The computer you are using to read this page uses a microprocessor to do its work. The microprocessor is the heart of any normal computer, whether it is a desktop machine, a server or a laptop. The microprocessor you are using might be a Pentium, a K6, a PowerPC, a Sparc or any of the many other brands and types of microprocessors, but they all do approximately the same thing in approximately the same way.

A microprocessor -- also known as a CPU or central processing unit -- is a complete computation engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful -- all it could do was add and subtract, and it could only do that 4 bits at a time. But it was amazing that everything was on one chip. Prior to the 4004, engineers built computers either from collections of chips or from discrete components (transistors wired one at a time). The 4004 powered one of the first portable electronic calculators.

If you have ever wondered what the microprocessor in your computer is doing, or if you have ever wondered about the differences between types of microprocessors, then read on. In this article, you will learn how fairly simple digital logic techniques allow a computer to do its job, whether its playing a game or spell checking a document!

Microprocessor Progression: Intel

The Intel 8080 was the first microprocessor in a home computer.

The first microprocessor to make it into a home computer was the Intel 8080, a complete 8-bit computer on one chip, introduced in 1974. The first microprocessor to make a real splash in the market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which first appeared around 1982). If you are familiar with the PC market and its history, you know that the PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. All of these microprocessors are made by Intel and all of them are improvements on the basic design of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster!

The following table helps you to understand the differences between the different processors that Intel has introduced over the years.

Name	Date	Transistors	Microns	Clock speed	Data width	MIPS
8080	1974	6,000	6	2 MHz	8 bits	0.64
8088	1979	29,000	3	5 MHz	16 bits 8-bit bus	0.33
80286	1982	134,000	1.5	6 MHz	16 bits	1
80386	1985	275,000	1.5	16 MHz	32 bits	5
80486	1989	1,200,000	1	25 MHz	32 bits	20
Pentium	1993	3,100,000	0.8	60 MHz	32 bits 64-bit bus	100
Pentium II	1997	7,500,000	0.35	233 MHz	32 bits 64-bit bus	~300
Pentium III	1999	9,500,000	0.25	450 MHz	32 bits 64-bit bus	~510
Pentium 4	2000	42,000,000	0.18	1.5 GHz	32 bits 64-bit bus	~1,700
Pentium 4 "Prescott"	2004	125,000,000	0.09	3.6 GHz	32 bits 64-bit bus	~7,000

Information about this table:

The date is the year that the processor was first introduced. Many processors are re-introduced at higher clock speeds for many years after the original release date.
Transistors is the number of transistors on the chip. You can see that the number of transistors on a single chip has risen steadily over the years.
Microns is the width, in microns, of the smallest wire on the chip. For comparison, a human hair is 100 microns thick. As the feature size on the chip goes down, the number of transistors rises.
Clock speed is the maximum rate that the chip can be clocked at. Clock speed will make more sense in the next section.
Data Width is the width of the ALU. An 8-bit ALU can add/subtract/multiply/etc. two 8-bit numbers, while a 32-bit ALU can manipulate 32-bit numbers. An 8-bit ALU would have to execute four instructions to add two 32-bit numbers, while a 32-bit ALU can do it in one instruction. In many cases, the external data bus is the same width as the ALU, but not always. The 8088 had a 16-bit ALU and an 8-bit bus, while the modern Pentiums fetch data 64 bits at a time for their 32-bit ALUs.
MIPS stands for "millions of instructions per second" and is a rough measure of the performance of a CPU. Modern CPUs can do so many different things that MIPS ratings lose a lot of their meaning, but you can get a general sense of the relative power of the CPUs from this column.

From this table you can see that, in general, there is a relationship between clock speed and MIPS. The maximum clock speed is a function of the manufacturing process and delays within the chip. There is also a relationship between the number of transistors and MIPS. For example, the 8088 clocked at 5 MHz but only executed at 0.33 MIPS (about one instruction per 15 clock cycles). Modern processors can often execute at a rate of two instructions per clock cycle. That improvement is directly related to the number of transistors on the chip.

Microprocessor Logic

IntelPentium 4 processor

To understand how a microprocessor works, it is helpful to look inside and learn about the logic used to create one. In the process you can also learn about assembly language -- the native language of a microprocessor -- and many of the things that engineers can do to boost the speed of a processor. A microprocessor executes a collection of machine instructions that tell the processor what to do. Based on the instructions, a microprocessor does three basic things:

Using its ALU (Arithmetic/Logic Unit), a microprocessor can perform mathematical operations like addition, subtraction, multiplication and division. Modern microprocessors contain complete floating point processors that can perform extremely sophisticated operations on large floating point numbers.
A microprocessor can move data from one memory location to another.
A microprocessor can make decisions and jump to a new set of instructions based on those decisions.

There may be very sophisticated things that a microprocessor does, but those are its three basic activities. The following diagram shows an extremely simple microprocessor capable of doing those three things:

This is about as simple as a microprocessor gets. This microprocessor has:

An address bus (that may be 8, 16 or 32 bits wide) that sends an address to memory
A data bus (that may be 8, 16 or 32 bits wide) that can send data to memory or receive data from memory
An RD (read) and WR (write) line to tell the memory whether it wants to set or get the addressed location
A clock line that lets a clock pulse sequence the processor
A reset line that resets the program counter to zero (or whatever) and restarts execution

Let's assume that both the address and data buses are 8 bits wide in this example. Here are the components of this simple microprocessor:

Registers A, B and C are simply latches made out of flip-flops.
The address latch is just like registers A, B and C.
The program counter is a latch with the extra ability to increment by 1 when told to do so, and also to reset to zero when told to do so.
The ALU could be as simple as an 8-bit adder,or it might be able to add, subtract, multiply and divide 8-bit values. Let's assume the latter here.
The test register is a special latch that can hold values from comparisons performed in the ALU. An ALU can normally compare two numbers and determine if they are equal, if one is greater than the other, etc. The test register can also normally hold a carry bit from the last stage of the adder. It stores these values in flip-flops and then the instruction decoder can use the values to make decisions.
There are six boxes marked "3-State" in the diagram. These are tri-state buffers. A tri-state buffer can pass a 1, a 0 or it can essentially disconnect its output (imagine a switch that totally disconnects the output line from the wire that the output is heading toward). A tri-state buffer allows multiple outputs to connect to a wire, but only one of them to actually drive a 1 or a 0 onto the line.
The instruction register and instruction decoder are responsible for controlling all of the other components.

Although they are not shown in this diagram, there would be control lines from the instruction decoder that would:

Tell the A register to latch the value currently on the data bus
Tell the B register to latch the value currently on the data bus
Tell the C register to latch the value currently output by the ALU
Tell the program counter register to latch the value currently on the data bus
Tell the address register to latch the value currently on the data bus
Tell the instruction register to latch the value currently on the data bus
Tell the program counter to increment
Tell the program counter to reset to zero
Activate any of the six tri-state buffers (six separate lines)
Tell the ALU what operation to perform
Tell the test register to latch the ALU's test bits
Activate the RD line
Activate the WR line

Coming into the instruction decoder are the bits from the test register and the clock line, as well as the bits from the instruction register.

Microprocessor Memory

The previous section talked about the address and data buses, as well as the RD and WR lines. These buses and lines connect either to RAM or ROM -- generally both. In our sample microprocessor, we have an address bus 8 bits wide and a data bus 8 bits wide. That means that the microprocessor can address (2⁸) 256 bytes of memory, and it can read or write 8 bits of the memory at a time. Let's assume that this simple microprocessor has 128 bytes of ROM starting at address 0 and 128 bytes of RAM starting at address 128.

ROM chip

ROM stands for read-only memory. A ROM chip is programmed with a permanent collection of pre-set bytes. The address bus tells the ROM chip which byte to get and place on the data bus. When the RD line changes state, the ROM chip presents the selected byte onto the data bus.

RAM chip

RAM stands for random-access memory. RAM contains bytes of information, and the microprocessor can read or write to those bytes depending on whether the RD or WR line is signaled. One problem with today's RAM chips is that they forget everything once the power goes off. That is why the computer needs ROM. By the way, nearly all computers contain some amount of ROM (it is possible to create a simple computer that contains no RAM -- many microcontrollers do this by placing a handful of RAM bytes on the processor chip itself -- but generally impossible to create one that contains no ROM). On a PC, the ROM is called the BIOS (Basic Input/Output System). When the microprocessor starts, it begins executing instructions it finds in the BIOS. The BIOS instructions do things like test the hardware in the machine, and then it goes to the hard disk to fetch the boot sector (see How Hard Disks Work for details). This boot sector is another small program, and the BIOS stores it in RAM after reading it off the disk. The microprocessor then begins executing the boot sector's instructions from RAM. The boot sector program will tell the microprocessor to fetch something else from the hard disk into RAM, which the microprocessor then executes, and so on. This is how the microprocessor loads and executes the entire operating system.

Microprocessor Instructions

Even the incredibly simple microprocessor shown in the previous example will have a fairly large set of instructions that it can perform. The collection of instructions is implemented as bit patterns, each one of which has a different meaning when loaded into the instruction register. Humans are not particularly good at remembering bit patterns, so a set of short words are defined to represent the different bit patterns. This collection of words is called the assembly language of the processor. An assembler can translate the words into their bit patterns very easily, and then the output of the assembler is placed in memory for the microprocessor to execute. Here's the set of assembly language instructions that the designer might create for the simple microprocessor in our example:

LOADA mem - Load register A from memory address
LOADB mem - Load register B from memory address
CONB con - Load a constant value into register B
SAVEB mem - Save register B to memory address
SAVEC mem - Save register C to memory address
ADD - Add A and B and store the result in C
SUB - Subtract A and B and store the result in C
MUL - Multiply A and B and store the result in C
DIV - Divide A and B and store the result in C
COM - Compare A and B and store the result in test
JUMP addr - Jump to an address
JEQ addr - Jump, if equal, to address
JNEQ addr - Jump, if not equal, to address
JG addr - Jump, if greater than, to address
JGE addr - Jump, if greater than or equal, to address
JL addr - Jump, if less than, to address
JLE addr - Jump, if less than or equal, to address
STOP - Stop execution

If you have read How C Programming Works, then you know that this simple piece of C code will calculate the factorial of 5 (where the factorial of 5 = 5! = 5 * 4 * 3 * 2 * 1 = 120):

a=1;
f=1;
while (a <= 5)
{
    f = f * a;
    a = a + 1;
}

At the end of the program's execution, the variable f contains the factorial of 5.

Assembly Language

A C compiler translates this C code into assembly language. Assuming that RAM starts at address 128 in this processor, and ROM (which contains the assembly language program) starts at address 0, then for our simple microprocessor the assembly language might look like this:

// Assume a is at address 128
// Assume F is at address 129
0   CONB 1      // a=1;
1   SAVEB 128
2   CONB 1      // f=1;
3   SAVEB 129
4   LOADA 128   // if a > 5 the jump to 17
5   CONB 5
6   COM
7   JG 17
8   LOADA 129   // f=f*a;
9   LOADB 128
10  MUL
11  SAVEC 129
12  LOADA 128   // a=a+1;
13  CONB 1
14  ADD
15  SAVEC 128
16  JUMP 4       // loop back to if
17  STOP

ROM

So now the question is, "How do all of these instructions look in ROM?" Each of these assembly language instructions must be represented by a binary number. For the sake of simplicity, let's assume each assembly language instruction is given a unique number, like this:

LOADA - 1
LOADB - 2
CONB - 3
SAVEB - 4
SAVEC mem - 5
ADD - 6
SUB - 7
MUL - 8
DIV - 9
COM - 10
JUMP addr - 11
JEQ addr - 12
JNEQ addr - 13
JG addr - 14
JGE addr - 15
JL addr - 16
JLE addr - 17
STOP - 18

The numbers are known as opcodes. In ROM, our little program would look like this:

// Assume a is at address 128
// Assume F is at address 129
Addr opcode/value
0    3             // CONB 1
1    1
2    4             // SAVEB 128
3    128
4    3             // CONB 1
5    1
6    4             // SAVEB 129
7    129
8    1             // LOADA 128
9    128
10   3             // CONB 5
11   5
12   10            // COM
13   14            // JG 17
14   31
15   1             // LOADA 129
16   129
17   2             // LOADB 128
18   128
19   8             // MUL
20   5             // SAVEC 129
21   129
22   1             // LOADA 128
23   128
24   3             // CONB 1
25   1
26   6             // ADD
27   5             // SAVEC 128
28   128
29   11            // JUMP 4
30   8
31   18            // STOP

You can see that seven lines of C code became 18 lines of assembly language, and that became 32 bytes in ROM.

Decoding

The instruction decoder needs to turn each of the opcodes into a set of signals that drive the different components inside the microprocessor. Let's take the ADD instruction as an example and look at what it needs to do:

During the first clock cycle, we need to actually load the instruction. Therefore the instruction decoder needs to:
- activate the tri-state buffer for the program counter
- activate the RD line
- activate the data-in tri-state buffer
- latch the instruction into the instruction register
During the second clock cycle, the ADD instruction is decoded. It needs to do very little:
- set the operation of the ALU to addition
- latch the output of the ALU into the C register
During the third clock cycle, the program counter is incremented (in theory this could be overlapped into the second clock cycle).

Every instruction can be broken down as a set of sequenced operations like these that manipulate the components of the microprocessor in the proper order. Some instructions, like this ADD instruction, might take two or three clock cycles. Others might take five or six clock cycles.

Microprocessor Performance and Trends

The number of transistors available has a huge effect on the performance of a processor. As seen earlier, a typical instruction in a processor like an 8088 took 15 clock cycles to execute. Because of the design of the multiplier, it took approximately 80 cycles just to do one 16-bit multiplication on the 8088. With more transistors, much more powerful multipliers capable of single-cycle speeds become possible.

More transistors also allow for a technology called pipelining. In a pipelined architecture, instruction execution overlaps. So even though it might take five clock cycles to execute each instruction, there can be five instructions in various stages of execution simultaneously. That way it looks like one instruction completes every clock cycle.

Many modern processors have multiple instruction decoders, each with its own pipeline. This allows for multiple instruction streams, which means that more than one instruction can complete during each clock cycle. This technique can be quite complex to implement, so it takes lots of transistors.

Trends

The trend in processor design has primarily been toward full 32-bit ALUs with fast floating point processors built in and pipelined execution with multiple instruction streams. The newest thing in processor design is 64-bit ALUs, and people are expected to have these processors in their home PCs in the next decade. There has also been a tendency toward special instructions (like the MMX instructions) that make certain operations particularly efficient, and the addition of hardware virtual memory support and L1 caching on the processor chip. All of these trends push up the transistor count, leading to the multi-million transistor powerhouses available today. These processors can execute about one billion instructions per second!

64-bit Microprocessors

Sixty-four-bit processors have been with us since 1992, and in the 21st century they have started to become mainstream. Both Intel and AMD have introduced 64-bit chips, and the Mac G5 sports a 64-bit processor. Sixty-four-bit processors have 64-bit ALUs, 64-bit registers, 64-bit buses and so on.

Photo courtesy AMD

One reason why the world needs 64-bit processors is because of their enlarged address spaces. Thirty-two-bit chips are often constrained to a maximum of 2 GB or 4 GB of RAM access. That sounds like a lot, given that most home computers currently use only 256 MB to 512 MB of RAM. However, a 4-GB limit can be a severe problem for server machines and machines running large databases. And even home machines will start bumping up against the 2 GB or 4 GB limit pretty soon if current trends continue. A 64-bit chip has none of these constraints because a 64-bit RAM address space is essentially infinite for the foreseeable future -- 2^64 bytes of RAM is something on the order of a billion gigabytes of RAM.

With a 64-bit address bus and wide, high-speed data buses on the motherboard, 64-bit machines also offer faster I/O (input/output) speeds to things like hard disk drives and video cards. These features can greatly increase system performance.

Servers can definitely benefit from 64 bits, but what about normal users? Beyond the RAM solution, it is not clear that a 64-bit chip offers "normal users" any real, tangible benefits at the moment. They can process data (very complex data features lots of real numbers) faster. People doing video editing and people doing photographic editing on very large images benefit from this kind of computing power. High-end games will also benefit, once they are re-coded to take advantage of 64-bit features. But the average user who is reading e-mail, browsing the Web and editing Word documents is not really using the processor in that way.

Active Memory or RAM

Memory

Memory. At the word we think of human memory, and of machine memory(i.e)Computer memory.

Memory is a vital part of both machine and animal. Without it we as humans would not have any consciousness and the ability to create. Animals would not be able to survive.

In machines, and especially in computers, memory allows the machine to function in various ways, for example software to be run and data to be saved and processed.

What is memory ?

Memory. Something just about everyone could use more of, including your computer. Memory is the ability to retain data for a period of time, short or long. This data can be of a complexity including imagery, sounds, sensations, smells and other sensations like human memory, or it can be predetermined data as in computer memory.

One of the differences between human and machine memory is that we can program and access machine memory through the use of software, but we cannot access human memory in the same straightforward manner.

Lets now talk about computer memory.

To start with there are basically two types of memory for a computer: storage space (hard drive) and active memory (RAM).

We will focus on active memory or RAM.

Computer Memory - RAM

People in the computer industry commonly use the term "memory" to refer to RAM (Random Access Memory). As your processor cranks on your game, it uses RAM to store some of the data needed to make your game work. While all forms of memory work together, RAM is considered the main memory since most data, regardless of its source, is stored in RAM before it is registered in any other storage device. Consequently, RAM is used millions of times every second. A computer uses Ram to hold temporary instructions and data needed to complete tasks. This enables the computer's CPU (Central Processing Unit), to access instructions and data stored in memory very quickly.

Computer memory is extremely important to computer operation. Files and programs are loaded into memory from external media like fixed disks (hard drives) and removable disks (floppies tapes). Memory can be built right into a system board, but it is more typically attached to the system board in the form of a chip or module. Inside these chips are microscopic digital switches which are used to represent binary data.

A good example of this is when the CPU loads an application program - such as a word processing or page layout program - into memory, thereby allowing the application program to work as quickly and efficiently as possible. In practical terms, having the program loaded into memory means that you can get work done more quickly with less time spent waiting for the computer to perform tasks.

The process begins when you enter a command from your keyboard. The CPU interprets the command and instructs the hard drive to load the command or program into memory. Once the data is loaded into memory, the CPU is able to access it much more quickly than if it had to retrieve it from the hard drive.

This process of putting things the CPU needs in a place where it can get at them more quickly is similar to placing various electronic files and documents you're using on the computer into a single file folder or directory. By doing so, you keep all the files you need handy and avoid searching in several places every time you need them.

In general the more RAM a computer has the faster the computer operates. Why? RAM is where all the information is kept just before the computer needs to use it.

Think of it this way. During a conversation a person can speak without interruption if everything being talked about is in his or her memory. However, if a person does not have enough memory and has to look something up during the course of the conversation, in a book or newspaper, then the conversation stops until the needed information is found.

Computers are very similar; they can continue processing without interruption as long as all needed information is in memory (RAM). When that is not the case, the computer stops, retrieves the needed information from storage (i.e. Hard drive, CD, disk) and places it into memory and then continues processing. The more interruptions the computer receives to retrieve information the slower the computer. The more memory a computer has, the fewer interruptions and the faster the computer operates. More memory equates to more speed.

These days, no matter how much memory your computer has, it never seems to be quite enough. Not long ago, it was unheard of for a PC (Personal Computer), to have more than 1 or 2 MB (Megabytes) of memory. Today, most systems require 64MB to run basic applications. And up to 256MB or more is needed for optimal performance when using graphical and multimedia programs.

As an indication of how much things have changed over the past two decades, consider this: in 1981, referring to computer memory, Bill Gates said, "640K (roughly 1/2 of a megabyte) ought to be enough for anybody."

For some, the memory equation is simple: more is good; less is bad. However, for those who want to know more, this reference guide contains answers to the most common questions, plus much, much more.

note:A computer's system RAM alone is not fast enough to match the speed of the CPU. That is why you need a cache memory.
Different Types Of RAM'S

1)Dynamic RAM:Dynamic RAM is the most common type of memory in use today. Inside a dynamic RAM chip, each memory cell holds one bit of information and is made up of two parts: a transistor and a capacitor. These are, of course, extremely small transistors and capacitors so that millions of them can fit on a single memory chip. The capacitor holds the bit of information -- a 0 or a 1 (see How Bits and Bytes Work for information on bits). The transistor acts as a switch that lets the control circuitry on the memory chip read the capacitor or change its state.

A capacitor is like a small bucket that is able to store electrons. To store a 1 in the memory cell, the bucket is filled with electrons. To store a 0, it is emptied. The problem with the capacitor's bucket is that it has a leak. In a matter of a few milliseconds a full bucket becomes empty. Therefore, for dynamic memory to work, either the CPU or the memory controller has to come along and recharge all of the capacitors holding a 1 before they discharge. To do this, the memory controller reads the memory and then writes it right back. This refresh operation happens automatically thousands of times per second.
This refresh operation is where dynamic RAM gets its name. Dynamic RAM has to be dynamically refreshed all of the time or it forgets what it is holding. The downside of all of this refreshing is that it takes time and slows down the memory.

2)Static RAM:Static RAM uses a completely different technology. In static RAM, a form of flip-flop holds each bit of memory . A flip-flop for a memory cell takes 4 or 6 transistors along with some wiring, but never has to be refreshed. This makes static RAM significantly faster than dynamic RAM. However, because it has more parts, a static memory cell takes a lot more space on a chip than a dynamic memory cell. Therefore you get less memory per chip, and that makes static RAM a lot more expensive.

note:static RAM is fast and expensive, and dynamic RAM is less expensive and slower. Therefore static RAM is used to create the CPU's speed-sensitive cache, while dynamic RAM forms the larger system RAM space.

Cache memory

Cache (pronounced cash) memory is extremely fast memory that is built into a computer’s central processing unit (CPU), or located next to it on a separate chip. The CPU uses cache memory to store instructions that are repeatedly required to run programs, improving overall system speed.

The cache is a smaller, faster memory which stores copies of the data from the most frequently used main memory (RAM) locations. As long as most memory accesses are to cached memory locations, the average latency of memory accesses will be closer to the cache latency than to the latency of main memory (RAM).

As it happens, once most programs are open and running, they use very few resources. When these resources are kept in cache, programs can operate more quickly and efficiently.cache is so effective in system performance that a computer running a fast CPU with little cache can have lower benchmarks than a system running a somewhat slower CPU with more cache. Cache built into the CPU itself is referred to as Level 1 (L1) cache. Cache that resides on a separate chip next to the CPU is called Level 2 (L2) cache. Some CPUs have both L1 and L2 cache built-in and designate the separate cache chip as Level 3 (L3) cache.

The program and data is loaded from the hard drive into RAM. From RAM it is loaded into cache RAM, and from there it is executed by theCPU.Hard drives are very slow compared to the CPU. RAM is much faster than a hard drive, but still 4-5 times slower than your CPU. Cache RAM is extremely fast--it is capable of delivering data at or near the speed of the CPU.Cache RAM and normal RAM are very similar in the way they work. Cache isjust extremly fast, and expensive.That is why there is so very little of cache RAM available--it is expensive.

When the processor needs to read from a location in main memory (RAM), it first checks whether a copy of that data is in the cache. If so, the processor immediately reads from the cache, which is much faster than reading from main memory . If the cache doesn’t have that data, the processor is halted while it is loaded from main memory into the cache.

CACHE READ OPERATION

Most modern desktop and server CPUs have at least three independent caches: an instruction cache to speed up executable instruction fetch, a data cache to speed up data fetch and store, and a translation lookaside buffer used to speed up virtual-to-physical address translation for both executable instructions and data.

Read-only memory (ROM)

There is a type of memory that stores data without electrical current; it is the ROM (Read Only Memory) or is sometimes called non-volatile memory as it is not erased when the system is switched off.

This type of memory lets you stored the data needed to start up the computer. Indeed, this information cannot be stored on the hard disk since the disk parameters (vital for its initialisation) are part of these data which are essential for booting.
Different ROM-type memories contain these essential start-up data, i.e.:

The BIOS is a programme for controlling the system's main input-output interfaces, hence the name BIOS ROM which is sometimes given to the read-only memory chip of the mother board which hosts it.
The bootstrap loader: a programme for loading (random access) memory into the operating system and launching it. This generally seeks the operating system on the floppy drive then on the hard disk, which allows the operating system to be launched from a system floppy disk in the event of malfunction of the system installed on the hard disk.
The CMOS Setup is the screen displayed when the computer starts up and which is used to amend the system parameters (often wrongly referred to as BIOS).
The Power-On Self Test (POST), a programme that runs automatically when the system is booted, thus allowing the system to be tested (this is why the system "counts" the RAM at start-up).

Given that ROM are much slower than RAM memories (access time for a ROM is around 150 ns whereas for SDRAM it is around 10 ns), the instructions given in the ROM are sometimes copied to the RAM at start-up; this is known as shadowing, though is usually referred to as shadow memory).

Types of ROM

ROM memories have gradually evolved from fixed read-only memories to memories than can be programmed and then re-programmed.

ROM

The first ROMs were made using a procedure that directly writes the binary data in a silicon plate using a mask. This procedure is now obsolete.

PROM

PROM (Programmable Read Only Memory) memories were developed at the end of the 70s by a company called Texas Instruments. These memories are chips comprising thousands of fuses (or diodes) that can be "burnt" using a device called a " ROM programmer", applying high voltage (12V) to the memory boxes to be marked. The fuses thus burnt correspond to 0 and the others to 1.

EPROM

EPROM (Erasable Programmable Read Only Memory) memories are PROMs that can be deleted. These chips have a glass panel that lets ultra-violet rays through. When the chip is subjected to ultra-violet rays with a certain wavelength, the fuses are reconstituted, meaning that all the memory bits return to 1. This is why this type of PROM is called erasable.

EEPROM

EEPROM (Electrically Erasable Read Only Memory memories are also erasable PROMs, but unlike EPROMs, they can be erased by a simple electric current, meaning that they can be erased even when they are in position in the computer.

There is a variant of these memories known as flash memories (also Flash ROM or Flash EPROM). Unlike the classic EEPROMs that use 2 to 3 transistors for each bit to be memorised, the EPROM Flash uses only one transistor. Moreover, the EEPROM may be written and read word by word, while the Flash can be erased only in pages (the size of the pages decreases constantly).

Lastly, the Flash memory is denser, meaning that chips containing several hundred mega octets can be produced. EEPROMs are thus used preferably to memorise configuration data and the Flash memory is used for programmable code (IT programmes).

Types of Memorie's

The memory of the computer is basically divided into three; the Cache, the RAM, and the ROM.

* Cache: this is usually pronounced as ‘cash’. It is a small, high-speed memory area that stores the most recently used instructions or data. The basic idea behind the cache memory is to make it possible for the computer to save the most recently used or accessed segments of a program, data, instruction or process, in the Central Processing Unit, where they can be accessed almost instantaneously. Cache is usually identified as L1, L2, or L3, where the ‘L’ stands for levels away from the CPU. The L1 cache is built into the CPU itself, is the fastest possible storage and is quite expensive. The L2 is outside the CPU but on a high-speed, direct connection to the CPU. Levels beyond 2 are rarely found.

* RAM: Random Access Memory (RAM) is the ‘regular’ memory in a computer. RAM is the memory you can buy and insert in your computer and is usually what people mean when they say, “I’ve got to get more memory”. Today most computers have 64 Mbytes (or more) of RAM. RAM comes in dozens of different configurations, types, pin-outs, and other forms.

* ROM: Read Only Memory (ROM) is used to store information about the computer system that never changes. ROM contains for example, the boot-up routines for the computer.

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