MICROPROCESSORS AND MEMORY

Microprocessor Basics

A microprocessor (or a “processor”) is an integrated circuit designed to process instructions. It is the most importantcomponent of a computer. It can be referred to as “a computer on a chip” or “a CPU on a chip” because it contains – on a single chip – circuitry that performs essentially the same tasks as the central processing unit (CPU) of a classic mainframe computer.

A microprocessor is a very complex integrated circuit, containing as many as 400 million miniaturized electronic components. The miniaturized circuitry in a microprocessor is grouped into important functional areas, such as the ALU and the control unit. The ALU (arithmetic logic unit) is the part of the microprocessor that performs arithmetic operations, such as addition and subtraction. It also performs logical operations, such as comparing two numbers to see if they are the same. The ALU uses registers to hold data that is being processed, and the microprocessor’s control unit fetches the necessary instructions. After the computer loads data into the ALU’s registers, the control unit allows the ALU to begin processing.

A microprocessor executes instructions provided by a computer program. The list of instructions that a microprocessor can perform is called its instruction set. These instructions are hard-wired into the processor’s circuitry and include basic arithmetic and logical operations, fetching data, and clearing registers. A computer can perform very complex tasks, but it does so by performing a combination of simple tasks from its instruction set.

Microprocessor Performance Factors

A microprocessor’s performance is affected by several factors, including clock speed, word size, cache size, instruction set, and processing techniques.

The microprocessor clock is a timing device that sets the pace for executing instructions. The speed of a microprocessor is usually specified in megahertz(MHz) gigahertz. A cycle is the smallest unit of time in a microprocessor’s universe. Every action a processor performs is measured by these cycles. The clock speed is not equal to the number of instructions a processor can execute in one second. In many computers, some instructions occur within one cycle, but other instructions might require multiple cycles. Some processors can execute several instructions in a single clock cycle.

Word size refers to the number of bits that a microprocessor can manipulate at one time. Word size is based on the size of registers in the ALU and the capacity of circuits that lead to those registers. A processor with a 32-bit word size, for example, has 32-bit registers, processes 32 bits at a time, and is referred to as a “32-bit processor”. Processors with a larger word size can process more data during each processor cycle that leads to increased computer performance.

Cache or “RAM cache” or “cache memory” is special high-speed memory that allows a microprocessor to access data more rapidly than from memory located elsewhere on the system board. Cache capacity is usually measured in kilobytes.

As chip designers developed various instruction sets for microprocessors, they tended to add increasingly complex instructions, each requiring several clock cycles for execution. A microprocessor with such an instruction set uses CISC (complex instruction set computer) technology. A microprocessor with a limited set of simple instructions uses RISC (reduced instruction set computer) technology. A RISC processor performs most instructions faster than a CISC processor. It might, however, require more of these simple instructions to complete a task than a CISC processor requires for the same task. Most processors in today’s personal computer use CISC technology.

Some processors execute instruction “serially” – that is, one instruction at a time. With serial processing, the processor must complete all steps in the instruction cycle before it begins to execute the next instruction. However, using a technology called pipelining, a processor can begin executing an instruction before it completes the previous instruction. Many of today’s microprocessors also perform parallel processing, in which multiple instructions are executed at the same time. Pipelining and parallel processing enhance processor performance.

Some computers have a single chip containing the circuitry for two microprocessors. A dual core processor is faster than one with a single core.

Various testing laboratories run a series of tests to gauge the overall speed of a microprocessor. The results of these tests – called benchmarks– can then be compared to the results for other microprocessors.

Random Access Memory

RAM(random access memory) is a temporary holding area for data, application program instructions, and the operating system. RAM is usually several chips or small circuit boards that plug into the system board within the computer’s system unit. RAM is the “waiting room” for the computer’s processor. It holds raw data waiting to be processed as well as the program instructions for processing that data.

RAM also holds the results of processing until they can be stored more permanently on disk or tape, it also holds data and application software instructions, operating system instructions that control the basic functions of a computer system. These instructions are loaded into RAM every time you start your computer, and they remain there until you turn off your computer.

People sometimes tend to confuse RAM and hard-disk storage, maybe because both components hold data, because they typically are “hidden” inside the system unit, or because they can both be measured in gigabytes. To differentiate between RAM and hard-disk storage, remember that RAM holds data in circuitry that’s directly connected to the system board, whereas hard-disk storage places data on magnetic media. RAM is temporary storage; hard-disk storage is more permanent. Besides, RAM usually has less storage capacity than hard-disk storage.

In RAM, microscopic electronic parts, called capacitorshold the bits that represent data. You can visualize the capacitors as microscopic lights that can be turned on and off. A charged capacitor is “turned on” and represents a”1” bit. A discharged capacitor is “turned off” and represents a “0” bit. Each bank of capacitors holds eight bits – one byte of data. A RAM address on each bank helps the computer locate data as needed, for processing.

Each RAM location has an address and uses eight capacitors to hold the eight bits that represent a byte. See Figure 1:

Fig. 1: The way RAM represents different characters

In some respects, RAM is similar to a chalkboard. You can use a chalkboard to write mathematical formulas, erase them, and then write an outline for a report. RAM contents can be changed just by changing the charge of the capacitors. Unlike disk storage, most RAM is volatile, which means it requires electrical power to hold data. If the computer is turned off or the power goes out, all data stored in RAM instantly and permanently disappears.

Today’s personal computer operating systems are quite adept at allocation RAM space to multiple programs. If a program exceeds its allocated space, the operating system uses an area of the hard disk, called virtual memory,to store parts of programs or data files until they are needed. By selectively exchanging the data in RAM with the data in virtual memory, your computer effectively gains almost unlimited memory capacity.

RAM components vary in speed, technology, and configuration. RAM speed is often expressed in nanoseconds or megahertz. Onenanosecond (ns) is 1 billionth of a second. RAM speed can also be expressed in MHz (millions of cycles per second). Most of today’s personal computers use SDRAM (synchronous dynamic RAM), fast and relatively inexpensive, it is typically available on a small circuit board called a DIMM (dual inline memory module) or RDRAM (Rambus dynamic RAM), first developed for a game system and then adapted for use in personal computers.

Read-Only Memory

ROM (read-only memory) is a type of memory circuitry that holds the computer’s startup routine. Whereas RAM is temporary and volatile, ROM is permanent and non-volatile. ROM circuitry holds “hard-wired” instructions that are a permanent part of the circuitry and remain in place even when the computer power is turned off.

When you turn on your computer, the microprocessor receives electrical power and is ready to begin executing instructions. As a result of the power being off, however, RAM is empty and doesn’t contain any instructions for the microprocessor to execute. Now ROM plays its part. ROM contains a small set of instruction called the ROM BIOS (basic input/output system).These instructions tell the computer how to access the hard disk, find the operating system, and load it into RAM. After the operating system is loaded, the computer can understand your input, display output, run software, and access your data.

CMOS Memory

To operate correctly, a computer must have some basic information about storage, memory, and display configurations. The information is held in CMOS (pronounced “SEE moss”), a type of chip that requires very little power to hold data. It can be powered by a small battery that is integrated into the system board and automatically recharges while your computer power is on. The battery trickles power to the CMOS chip so that it can retain vital data about your computer system configuration even when your computer is turned off. When you change the configuration of your computer system, the data in CMOS must be updated.

The more data and programs that can fit into RAM, the less time your computer will spend moving data to and from virtual memory.

Comprehension check. Find the paragraph where the following ideas are found in the text.

1. It is the most significant component of the computer.

2. It executes logical operations, as well as arithmetic operations.

3. With this type of processing the processor performs one instruction at a time.

4. The results of the tests must match the results for model microprocessors.

5. If the allotted space is surpassed, the operating system employs an area of the hard disk.

6. The chip can hold the vital data about the configuration of your computer even when there is no energy supply.

Date: 2016-01-03; view: 1316