machine cycle, the CPU: (a) Fetches an instruction, (b) Decodes the instruction, (c) Executes the instruction, and (d) Stores the result.

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1 Central Processing Unit (CPU) A processor is also called the CPU, and it works hand in hand with other circuits known as main memory to carry out processing. The CPU is the "brain" of the computer; it follows the instructions of the software (program) to manipulate data into information. The CPU consists of three parts:- (1) The control unit (CU) (2) the arithmetic/logic unit (ALU), which both contains (3) registers, or high-speed storage areas. All are linked by a kind of electronic "roadway" called a bus. The control unit - for directing electronic signals: The control unit deciphers each instruction stored in it and then carries out the instruction. It directs the movement of electronic signals between main memory and the arithmetic/logic unit. It also directs these electronic signals between main memory and the input and output devices. For every instruction, the control unit carries out four basic operations, known as the machine cycle. In the machine cycle, the CPU: (a) Fetches an instruction, (b) Decodes the instruction, (c) Executes the instruction, and (d) Stores the result. 19

2 The arithmetic/logic unit -for arithmetic and logical operations: The arithmetic/logic unit (ALU) performs arithmetic operations and logical operations and controls the speed of those operations. As you might guess, arithmetic operations are the fundamental math operations: addition, subtraction, multiplication, and division. Logical operations are comparisons. That is, the ALU compares two pieces of data to see whether one is equal to (=), greater than (>), or less than (<) the other. (The comparisons can also be combined, as in "greater than or equal to" and "less than or equal to.") Registers -special high-speed storage areas: The control unit and the ALU also use registers, special areas that enhance the computer's performance. Registers are high-speed storage areas that temporarily store data during processing. They may store a program instruction while it is being decoded, store data while it is being processed by the ALU, or store the results of a calculation. The most important registers are: - 1. Program Counter (PC): It contains the address of the next instruction to be executed. 2. Instruction Register (IR): It contains the instruction being executed. 3. Address Register (AR) or memory Address Register (MAR): holds the address of memory location. 4. Accumulator (A): it is register that holds the data to be used in arithmetic and logic operation and also holds the results of processing operations. 5. (B, C, D, E, F, H, and L) register: these registers contain the data to execute the operation. 20

3 Buses -data roadways: Buses or bus lines are electrical data roadways through which bits are transmitted within the CPU and between the CPU and other components of the motherboard. A bus resembles a multilane highway: The more lanes it has, the faster the bits can be transferred. The oldfashioned 8-bit-word bus of early microprocessors had only eight pathways. Data is transmitted four times faster in a computer with a 32-bit bus, which has 32 pathways, than in a computer with an 8-bit bus. There are several principal expansion bus standards, or "architectures," for microcomputers. The various components of the CPU are connected together with lines called Internal lines (Internal Buses). The lines connected the CPU to the reminder of the computer components are called External lines (External Buses). A computer main memory is interfaced to the CPU through three groups of lines or buses: Data bus, Address bus, Control bus. Memory The memory unit is an essential component in any digital computer since it is needed for storing programs and data. A memory unit is a collection of storage cells together with associated circuits needed to transfer information in and out of storage. The memory stores binary information in groups of bits called words. A word in a memory is an entity of bits that move in and out of storage as a unit. A memory word is a group of 1 s and 0 s and may represent a number, an instruction code, one or more alphanumeric characters, or any other binary-coded information. Most computer memories use words whose number of bits is a multiple of 8. Thus a 16-bit word contains two bytes, and 32bit word is made up of four bytes. The capacity of memories in commercial computers is usually stated as the total number of bytes that can be stored. The number of words in the memory and the number of bits in each word specify the internal structure of a memory unit. 21

4 Special input lines called address lines select one particular word. Each word in a memory is assigned an identification number, called an address, starting from 0 and continuing with 1, 2, 3, up to 2 k - 1 where k is the number of address lines. The selection of a specific word inside the memory is done by applying the k-bit binary address to the address lines. A decoder inside the memory accepts this address and opens the paths needed to select the bits of the specified word. Computer memories may range from 1024 words, requiring an address of 10 bits, to 4G words, requiring 32 address bits. The memory components of a computer system can be divided into four main groups:- 1. Internal processor memory: This comprises a small set of highspeed registers used as a working memory for temporary storage of instructions and data. 2. Main memory (also called primary memory): This is a relatively large fast memory used for program and data storage during computer operation. It is characterized by the fact that locations in main memory can be accessed directly and rapidly by the CPU instruction set. The principle technology used for main memory is based on semiconductor integrated circuits (ICs). 3. Secondary memory (also called auxiliary memory): This is generally much larger in capacity but also much slower than main memory. It is used for storing system programs, large data files, and the like which are not continually required by the CPU; it is also serves as an overflow memory when the capacity of the main memory is exceeded. Information in secondary storage is accessed indirectly via inputoutput programs that first transfer the required information to main memory. Representative technologies used for secondary memory are magnetic disks and tapes. 22

5 4. Cache Memory, which serves as an intermediate temporary storage unit logically positioned between the processor registers and main memory. Unlike the other memory levels mentioned above, caches are transparent to the programmer. The total memory capacity of a computer can be visualized as being a hierarchy of components. The memory hierarchy system consists of all storage devices employed in a computer system from the slow but highcapacity auxiliary (secondary) memory to a relative faster main memory, to an even smaller and faster cache memory accessible to the high-speed processing logic. The reason of having two or three levels of memory hierarchy is economics. As the storage capacity of the memory increases, the cost per bit for storing binary information decreases and the access time of the memory becomes longer. Figure below illustrates the components in a typical memory hierarchy. Memory hierarchy in a computer system In addition, most current computer operating systems allow for the use of virtual memory -that is, some free hard-disk space is used to extend the 23

6 capacity of RAM. First L1, then L2, then RAM, then hard disk (or CD- ROM)-that is the order in which the processor goes about searching for data or program instructions. In this progression, each kind of memory or storage is slower than its predecessor. Primary storage Is temporary or working storage and is often called memory or main memory. The four principal types of memory chips are RAM, ROM, CMOS, and flash. 1. RAM chips -to temporarily store program instructions and data: RAM chips-ram stands for random access memory-are for primary storage; they temporarily hold (1) software instructions and (2) data before and after it is processed by the CPU. Because its contents are temporary, RAM is said to be volatile-the contents are lost when the power is turned off. This is why you should frequently-every 5-10 minutes, say transfer (save) your work to a secondary-storage medium such as your hard disk, in case the electricity goes off while you're working. Two types of RAM chips used in personal computers: DRAM (dynamic RAM) must be constantly refreshed by the CPU or it will lose its contents. i. SDRAM (synchronous DRAM) is the most popular type of DRAM. SDRAM has a synchronous interface, meaning that it waits for a clock signal before responding to its control inputs. It is synchronized with the computer's system bus, and thus with the processor. 24

7 168-pin SDRAM ii. DDR-SDRAM (Double Data Rate SDRAM) (sometimes called "DDR1" for greater clarity) doubles the minimum read or write unit; every access refers to at least two consecutive words. Typical DDR-SDRAM clock speeds are 133, 166 and 200 MHz (7.5, 6, and 5 ns/cycle), generally described as DDR-266, DDR-333 and DDR- 400 (3.75, 3, and 2.5 ns per beat). Corresponding 184-pin DIMMS are known as PC2100, PC2700 and PC3200. Speeds up to DDR-550 (PC4400) are available. 184-pin DDR iii. DDR2-SDRAM is very similar to DDR-SDRAM, but doubles the minimum read or write unit again, to 4 consecutive words. This allows the bus speed of the SDRAM to be doubled without increasing the speed of internal RAM operations; instead, internal operations are performed in units 4 times as wide as SDRAM. Typical DDR2-SDRAM clock speeds are 200, 266, 333 or 400 MHz (5, 3.75, 3 and 2.5 ns/cycle), generally described as 25

8 DDR2-400, DDR2-533, DDR2-667 and DDR2-800 (2.5, 1.875, 1.5 and 1.25 ns per beat). Corresponding 240-pin DIMMS are known as PC through PC Speeds up to DDR (PC ) are available. 240-pin DDR2 iv. DDR3-SDRAM continues the trend, doubling the minimum read or write unit to 8 consecutive words. This allows another doubling of bandwidth and external bus speed without having to change the speed of internal operations, just the width. SRAM (static RAM) is faster than any DRAM and will retain its contents without having to be refreshed by the CPU. If you're short on memory capacity, you can usually add more RAM chips by plugging them into the motherboard. Chips can be bought single or in so-called memory modules, circuit boards that can be plugged into expansion slots on the motherboard. There are two types of such modules: SIMMs and DIMMS, both of which use DRAM chips. A SIMM (single inline memory module) has RAM chips on only one side. A DIMM (dual inline memory module) has RAM chips on both sides. 26

9 2. ROM chips - to store fixed start-up instructions: Unlike RAM, to which data is constantly being added and removed, ROM (read-only memory) cannot be written on or erased by the computer user without special equipment. ROM chips contain fixed start-up instructions. That is, ROM chips are loaded, at the factory, with programs containing special instructions for basic computer operations, such as those that start the computer. These chips are nonvolatile; their contents are not lost when power to the computer is turned off. 3. CMOS chips - to store flexible start-up instruction: Pronounced "seemoss," CMOS (complementary metal-oxide semiconductor) chips are powered by a battery and thus don't lose their contents when the power is turned off. CMOS chips contain flexible start-up instructions, such as time, date, and calendar, which must be kept current even when the computer is turned off. Unlike ROM chips, CMOS chips can be reprogrammed, as when you need to change the time for daylight savings time. 4. Flash memory chips-to store flexible programs: Also a nonvolatile form of memory, flash memory chips can be erased and reprogrammed more than once (unlike PROM chips, which can be programmed only once). Flash memory, which can range up to 32 gigabytes in capacity, is used to store programs not only in personal computers but also in cell phones, printers, and digital cameras. 27

10 How Cache Works: Pronounced "kash," cache is a special very high-speed memory sometimes used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate. The cache memory is employed in computer systems to compensate for the speed differential between main memory access time and processor logic. CPU logic is usually faster than main memory access time, with the result that processing speed is limited primarily by the speed of main memory. A technique used to compensate for the mismatch in operating speeds is to employ an extremely fast, small cache between the CPU and main memory whose access time is close to processor logic clock cycle time. The cache is used for storing segments of programs currently being executed in the CPU and temporary data frequently needed in the present calculations. By making programs and data available at a rapid rate, it is possible to increase the performance rate of the computer. While the I/O processor manages data transfers between auxiliary memory and main memory, the cache organization is concerned with the transfer of information between main memory and CPU. Thus each is involved with a different level in the memory hierarchy system. There are two kinds of cache-level 1 and Level 2: (1) Level 1 (L1) cache-part of the microprocessor chip: Level 1 (L1) cache, also called internal cache, is built into the processor chip. Ranging from 8 to 512 kilobytes, its capacity is less than that of Level 2 cache, although it operates faster. (2) Level 2 (L2) cache-not part of the microprocessor chip: This is the kind of cache usually referred to in computer ads. Level 2 (L2) cache, also called 28

11 external cache, resides outside the processor chip and consists of SRAM chips. Capacities range from 64 kilobytes to 2 megabytes. (Intel ads call L2 Advanced Transfer Cache.) 29