OpenPA.net
PA-RISC information - since 1999

PA-RISC Architecture

This is partially old content, from early 2002/2003.

Precision Architecture RISC

PA-RISC is Hewlett Packard’s Reduced Instruction Set Computing (RISC) architecture from the 1980s and an offspring from active HP research and development undertakings from that time. The aim of the Precision Architecture was to replace 16-bit stack-based CPUs in HP 3000 servers and Motorola 680x0 CPUs in HP’s Unix systems with a common system architecture.

An earlier commercial design from HP from the early 1980s was the HP FOCUS architecture.

Overall PA-RISC was a rather conservative RISC design for that time:

Compared to other RISC architectures original PA-RISC was rather unspectacular — it had fewer features but remained always at competitive speeds, especially in Floating Point and multiprocessing. HP was the first to include multimedia extension in commercially available microprocessors, MAX-1 in the PA-7100LC and MAX-2 64-bit in the PA-8000, which allowed vector operations on two or four 16-bit subwords in 32-bit or 64-bit integer registers.

PA-RISC 1.0

The original PA-RISC 1.0 architecture was 32-bit and included a single instruction/data bus. PA-RISC later on moved to a Harvard-style architecture with seperate instruction and data buses.

PA-RISC 1.0 has thirty-two 32-bit integer general purpose registers (GR0-GR31), seven shadow registers (SR0-SR6) for fast-interrupts and thirty-two 64-bit Floating Point registers for the FPU, which also could be combined to 64×32-bit and 16×128-bit. The FPU is able to execute a Floating Point instruction simultaneously to the ALU.

The original addressing was 48-bit wide, it was later on expanded to 64-bit (with the introduction of the PA-8000 line).

PA-RISC 1.1

The PA-RISC architecture was extended to version 1.1 with the PA-7000 processor in 1991. The major change in PA-RISC 1.1 was the inclusion of a MMU (memory management unit), that enabled PA-RISC computers to use virtual memory. From the the second PA-RISC 1.1 processor, the PA-7100 onward all processors implement superscalar instruction execution — the ability to execute multiple instructions simultaneously.

The 32-bit PA-RISC 1.1 processors are up to two-way superscalar, later 64-bit processors up to four-way. Other significant developments in PA1.1 include the PA-7100LC and PA-7300LC processors (LC for low cost) , which integrated the memory and I/O controller onto the processor die, on the PA-7300LC additionally the cache controller and first-level cache.

PA-RISC 2.0

In 1996 the 64-bit redesign of PA-RISC was introduced with the PA-RISC 2.0 PA-8000 processor. The architectural changes were rather intrusive but stayed compatible with 32-bit PA-RISC 1.1. On a side note, the PA-RISC 2.0 and the PA-8000 were introduced before the last 32-bit PA-RISC processor — the PA-7300LC — shipped.

Main changes and features of PA-RISC 2.0 include:

The later PA-8x00 processors of the 2000s did not introduce significant changes to the architecture or logic, besides higher integration of large L1 caches in the PA-8600 and dual-core PA-8800 and PA-8900. The processors after the PA-8000 were mostly redesigns and extensions of that processor core.

Post-PA-RISC

From the mid-1990s on a parallel track to PA-RISC 2.0 development HP joined Intel in developing the VLIW Itanium architecture from its own R&D projects, called EPIC, which resulted in the Intel/HP IA64 architecture.

Since the early-2000s HP sold two lines of Unix computers and servers in parallel — PA-RISC 2.0 and Itanium. These competing designs were apparent in the Integrity servers — with the rp servers (PA-RISC) and rx servers (Itanium).

These post-PA-RISC designs were not the success many hoped and HP after the turn of the century switched to standard Intel x86 fare.

Pre-PA-RISC

The predecessor of PA-RISC in the early 1980s was the HP FOCUS architecture from the HP 9000 Series 500. FOCUS was a stack architecture, with 230 instructions both 32 bits and 16 bits wide, a segmented memory model, and no general purpose programmer-visible registers. There are thirty-nine 32-bit registers in the CPU hardware, thirty-one internal 32-bit general purpose registers, two 32-bit ALU registers, and others.

↑ up

Floating Point Unit (FPU)

The Floating Point Unit is an assist processor logically added to a system to improve the performance on floating-point operations. The processor can be on a seperate chip (e.g., PA-7000) or integrated onto the central CPU die (all PA-RISC CPUs upwards). The FPU executes special floating point instruction to perform arithmetic on its own set of independent registers (register file) and to move data between its own registers and the system’s lower memory hierarchy. The FPU execution stage is pipelined. All PA-RISC FPUs contain thirty-two 64-bit registers, which can also be used as sixty-four 32-bit registers and sixteen 128-bit registers.

↑ up

Transition Lookaside Buffer (TLB)

The Translation Lookaside Buffer is a hardware structure doing virtual-to-physical memory address translations. The TLB takes virtual page numbers and returns the corresponding physical page number. The PA-7000 is the last PA-RISC processor to use seperate I/D TLBs, all later PA 1.1 and 2.0 CPUs use a combined TLB structure.

Hitachi’s PA-RISC 1.1 derivates also used split TLBs:

Most interestingly, the older PA-RISC 1.0 processors (pre-PA-7000) have huge TLBs (even for today’s standards):

The TLB memory on these earlier CPUs was implemented mostly off-chip/off-die via separate memory (SRAM) chips.

Translation process

TLB miss handling implementations

↑ up

Block Transition Lookaside Buffer (BTLB)

Similar to the TLB, the BTLB provides virtual-to-physical address translations. The BTLB however maps large address ranges rather that single pages as the TLB. These large address ranges are block translations and therefore stored in the Block Translation Lookaside Buffer. These block translations are useful for virtual address ranges that do not get paged in or out.

BTLBs were only implemented on 32-bit PA-RISC processors (PA-7x00), 64-bit PA-RISC instead implemented variable page sizes, thus any entry can be of >4k mapping.

↑ up

Superscalar execution

Overview

A superscalar processor implementation decodes, dispatches and executes multiple instructions per cycle if dependencies between the instructions permit. This is possible if the instruction stream contains independent instructions. Superscalarity can be gained from a decoupled floating point unit (FPU) which executes floating point operations indepently from the integer ALU. More complicated variations allow for parallel load/store operations, integer calculations and so on, which need a more complex CPU design that analyzes the instructions/branches.

Every PA-RISC processor from the PA-7100 on implements superscalar execution. Instructions proceed together through the execution pipeline, which is called instruction bundling. The superscalar execution is functionally transparent to the software, the effects of any given instruction are the same whether it was executed as part of a bundle or alone. Bundling rules are applied at run-time by the hardware; optimal performance may only be gained by proper ordering of the instructions so the processor can use its full superscalar potential.

Several kinds of restrictions are placed upon the instruction bundling in PA-RISC:

For bundling purposes instructions are divided into classes:

PA-RISC superscalar instruction classes
Class Description
FLOP Floating point operation
LDST Loads and stores
ALU Integer ALU
MM Shifts, extracts, deposits
NUL Might nullify successor
BV Branch Vectored (BV) local, Branch (BE) external
BR Other branches
FSYS FTEST and FP status/exception
SYS System control instructions

PA-7100 superscalar capabilities

The PA-7100 is two-way superscalar with one integer ALU and one FPU.

Allowed bundles

PA-7100 allowed instruction bundles
First instruction Second instruction
ALU  + FLOP
LDST  + FLOP
FLOP  + ALU/LDST/Branch

PA-7100LC/PA-7300LC superscalar capabilities

These are 2-way superscalar processor implementations with two integer ALUs and one FPU. Notably only one of the two ALUs is capable to handle loads, stores and shifts.

Allowed bundles

PA-7100LC/PA-7300LC allowed instruction bundles
First instruction Second instruction
FLOP  + LDST/ALU/MM/NUL/BV/BR
LDST  + FLOP/ALU/MM/NUL/BR
ALU  + FLOP/LDST/ALU/MM/NUL/BR/FSYS
MM  + FLOP/LDST/ALU/FSYS
NUL  + FLOP
SYS Never bundled

Besides from these bundles, LDST + LDST bundles are under certain circumstances also possible. These are then called double word load/store.

Data dependencies

Several kinds of instructions cannot be bundled together because of inter-instruction data dependencies:

Control Flow

PA-7200 superscalar capabilities

This is a 2-way superscalar processor implementation. It has two integer ALUs and one FPU. Similar to the PA-7100LC, shift-merge and test condition units are not duplicated in the second ALU. To support the superscalar capabilities one additional write port and two additional read ports were added to the general registers (GR*).

Allowed bundles

PA-7100LC/PA-7300LC allowed instruction bundles
First instruction Second instruction
FLOP  + LDST/ALU/MM/NUL/BV/BR
LDST  + FLOP/ALU/MM/NUL/BR
ALU  + FLOP/LDST/ALU/MM/NUL/BR/FSYS
MM  + FLOP/LDST/ALU/FSYS
NUL  + FLOP

PA-8x00 superscalar capabilities

To be described.

↑ up

Multimedia Acceleration eXtensions (MAX-1 and MAX-2)

MAX-1 (32-bit)

MAX-1 are the original multimedia extensions from the 1990s introduced with the HP PA-7100LC processor and later also the PA-7300LC. The aim from HP in its design was to enable contemporary workstations with these CPUs to provide real-time MPEG video decompression and playback at a rate of 30 frames/second without the need for a special DSP (digital signal processing) chip, not an easy feat.

The HP design process for the PA-7100LC processor in the early 1990s included for the first time multimedia benchmarks for analyzing optimizations in the instruction set design.

The actual implementation used a small set of SIMD-MIMD instructions to faciliate the application of instructions on bundled subword data. Since these instructions use the same data paths and execution units within the processor as the regular instructions, the design team termed this intrinsic signal processing (ISP).

Sticking to conventional RISC principles, the design team decided against adding complex special-purpose instructions to the design but opted for the elegant use of the existing facilities in the CPU, which were slightly modified to understand new, packed subword data.

In 1994, the MAX-1 extensions made their way into the final PA-7100LC product and as such were the first SIMD instructions found in a general microprocessor. Less than 0.2 percent of the processor silicon area had to be used for MAX-1 additions and modifications, while allowing a very significant performance boost in affected applications.

As an example, the then-highend HP 9000 735/99 workstation with a 99  MHz processes and 512 KB cache achieved 18.7 FPS at MPEG decompression benchmarks — the new entry-level 712 workstation at 60 MHz and 64 KB cache achieved 26 FPS, an impressive feat for the time an 1990s information technology.

New MAX-1 multimedia instructions include: parallel add, parallel subtract, parallel shift left & add (i.e. multiply with integer), parallel shift right & add (i.e. division), parallel average.

MAX-2 (64-bit)

With the introduction of the new 64-bit PA-RISC 2.0 architecture in 1996 HP unveiled a new set of multimedia-oriented instructions aimed at using the processor’s resources more effectively for sub-word data. The basic components of the contemporary multimedia data were often represented as 8, 12 or 16-bit integers, for example audio sampling and pixel color depth. Doing arithmetic with data of this length would waste an considerable amount of the processor’s execution capacities, a simple addition of 16-bit data would only use one quarter of the 64-bit wide integer units datapath. To remedy this situation, MAX allows for packing of these subword data into larger words near the processor’s natural word width (64-bit on PA-RISC 2.0 processors) and using parallel instructions on them. An example would be four 16-bit additions by the 64-bit adder on four 16-bit packed subwords.

The basic functionality from the earlier 32-bit MAX-1 was taken over and four more instructions added for MAX-2. Additionally, due to the wider integer registers (now 64-bit) more subwords can be packed in one cycle, doubling the effective speed of these multimedia instructions. The MAX-2 multimedia instructions include (new in MAX-2 are in bold): parallel add, parallel subtract, parallel shift left & add (i.e. multiply with integer), parallel shift right & add (i.e. division), parallel average, parallel shift right, parallel shift left, mix and permute.

MAX-2 debuted 1996 with the PA-8000 processor and later featured on all subsequent PA-RISC 2.0 processors (PA-8x00). In contrast to contemporary multimedia extensions, MAX-2 required only very little die space (0.1 percent on the PA-8000).

↑ up

Further reading

Selected papers and articles for further reading on the PA-RISC architecture and platform

↑ up