Processing Unit. Backing Store

Transcription

1 SYSTEM UNIT Basic Computer Structure Input Unit Central Processing Unit Main Memory Output Unit Backing Store The Central Processing Unit (CPU) is the unit in the computer which operates the whole computer following instructions and performs all logical computations and numerical calculations The main memory (primary memory) stores data temporarily It may accept data from input device and send data to output device Backing store (secondary memory) is also connected with the main memory It stores data permanently Types of Computer Systems Mainframe computer A mainframe (or mainframe computer) is a large and expensive computer system It has a high speed CPU, a large main memory and a host of input/output and backup storage devices Up to 100 or more users can use the computer at the same time through different terminals Minicomputer Minicomputer is smaller than mainframe in size and in processing capacity, for about 10 users to use The hardware organization of minicomputer is similar to that of mainframe Microcomputer A microcomputer is much smaller The whole CPU is built in a small chip called microprocessor Also, microcomputer contains only one terminal So only one user can use it at a time Central Processing Unit (CPU) The CPU is a device to accept and execute sequentially the instructions stored in the computer s main memory The CPU is composed of control unit ( CU ) and arithmetic and logic unit ( ALU ) The CU sends control signals to all parts of the computer The ALU calculates the data from input device or main memory The registers are locations inside CPU where data are held temporarily during calculations or other operations SYSTEM UNIT page 1

2 Inside the CPU, there are special memory cells called registers They are used by the CPU for temporarily storing data or instructions during the execution of an instruction The data bus, address bus and control bus are the main communication links within the CPU Control unit A group of electronic circuits that has the all-important function of controlling all the operations within the CPU It is also the function of the control unit to ensure that all the correct electronic path are set up within computer Arithmetic and Logic Unit (ALU) It carries out all arithmetic and logic operations i Arithmetic operations include, a basic operation : +, -, *, / b comparison : >, <, =, >=, <=, <> ii Logic operations include: NOT, AND, OR, NAND, NOR System clock An accurate electronic timer used for synchronization of all the chips inside the computer It is an electronic system which produces a train of binary pulses which are represented by the pattern etc and represents the square wave as follows One pulse is usually needed to fetch an instruction from memory - it is then decoded automatically by the electronic inside the microprocessor chip - then the next clock pulse might cause the instruction to be executed The speed of the clock is measured in Hz(cycle per second) or GHz A CPU with a higher clock speed should be faster than another one with lower speed if they are of similar architecture Registers i Accumulator (ACC) It is used to accumulate results Thus it is a place where the answers from many operations are stored temporarily before being put out to the computer s memory, for example ii Program counter / sequence control register (PC) a Its job is to keep a count of the place from where the next instruction is to be fetched b Actually, it stores the address of the memory location where the next instruction is located iii Current instruction register (CIR) or instruction register (IR) It stores the most-recently-fetched from memory instruction iv Memory address register (MAR) A register contains the address of the data that is being accessed in main memory (inaccessible to programmers) v Memory data register (MDR) A register holds the data of the last selected word read from, or written to, the main memory (inaccessible to programmers) vi Stack pointer (SP) A register stores the address of the current top element of the stack SYSTEM UNIT page 2

3 vii Process status register A register which is used by control unit as a means of detecting condition which have occurred such as the ALU detecting the arithmetic error of division by zero (The word held in it is known as Processor Status Word, PSW ) B A Instruction decoder Internal CPU bus Instruction decoder Internal CPU bus IR IR PC PC Memory bus Address Data MAR MDR R 0 Memory bus Address Data MAR MDR R 0 M M R (n-1) R (n-1) Y Y ALU control Add Sub M A ALU B A ALU B Z X New 32-bit names 16-bit names Old 8-bit names EAX * AH A X AL Accumulator EBX * BH B X BL Base index ECX * CH C X CL Count EDX * DH D X DL Data 8-bits ESP * SP Stack Pointer EBP * BP Base Pointer EDI * DI Destination Index ESI * SI Source Index 32 bits EIP * IP Instruction Pointer EFLAGS * FLAGS Flags 16 bits CS Code Segment DS Data Segment ES Extra Segment SS Stack Segment FS * GS * No special name Fig 2 The registers in 80x86 SYSTEM UNIT page 3

4 viii General-purpose register sets These registers are used for general-purpose temporary storage They are usually accessible to programmers The general-purpose registers are labelled as R 0 TO R (N-1) where the number, N, and functions vary considerably from one machine to another They may be provided for general-purpose use by the programmer; or dedicated as special-purpose registers, such as index registers or stack pointers ** A stack can be stored in the main memory of a computer with successive elements in the stack occupying successive lower-address memory location SP points to the lowest address of the stack (Index register and many other registers will be discussed later) Memory Organization Each 0 or 1 of data is called a bit Each bit of data is stored in a memory cell 8 bits is called 1 byte 1 KB (kilobyte) means 1024 bytes of data (2 10 ) If a disk is said to 360 KB of disk space, it will be able to store about bytes of data A 8-bit machines means that a word in the computer contains 8 bits or 1 byte of data, stored in 8 memory cell The number of bits in a word is called word length (n-1) Memory address The longer the word length, the more information can be stored in each word On the other hand, the complexity of the electronic circuitry will also increase proportionately The whole memory can be considered as a long array of memory Each element may contain a word of data Each word is accessed by specifying the memory address Address start from 0 to the total number of words in memory minus one eg for 64 KB memory, the address starts from to FFFF 16 RAM and ROM are under the same addressing system word Figure 1 SYSTEM UNIT page 4

5 Machine Codes Machine language is a language in that the instruction are in a form that allows the computer to perform them immediately Machine code The patterns of binary digits that are used by the computer to perform its specific tasks Instruction set The set of instructions that the CPU can perform such as ADD, LOAD and STORE Instruction format The size and arrangement of a machine instruction's components The two major components are the function code ( opcode ) which specifies the function or operation performed, and the operand address, which specifies the locations of the operands used ** Example : An imaginary computer Here is the instruction of the computer : MACHINE CODE (BIN / OCT) FUNCTION (OPERATION) MNEM- ONIC 0000 / 00 LoaD Accumulator from a specified address LDA 0001 / 01 Store contents of Accumulator into a specified address STA 0010 / 02 LoaD specified Number into the accumulator LDN 0011 / 03 ADD contents of a specified address from accumulator contents ADD 0100 / 04 SUBtract contents of specified address from accumulator contents SUB 0101 / 05 ADd specified Number to contents of accumulator ADN 0110 / 06 SUbtract specified Number from contents of accumulator SUN 0111 / 07 perform a boolean AND on specified address and accumulator contents AND 1000 / 10 perform a boolean OR on specified address and accumulator contents OR 1001 / 11 Jump to specified address if Accumulator contents Zero JAZ 1010 / 12 JumP Unconditionally to specified address JPU 1011 / 13 Jump to specified address if Accumulator contents Greater than zero JAG 1100 / 14 Jump to specified address if Accumulator contents Less than zero JAL 1101 / 15 Jump to specified address to the content of Accumulator?? JAN 1110 / 16 perform specified Input / Output operation IO 1111 / 17 STOP the program ie HALT the processor STOP A 16 bit machine The opcode requires 4 bits so 12 bits remaining for operand addresses 1 accumulator In instruction where there is no memory reference, eg I/O operations, the operand address bits may be used to extend the function options * Most real computers have more than 1 instruction format ** A large instruction set implies that the function field of each instruction must have a larger number of bits Thus, i It may not be possible to store one instruction in a single word ii Most microcomputer and minicomputer instruction may each occupy more than one word SYSTEM UNIT page 5

6 FETCH-EXECUTE CYCLE All the sequence takes place under the guidance of the control unit and is timed (or synchronize) by the system clock First assume that each instruction occupies 1 memory word * Notation: [A] means the content stored in A B [A] means to copy the content of A into B Fetch cycle: MAR [PC] MDR [[MAR]] IR [MDR] PC [PC] +1 Decode [IR] (ie the current instruction) Execute cycle: i Load data from memory location 1000 into accumulator MAR 1000 MDR [[MAR]] Ri [MDR] ii Add the number in the accumulator and store the result in the accumulator MAR 1001 MDR [[MAR]] Rj [MDR] Ri [Ri] + [Rj] iii Store the contents of the accumulator in memory location 1002 MAR [Ri] [MAR] [MDR] iv Branch to the instruction at memory location nnnn PC nnnn Decoding Sometimes, the decoding step may be separated, then it becomes fetch-decode-execute cycle Micro-code i Instructions in micro-codes (micro-program) stored inside the instruction decoder ii A single machine code instruction is executed by several micro-code instruction Multi-layered Computer Architecture 7 Application Layer 6 SOFTWARE LEVEL Higher Order Software Layer 5 Operating System Layer 4 Machine Layer 3 HARDWARE LEVEL Microprogrammed Layer 2 Digital Logic Layer 1 Physical Device Layer Fig 4 Multilayer Computer Architecture SYSTEM UNIT page 6

7 Main memory There are two types of main memory, Read-Only Memory (ROM) and Random Access Memory (RAM) RAM ( Random Access Memory ) The part of memory used to store the program and data input to the computer Data in it are addressed directly (randomly) Data can be read from or written to RAM by sending signal to specify the location in memory ( address ) Also known as Read / Write Memory Data in RAM is volatile, ie will disappear if the power supply is turned off Types of RAM: i Dynamic RAM (DRAM) It needs be refreshed every few milliseconds ii Static RAM (SRAM) It needs not be refreshed and works faster It is more expensive and often used as cache memory iii Synchronous DRAM (SDRAM) It improves performance because it is synchronised with the CPU clock iv Double Data Rate Synchronous DRAM (DDR SDRAM) It doubles the speed of SDRAM ROM ( Read-Only Memory ) Data CANNOT be written into a ROM but CAN be read from it (READ-ONLY / non-volatile ) It usually stores firmware (permanently written) such as BIOS (Basic Input / Output System) including the bootstrap loader for system start-up The part of memory used only by the computer itself Information is masked on the ROM permanently while it is manufactured in the factory It is also accessed randomly CMOS ( Complementary Metal-oxide Semiconductor ) It uses very little power to retain information (a small battery for long time) It stores system configuration data such as the type of hard disk, the system date and time and other BIOS setting Data in it is volatile Cache memory It is a small fast memory, too expensive to be used for the whole of RAM, that acts as an intermediate store between the CPU and memory, is used to improve the overall speed of the computer by pre-fetching instructions and data from slower main memory Level 1cache inside CPU / Level 2 cache on the motherboard SYSTEM UNIT page 7

8 Bus Bus is a group of wires interconnecting the various parts of computer system There are 3 kinds of buses: i data bus ii address bus iii control bus Data bus used for the transfer of data between units The routing of information occurs a whole word at a time in a data bus Eg An 8-bit machine sending 8 bits of data at time in a data bus Data bus is often bi-directional Address bus used for carrying the address of the memory location specified by the control unit The number of wires in the address bus depends on the size of the main memory For 2 16 = 64 K memory locations, 16 are necessary Control bus used for carrying the control signals to and from control unit in CPU The number of wires varies between processors but not normally be less than 10 ** Other than classifying buses by function, it is also customary to classify them by the way they are connected to the CPU, such as I/O bus or memory bus Example A 32-bit microcomputer system has a 128 MB memory How many will be most possibly present in the data bus and address bus? No of in the data bus = word length = 32 No of words in memory = No of bytes in memory / No of bytes in a word = 128 MB / 4 bytes = 2 27 / 2 2 = 2 25 No of in the address bus = 25 ** Note that : A memory of 2 n locations needs an address n bits A memory of N locations needs an address of m bits where m is an integer log 2 N Questions : Find the most probable number of in data bus and address bus in the following cases, i A 32-bit byte-addressing microcomputer has 4 M memory ii A 8-bit word-addressing microcomputer has 128 M memory SYSTEM UNIT page 8