-RISC to move all of their non-PC compatible machines to a single RISC CPU family.
Precision Architecture was introduced in 1986. It had thirty-two 32-bit integer registers and sixteen 64-bit floating-point registers. The number of floating-point registers was doubled in the 1.1 version to 32 once it became apparent that 16 were inadequate and restricted performance. The architects included Allen Baum, Hans Jeans, Michael J. Mahon, Ruby Bei-Loh Lee, Russel Kao, Steve Muchnick, Terrence C. Miller, David Fotland, and William S. Worley.
The first implementation was the TS1, a central processing unit built from discrete transistor-transistor logic (74F TTL) devices. Later implementations were multi-chip VLSI designs fabricated in NMOS processes (NS1 and NS2) and CMOS (CS1 and PCX). They were first used in a new series of HP 3000 machines in the late 1980s – the 930 and 950, commonly known at the time as Spectrum systems, the name given to them in the development labs. These machines ran MPE/iX. The HP 9000 machines were soon upgraded with the PA-RISC processor as well, running the HP-UX version of UNIX.
An interesting aspect of the PA-RISC line is that most of its generations have no Level 2 cache. Instead large Level 1 caches are used, formerly as separate chips connected by a bus, and now integrated on-chip. Only the PA-7100LC and PA-7300LC had L2 caches. Another innovation of the PA-RISC was the addition of vectorized instructions (SIMD) in the form of MAX, which were first introduced on the PA-7100LC.
The ISA was extended in 1996 to 64-bits, with this revision named PA-RISC 2.0. PA-RISC 2.0 also added fused multiply–add instructions, which help certain floating-point intensive algorithms, and the MAX-2 SIMD extension, which provides instructions for accelerating multimedia applications. The first PA-RISC 2.0 implementation was the PA-8000, which was introduced in January 1996.
CPU specifications 
|Model||Marketing name||Year||Frequency [MHz]||Memory Bus [MB/s]||Process [µm]||Transistors [millions]||Die size [mm²]||Power [W]||Dcache [kB]||Icache [kB]||L2 cache [MB]||ISA||Notes|
|Mako||PA-8800||2003||800–1000||6400||0.13||300||361||?||768/core||768/core||0 or 32||2.0|
See also 
- "One Year Ago". (26 February 1987). Computer Business Review.
- Hewlett-Packard Company (September 1987). Hewlett-Packard Journal 38 (9): p. 3.
- HP Completes Its PA-RISC Road Map With Final Processor Upgrade - PA-RISC Processor
- How long will HP continue to support HP 9000 systems?
- Smotherman, Mark (2 July 2009). Recent Processor Architects.
- Marston, A. et al. (1987). "A 32b CMOS single-chip RISC type processor". ISSCC Digest of Technical Papers. pp. 28–29.
- Yetter, J. et al. (1987). "A 15 MIPS 32b Microprocessor". ISSCC Digest of Technical Papers.
- Boschma, Brian D. et al. (1989). "A 30 MIPS VLSI CPU". ISSCC Digest of Technical Papers. pp. 82–83, 299
- "HP L1000 & L2000 (rp5400/rp5450) Servers", openpa.net
- LostCircuits Hewlett Packard PA8800 Risc Processor overview.
- HP's documentation - page down for PA-RISC, Architecture PDFs available.
- OpenPA.net Comprehensive PA-RISC chip and computer information.
- chipdb.org Images of different PA-RISC processors