PA-RISC is a microprocessor architecture developed by Hewlett-Packard's Systems & VLSI Technology Operation. As the name implies, it is an implementation using a RISC design, where the PA stands for Precision Architecture. The design is also referred to as HP/PA for Hewlett Packard Precision Architecture.
In the late 1980s HP was building two series of computers, both on CISC CPUs. One was the HP-9000 Unix workstations, designs using the Motorola 68000 family they had aquired when they purchased Apollo. The other was the HP-3000 series minicomputers, based on an HP designed 16-bit CPU. HP was looking to use the PA-RISC to move all of their machines to a single RISC CPU family.
The first PA-RISC 7000 family were 32-bit designs, and were first used in a new series of HP-3000 machines in the late 1980's, the 930 and 950. They were commonly known at the time as Spectrum systems, the name given to them in the development labs. The HP-9000 machines were soon upgraded with the PA-RISC as well, running the HP/UX version of UNIX (although other operating systems, such as Linux and MPE/iX, the HP-3000's OS, can be used).
An interesting aspect of PA-RISC is that HP has typically designed each generation without any Level 2 cache. Instead large Level 1 caches have been used, as separate chips connected by a bus. Another innovation on the PA-RISC was the addition of multimedia instructions (SIMD) in the form of MAX which were first introduced on the 7100.
The design was upgraded in 1996 to a full 64-bit architechture, referred to as the 8000 series. They also added no less than ten functional units and an agressive pipelining system. Another change was the splitting of the instruction cache, with separate caches for instructions that take long or short time to complete.
With the PA-8500 design, HP produced the first commercial microprocessor with a large on chip cache. The 8500 has 1.5 MB of Level 1 cache on chip. Consequently, it was a great performer for its time. The 8600 is essentially a higher clocked 8500 with a few architechtural improvements. The 8700 is clocked higher than the 8600, to which it is similar, and has 2.25 MB of L1 cache.
The PA 8800, codenamed Mako, will feature 2 independant microprocessors on a single die. Thus each "chip" will be a 2-way SMP set. Each processor on the Mako has a large L1 cache, but HP is breaking with its tradition in a big way by including 32 MB of L2 cache using separate chips. The processor will be packaged in a "slot" format and is expected to have excellent performance. After the PA-8900, an improved Mako, HP is expected to retire the PA-RISC line in favor of Intel's Itanium.