CPU design

CPU design is the design engineering task of creating a central processing unit (CPU), a component of computer hardware. It is a subfield of electronics engineering and computer engineering.

Overview

CPU design focuses on these areas:

datapaths (such as ALUs and pipelines)

control unit: logic which controls the datapaths

Memory components such as register files, caches

Clock circuitry such as clock drivers, PLLs, clock distribution networks

Pad transceiver circuitry

Logic gate cell library which is used to implement the logic

CPUs designed for high-performance markets might require custom designs for each of these items to achieve frequency, power-dissipation, and chip-area goals.

CPUs designed for lower performance markets might lessen the implementation burden by:

Acquiring some of these items by purchasing them as intellectual property

Use control logic implementation techniques (logic synthesis using CAD tools) to implement the other components - datapaths, register files, clocks

Common logic styles used in CPU design include:

Unstructured random logic

Finite-state machines

Microprogramming (common from 1965 to 1985)

Programmable logic array (common in the 1980s, no longer common)

Device types used to implement the logic include:

Transistor-transistor logic Small Scale Integration logic chips - no longer used for CPUs

Programmable Array Logic and Programmable logic devices - no longer used for CPUs

Emitter-coupled logic (ECL) gate arrays - no longer common

CMOS gate arrays - no longer used for CPUs

CMOS ASICs - what's commonly used today, they're so common that the term ASIC is not used for CPUs

Field-programmable gate arrays (FPGA) - common for soft microprocessors, and more or less required for reconfigurable computing

A CPU design project generally has these major tasks:

Programmer-visible instruction set architecture, which can be implemented by a variety of microarchitectures

Architectural study and performance modeling in ANSI C/C++ or SystemC

High-level synthesis (HLS) or RTL (eg. logic) implementation

RTL Verification

Circuit design of speed critical components (caches, registers, ALUs)

Logic synthesis or logic-gate-level design

Timing analysis to confirm that all logic and circuits will run at the specified operating frequency

Physical design including floorplanning, place and route of logic gates

Checking that RTL, gate-level, transistor-level and physical-level representations are equivalent

Checks for signal integrity, chip manufacturability

As with most complex electronic designs, the logic verification effort (proving that the design does not have bugs) now dominates the project schedule of a CPU.

Key CPU architectural innovations include index register, cache, virtual memory, instruction pipelining, superscalar, CISC, RISC, virtual machine, emulators, microprogram, and stack.

Goals

The first CPUs were designed to do mathematical calculations faster and more reliably than human computers.

Each successive generation of CPU might be designed to achieve some of these goals:

higher performance levels of a single program or thread

higher throughput levels of multiple programs/threads

less power consumption for the same performance level

lower cost for the same performance level

greater connectivity to build larger, more parallel systems

more specialization to aid in specific targeted markets

Re-designing a CPU core to a smaller die-area helps achieve several of these goals.

Shrinking everything (a "photomask shrink"), resulting in the same number of transistors on a smaller die, improves performance (smaller transistors switch faster), reduces power (smaller wires have less parasitic capacitance) and reduces cost (more CPUs fit on the same wafer of silicon).

Releasing a CPU on the same size die, but with a smaller CPU core, keeps the cost about the same but allows higher levels of integration within one VLSI chip (additional cache, multiple CPUs, or other components), improving performance and reducing overall system cost.

Performance analysis and benchmarking

Main article: Computer performance

Because there are too many programs to test a CPU's speed on all of them, benchmarks were developed. The most famous benchmarks are the SPECint and SPECfp benchmarks developed by Standard Performance Evaluation Corporation and the ConsumerMark benchmark developed by the Embedded Microprocessor Benchmark Consortium EEMBC.

Some important measurements include:

Instructions per second - Most consumers pick a computer architecture (normally Intel IA32 architecture) to be able to run a large base of pre-existing pre-compiled software. Being relatively uninformed on computer benchmarks, some of them pick a particular CPU based on operating frequency (see Megahertz Myth).

FLOPS - The number of floating point operations per second is often important in selecting computers for scientific computations.

Performance per watt - System designers building parallel computers, such as Google, pick CPUs based on their speed per watt of power, because the cost of powering the CPU outweighs the cost of the CPU itself. 

Some system designers building parallel computers pick CPUs based on the speed per dollar.

System designers building real-time computing systems want to guarantee worst-case response. That is easier to do when the CPU has low interrupt latency and when it has deterministic response. (DSP)

Computer programmers who program directly in assembly language want a CPU to support a full featured instruction set.

Low power - For systems with limited power sources (e.g. solar, batteries, human power).

Small size or low weight - for portable embedded systems, systems for spacecraft.

Environmental impact - Minimizing environmental impact of computers during manufacturing and recycling as well during use. Reducing waste, reducing hazardous materials. (see Green computing).

Some of these measures conflict. In particular, many design techniques that make a CPU run faster make the "performance per watt", "performance per dollar", and "deterministic response" much worse, and vice versa.

Markets

There are several different markets in which CPUs are used. Since each of these markets differ in their requirements for CPUs, the devices designed for one market are in most cases inappropriate for the other markets.

General purpose computing

The vast majority of revenues generated from CPU sales is for general purpose computing. That is, desktop, laptop and server computers commonly used in businesses and homes. In this market, the Intel IA-32 architecture dominates, with its rivals PowerPC and SPARC maintaining much smaller customer bases. Yearly, hundreds of millions of IA-32 architecture CPUs are used by this market.

Since these devices are used to run countless different types of programs, these CPU designs are not specifically targeted at one type of application or one function. The demands of being able to run a wide range of programs efficiently has made these CPU designs among the more advanced technically, along with some disadvantages of being relatively costly, and having high power consumption.

High-end processor economics

In 1984, most high-performance CPUs required four to five years to develop.

This section may require cleanup to meet Wikipedia's quality standards. Please improve this section if you can. The talk page may contain suggestions. (December 2009)

Developing new, high-end CPUs is a very costly proposition. Both the logical complexity (needing very large logic design and logic verification teams and simulation farms with perhaps thousands of computers) and the high operating frequencies (needing large circuit design teams and access to the state-of-the-art fabrication process) account for the high cost of design for this type of chip. The design cost of a high-end CPU will be on the order of US $100 million. Since the design of such high-end chips nominally takes about five years to complete, to stay competitive a company has to fund at least two of these large design teams to release products at the rate of 2.5 years per product generation.

As an example, the typical loaded cost for one computer engineer is often quoted to be $250,000 US dollars/year. This includes salary, benefits, CAD tools, computers, office space rent, etc. Assuming that 100 engineers are needed to design a CPU and the project takes 4 years.

Total cost = $250,000 / Engineer-Man/Year x 100 engineers x 4 years = $100,000,000 USD.

The above amount is just an example. The design teams for modern day general purpose CPUs have several hundred team members.

Scientific computing

Main article: Supercomputer

A much smaller niche market (in revenue and units shipped) is scientific computing, used in government research labs and universities. Previously much CPU design was done for this market, but the cost-effectiveness of using mass markets CPUs has curtailed almost all specialized designs for this market. The main remaining area of active hardware design and research for scientific computing is for high-speed system interconnects.

Embedded design

As measured by units shipped, most CPUs are embedded in other machinery, such as telephones, clocks, appliances, vehicles, and infrastructure. Embedded processors sell in the volume of many billions of units per year, however, mostly at much lower price points than that of the general purpose processors.

These single-function devices differ from the more familiar general-purpose CPUs in several ways:

Low cost is of utmost importance.

It is important to maintain a low power dissipation as embedded devices often have a limited battery life and it is often impractical to include cooling fans.

To give lower system cost, peripherals are integrated with the processor on the same silicon chip.

Keeping peripherals on-chip also reduces power consumption as external GPIO ports typically require buffering so that they can source or sink the relatively high current loads that are required to maintain a strong signal outside of the chip.

Many embedded applications have a limited amount of physical space for circuitry; keeping peripherals on-chip will reduce the space required for the circuit board.

The program and data memories are often integrated on the same chip. When the only allowed program memory is ROM, the device is known as a microcontroller.

For many embedded applications, interrupt latency will be more critical than in some general-purpose processors.

Embedded processor economics

As of 2009, more CPUs are produced using the ARM architecture instruction set than any other 32-bit instruction set. The ARM architecture and the first ARM chip were designed in about one and a half years and 5 man years of work time.

The 32-bit Parallax Propeller microcontroller architecture and the first chip were designed by two people in about 10 man years of work time.

It is believed[weasel words] that the 8-bit AVR architecture and first AVR microcontroller was conceived and designed by two students at the Norwegian Institute of Technology.

The 8-bit 6502 architecture and the first MOS Technology 6502 chip were designed in 13 months by a group of about 9 people.

Research and educational CPU design

The 32 bit Berkeley RISC I and RISC II architecture and the first chips were mostly designed by a series of students as part of a four quarter sequence of graduate courses.This design became the basis of the commercial SPARC processor design.

For about a decade, every student taking the 6.004 class at MIT was part of a team—each team had one semester to design and build a simple 8 bit CPU out of 7400 series integrated circuits. One team of 4 students designed and built a simple 32 bit CPU during that semester. 

Some undergraduate courses require a team of 2 to 5 students to design, implement, and test a simple CPU in a FPGA in a single 15 week semester. 

Soft microprocessor cores

For embedded systems, the highest performance levels are often not needed or desired due to the power consumption requirements. This allows for the use of processors which can be totally implemented by logic synthesis techniques. These synthesized processors can be implemented in a much shorter amount of time, giving quicker time-

to-market.

Main article: Soft microprocessor

Micro-architectural concepts

Main article: Microarchitecture

Research Topics

Main article: History of general purpose CPUs#1990 to today: looking forward

A variety of new CPU design ideas have been proposed, including reconfigurable logic, clockless CPUs, and optical computing.

(source:wikipedia)