Microprocessor Design/Microprocessors

Microprocessors

Microprocessors are the devices in a computer which make things happen. Microprocessors are capable of performing basic arithmetic operations, moving data from place to place, and making basic decisions based on the quantity of certain values.

The components of a PC computer. Part number 3 is the CPU.

Types of Processors

The vast majority of microprocessors can be found in embedded microcontrollers. The second most common type of processors are common desktop processors, such as Intel's Pentium or AMD's Athlon. Less common are the extremely powerful processors used in high-end servers, such as Sun's SPARC, IBM's Power, or Intel's Itanium.

Historically, microprocessors and microcontrollers have come in "standard sizes" of 8 bits, 16 bits, 32 bits, and 64 bits. These sizes are common, but that does not mean that other sizes are not available. Some microcontrollers (usually specially designed embedded chips) can come in other "non-standard" sizes such as 4 bits, 12 bits, 18 bits, or 24 bits. The number of bits represent how much physical memory can be directly addressed by the CPU. It also represents the amount of bits that can be read by one read/write operation. In some circumstances, these are different; for instance, many 8 bit microprocessors have an 8 bit data bus and a 16 bit address bus.

• 8 bit processors can read/write 1 byte at a time and can directly address 256 bytes
• 16 bit processors can read/write 2 bytes at a time, and can address 65,536 bytes (64 Kilobytes)
• 32 bit processors can read/write 4 bytes at a time, and can address 4,294,967,295 bytes (4 Gigabytes)
• 64 bit processors can read/write 8 bytes at a time, and can address 18,446,744,073,709,551,616 bytes (16 Exabytes)

General Purpose Versus Specific Use

Microprocessors that are capable of performing a wide range of tasks are called general purpose microprocessors. General purpose microprocessors are typically the kind of CPUs found in desktop computer systems. These chips typically are capable of a wide range of tasks (integer and floating point arithmetic, external memory interface, general I/O, etc). We will discuss some of the other types of processor units available:

General Purpose
A general purpose processing unit, typically referred to as a "microprocessor" is a chip that is designed to be integrated into a larger system with peripherals and external RAM. These chips can typically be used with a very wide array of software.
DSP
A Digital Signal Processor, or DSP for short, is a chip that is specifically designed for fast arithmetic operations, especially addition and multiplication. These chips are designed with processing speed in mind, and don't typically have the same flexibility as general purpose microprocessors. DSPs also have special address generation units that can manage circular buffers, perform bit-reversed addressing, and simultaneously access multiple memory spaces with little to no overhead. They also support zero-overhead looping, and a single-cycle multiply-accumulate instruction. They are not typically more powerful than general purpose microprocessors, but can perform signal processing tasks using far less power (as in watts).
Embedded Controller
Embedded controllers, or "microcontrollers" are microprocessors with additional hardware integrated into a single chip. Many microcontrollers have RAM, ROM, A/D and D/A converters, interrupt controllers, timers, and even oscillators built into the chip itself. These controllers are designed to be used in situations where a whole computer system isn't available, and only a small amount of simple processing needs to be performed.
Programmable State Machines
The most simplistic of processors, programmable state machines are a minimalist microprocessor that is designed for very small and simple operations. PSMs typically have very small amount of program ROM available, limited scratch-pad RAM, and they are also typically limited in the type and number of instructions that they can perform. PSMs can either be used stand-alone, or (more frequently) they are embedded directly into the design of a larger chip.
Graphics Processing Units
Computer graphics are so complicated that functions to process the visuals of video and game applications have been offloaded to a special type of processor known as a GPU. GPUs typically require specialized hardware to implement matrix multiplications and vector arithmetic. GPUs are typically also highly parallelized, performing shading calculations on multiple pixels and surfaces simultaneously.

Types of Use

Microcontrollers and Microprocessors are used for a number of different types of applications. People may be the most familiar with the desktop PC, but the fact is that desktop PCs make up only a small fraction of all microprocessors in use today. We will list here some of the basic uses for microprocessors:

Signal Processing
Signal processing is an area that demands high performance from microcontroller chips to perform complex mathematical tasks. Signal processing systems typically need to have low latency, and are very deadline driven. An example of a signal processing application is the decoding of digital television and radio signals.
Real Time Applications
Some tasks need to be performed so quickly that even the slightest delay or inefficiency can be detrimental. These applications are known as "real time systems", and timing is of the utmost importance. An example of a real-time system is the anti-lock braking system (ABS) controller in modern automobiles.
Throughput and Routing
Throughput and routing is the use of a processor where data is moved from one particular input to an output, without necessarily requiring any processing. An example is an internet router, that reads in data packets and sends them out on a different port.
Sensor monitoring
Many processors, especially small embedded processors are used to monitor sensors. The microprocessor will either digitize and filter the sensor signals, or it will read the signals and produce status outputs (the sensor is good, the sensor is bad). An example of a sensor monitoring processor is the processor inside an antilock brake system: This processor reads the brake sensor to determine when the brakes have locked up, and then outputs a control signal to activate the rest of the system.
General Computing
A general purpose processor is like the kind of processor that is typically found inside a desktop PC. Names such as Intel and AMD are typically associated with this type of processor, and this is also the kind of processor that the public is most familiar with.
Graphics
Processing of digital graphics is an area where specialized processor units are frequently employed. With the advent of digital television, graphics processors are becoming more common. Graphics processors need to be able to perform multiple simultaneous operations. In digital video, for instance, a million pixels or more will need to be processed for every single frame, and a particular signal may have 60 frames per second! To the benefit of graphics processors, the color value of a pixel is typically not dependent on the values of surrounding pixels, and therefore many pixels can typically be computed in parallel.
   \mbox{Clock Time} = \frac{1}{\mbox{Clock Rate}}


Abstraction Layers

Computer systems are developed in layers known as layers of abstraction. Layers of abstraction allow people to develop computer components (hardware and software) without having to worry about the internal design of the other layers in the system. At the highest level are the user-interface programs that people use on their computers. At the lowest level are the transistor layouts of the individual computer components. Some of the layers in a computer system are (listed from highest to lowest):

1. Application
2. Operating System
3. Firmware
4. Instruction Set Architecture
5. Microprocessor Control Logic
6. Physical Circuit Layout

This book will be mostly concerned with the Instruction Set Architecture (ISA), and the Microprocessor Control Logic but we will also describe the Operating System (OS) in brief. Topics above these are typically the realm of computer programmers. The bottom layer, the Physical Circuit Layout is the job of hardware and VLSI engineers.

Operating System

Operating System is a program which acts as an interface between the system user and the computer hardware and controls the execution of application programs. It is the program running at all times on the computer, usually called the Kernel.

ISA

The Instruction Set Architecture is a long name for the assembly language of a particular machine, and the associated machine code for that assembly language. We will discuss this below.

Assembly Language

An assembly language is a small language that contains a short word or "mnemonic" for each individual command that a microcontroller can follow. Each command gets a single mnemonic, and each mnemonic corresponds to a single machine command. Assembly language gets converted (by a program called an "assembler") into the binary machine code. The machine code is specific to each different type of machine.

Common ISAs

Wikibooks contains books about programming in multiple different types of assembly language. For more information about Assembly language, or for books on a particular ISA, see Assembly Language.

Some of the most common ISAs, listed in order of popularity (most popular first) are:

• ARM
• IA-32 (Intel x86)
• MIPS
• Motorola 68K
• PowerPC
• Hitachi SH
• SPARC

Moore's Law

A common law that governs the world of microprocessors is Moore's Law. Moore's Law, originally by Dr. Carver Mead at Caltech, and summarized famously by Intel Founder Gordon Moore. Moore's Law states that the number of transistors on a single chip at the same price will double every 18 to 24 months. This law has held without fail since it was originally stated in 1965. Current microprocessor chips contain millions of transistors and the number is growing rapidly. Here is Moore's summarization of the law from Electronics Magazine in 1965:

The complexity for minimum component costs has increased at a rate of roughly a factor of two per year...Certainly over the short term this rate can be expected to continue, if not to increase. Over the longer term, the rate of increase is a bit more uncertain, although there is no reason to believe it will not remain nearly constant for at least 10 years. That means by 1975, the number of components per integrated circuit for minimum cost will be 65,000. I believe that such a large circuit can be built on a single wafer.
—Gordon Moore

Moore's Law has been used incorrectly to calculate the speed of an integrated circuit, or even to calculate its power consumption, but neither of these interpretations are true. Also, Moore's law is talking about the number of transistors on a chip for a "minimum component cost", which means that the number of transistors on a chip, for the same price, will double. This goes to show that chips for less price can have fewer transistors, and that chips at a higher price can have more transistors. On an economic note, a consequence of Moore's Law is that companies need to continue to innovate and integrate more transistors onto a single chip, without being able to increase prices.

Moore's Law does not require that the speed of the chip increase along with the number of transistors on the chip. However, the two measurements are typically related. Some points to keep in mind about transistors and Moore's Law are:

1. Smaller Transistors typically switch faster then larger transistors.
2. To get more transistors on a single chip, the chip needs to be made larger, or the transistors need to be made smaller. Typically, the transistors get smaller.
3. Transistors tend to leak electrical current as they get smaller. This means that smaller transistors require more power to operate, and they generate more heat.
4. Transistors tend to generate heat as a function of frequencies. Higher clock rates tend to generate more heat.

Moore's law is occasionally misinterpreted to mean that the speed of processors, in hertz will double every 18 months. This is not strictly true, although the speed of processors does tend to increase as transistors are made smaller and more compact. With the advent of multi-core processors, some people have used Moore's law to mean that processor throughput increases with time, which is not strictly the case either (although it is a likely side effect of Moore's law).

Clock Rates

Microprocessors are typically discussed in terms of their clock speed. The clock speed is measured in hertz (or megahertz, or gigahertz). A hertz is a "cycle per second". Each cycle, a microprocessor will perform certain tasks, although the amount of work performed in a single cycle will be different for different types of processors. The amount of work that a processor can complete in a single cycle is measured in "cycles per instruction". For some systems, such as MIPS, there is 1 cycle per instruction. For other systems, such as modern x86 chips, there are typically very many cycles per instruction.

The clock rate is equated as such:

$\mbox{Clock Time} = \frac{1}{\mbox{Clock Rate}}$

This means that the amount of time for a cycle is inversely proportional to the clock rate. A computer with a 1MHz clock rate will have a clock time of 1 microsecond. A modern desktop computer with a 3.2 GHz processor will have a clock time of approximately 3× 10-10 seconds, or 300 picoseconds. 300 picoseconds is an incredibly small amount of time, and there is a lot that needs to happen inside the processor in each clock cycle.

Basic Elements of a Computer

There are a few basic elements that are common to all computers. These elements are:

• CPU
• Memory
• Input Devices
• Output Devices

Depending on the particular computer architecture, these elements may be available in various sizes, and they may be accompanied by additional elements.