MACHINE ARCHITECTURE In its simplest form, a computer consists of five basic functional units: – the control unit, the arithmetic and logic unit, memory unit, the input unit and the output unit. INPUT CONTROL UNIT MEMORY UNIT UNIT OUTPUT ARITHMETIC UNIT & LOGIC UNIT I/O PROCESSOR
BASIC FUNCTIONAL UNITS OF COMPUTER Input Unit: is the means by which computers can accept coded information. Memory Unit: its function is to store programs and data that are currently in use. Semiconductor storage cells are used for primary storage. Arithmetic and Logic Unit (ALU): Execution of most operations within a computer takes place in the ALU. e.g. the addition of two numbers currently in main memory Output Unit: Its function is to return processed results to the outside world. Control Unit: Its function is to co-ordinate the operations of the other units in an organized way.
OPERATION OF A COMPUTERThe operation of a computer can be briefly described as follows: It accepts information (programs and data) through an input unit and transfers it to main memory Information stored in memory is fetched under program control into an ALU to be processed Processed information leaves the computer through an output unit The control unit directs all activities inside the machine. The system clock generates a continuous sequence of clock pulses to step the control unit through its operation.
OVERALL BLOCK STRUCTURE OF A COMPUTER Control System External Unit Internal clock Address/ Address/data Data Bus buses Arithmetic & Logic Unit Memory Unit Input/Output Input port Unit Output portThe functional units are connected by BUSES and control links. The latter are notshown on the above diagram.
WHAT IS A BUS A bus is a pathway along which data, addresses and control signals pass within the computer. Physically a bus is a parallel group of wires, usually 16, 32, 64 … bits wide. The capacity of a bus is determined by how many bits can travel through the circuitry at one time. The greater the capacity of a bus, the more powerful and faster the operation. The speed of the bus is another factor that influences the processing power of a computer To represent a bus we usually use the large arrow as shown in the diagram below. (i) (ii) Bus 16 bits wide
TYPES OF BUSES A data bus is used in order to transfer data from one functional unit to the other (say between the CPU and memory) inside the computer. The data bus is two way. The width of the data bus determines how many bits can be transferred simultaneously. It is usually (but not always) the same as the word size. o The word size of a computer is the number of bits that the CPU can process simultaneously. Word size may be 8, 16,32, 64, and processed as a single unit during input and output, arithmetic and logic instructions. Word size is an important factor in determining the speed of a processor. An address bus is used to transfer addresses from the control unit to other parts of the computer (say from CPU to main memory). The address bus is one way only. The width of the address bus determines the maximum address that can be directly referenced. Example: Given an 8-bit address bus, you can directly address 256 locations 0000 0000 – 1111 1111 (0-255) The control bus carries control signals to different parts of the computer such as ‗read‘, ‗write‘ (say from CPU to main memory). System bus is a general term for the bus system connecting the CPU to main memory.
BANDWIDTH AND BUS SYSTEMS The bandwidth of the bus, measured in bits/sec, signifies the number of bits that can be transmitted along the bus. There are several ways in which this physical limitation and others of a bus system may be overcome in order to improve system performance: Increase the bus system bandwidth The use of caching technique Multiple buses. Apart from the added buses this would require extra hardware so that a processor may switch from one bus to another according to which bus is not in use at the time.
THE SYSTEM CLOCK The System clock also called Control Pulse Generator. Processors have an internal clock which generates regularly timed pulses in order to synchronize the various operations being carried out inside the computer. Clock pulses Clock pulseThe clock is an electronic system which produces a train of binary pulseswhich represent the pattern 01010101 . Each clock pulse represents onecycle of the square wave as shown above.
CLOCK SPEED All processor activities, such as fetching an instruction, reading a data item from main memory into the CPU, etc., must begin on a clock pulse. Some activities may take more than one clock pulse to complete. The speed (frequency) of the clock is measured in Hz (hertz), i.e. cycles per second. Say, given an 800MHz Pentium processor, 800MHz = 800 * 106 Hz = 800 000 000 cycles per second i.e. the processor is being told to do different things at a rate of 800 million times per second Hence, the clock speed is one of the factors that influence the processing power of a computer.
MAIN MEMORY ORGANISATION Main memory is used to store data. Data may represent instructions, ASCII character codes, numeric codes … a host of different data codes. A single storage location is called a register. REGISTER n bits Storage consists of an array of such registers. Say, a 1 Mb memory bank with 8-bit registers: 1Mb = 1024*1024 bytes (220) = 1048576 bytes hence 1048576 memory registers of 1 byte each. Furthermore, a 20-bit address is required to address all registers directly.
MEMORY MAP Physically the storage used inside computer unit is of the semiconductor type. Different types of memory chips exist and are in use. Addresses (Hex) The main memory of a computer consists of Operating FFF001 FFFFF a mixture of semiconductor types. Different parts of main memory are used for specific system purposes. How the memory of the computer is utilized, is called the MEMORY MAP and is User 0F4241 FFF000 usually dictated by the operating system. programs Below is a simplistic example of a memory map. Operating 010000 – 0F4240 In order to be able to store and retrieve System information in memory locations, each location is identified by means of an identification number called ADDRESS. BIOS 000000 – 00FFFF Knowing the address, a memory location may be accessed immediately – hence random access (memory). We usually denote memory addresses in hex.
MEMORY CYCLE In order to read an item from store to the CPU, the address of the item is sent across to main memory via the address bus and the read signal is sent via the control bus so that the required item is retrieved into the CPU via the data bus. Similarly, in order to write an item to store, the address where to store the data item is sent across the address bus, the item to be stored is sent via the data bus, while the write signal is sent via the control bus. One sequence for reading/writing is called a MEMORY CYCLE. The complexity of an operation often depends on the number of memory cycles involved. The duration of a memory cycle is a determining factor of the overall speed of a computer.
INPUT/OUTPUT ORGANIZATIONBelow is a more elaborated block diagram of the computer.Most microcomputers use a single bus arrangement as shown below. Data bus RAM ROM VDU & HardDisk Printer Digital A/D CPU keyboard CD ROM Plotter I/O D/A Process Scanner controlSystem clock Address bus Control bus is not shown →—
HANDLING INPUT-OUTPUT The processor, memory and I/O devices are connected to the bus, which consists of three sets of lines used to carry address, data and control signals. Each I/O device is assigned a unique address. When the processor places a particular address on the address lines, the device that recognizes its address responds to the commands issued on the control lines. When I/O devices and the memory share the same address space, the arrangement is called MEMORY-MAPPED I/O. An alternative way of handling input-output is to have special instructions to perform I/O transfers and separate address space for I/O devices – ISOLATED I/O.
I/O INTERFACE The diagram below illustrates the I/O interface for an input device. Address lines Bus Data lines Control lines Address Control Data & status I/O decoder circuits registers Interface Input device In order to input data, the address decoder identifies its own address on the address bus and the read signal on the control line; the input device sends data to the data register and sets the status register to signal that the input is complete. The data is then copied from the input data register into the computer. A similar procedure is used to output data to an output device.
THE CENTRAL PROCESSING UNIT (PROCESSOR) Traditionally, in microcomputers the CPU or processor is contained on a single integrated circuit, and is known as a microprocessor. In larger computer systems, the term CPU is often referred to as the main processing unit and houses a number of processors. Nowadays, the processor of a personal computer may consist of a number of execution units or ‗cores‘ – dual core technology. More execution cores implies more processing power but it does not necessarily mean more speed. The control unit and arithmetic unit together, form the central processing unit (CPU). Popularly, the clock speed of the chip is quoted as CPU speed measurement; but the count of instructions processed per second (MIPS, BIPS, TIPS for Millions, Billions or Trillions of Instructions processed per Second) gives a better indication of CPU speed. The Arithmetic and logic unit is that part of the CPU where the actual manipulation of the data takes place. The task of the Control Unit is to direct the step-by-step workings of the processor as it carries out each instruction of a program
TASK OF CONTROL UNIT As already stated, the task of the Control Unit is to direct the step-by-step workings of the processor as it carries out each instruction of a program i.e. to control the sequence in which the instructions are executed to control access to the main store of the computer to regulate the timing of all operations carried out within the processor to send control signals to, and receive control signals from peripheral devices The CPU contains circuitry and registers to enable it to carry out its tasks.
CPU REGISTERS (1)The block diagram below represents the CPU with some of the registersinvolved. Note that some of the registers have a special purpose while othersare general purpose storage registers. Arithmetic and Instruction R0 Logic Unit Register (CIR) R1 R2 Accumulator Status Register R3 R4 R5 Program Counter (PC) Control Unit Memory address Memory Data Register (MAR) Register (MDR) External bus Memory
CPU REGISTERS (2) The program counter (PC) or Sequence Control register contains the address of the next program instruction to be fetched. It determines the sequence in which the program instructions are to be executed. After an instruction is fetched from main store, the content of the program counter is increased ready for the next instruction. Depending on the length of the current instruction, 1 or 2 or is added to the program counter in order to reset it. If an instruction transfers control to another part of the program, the address to which control is transferred, is loaded into the program counter. The Current Instruction Register (CIR) stores a copy of the current program instruction. The register is connected to a decoder that connects the control switches at various points throughout the processor according to the instruction in the instruction register.
CPU REGISTERS (3) The Memory Address Register (MAR) holds the address of the memory location currently being accessed (i.e. from which data is being read or data is being written. The Memory Data Register (MDR) or Memory Buffer Register (MBR) is used to temporarily store data read or written to main memory. (Note that, at any moment it time, it holds the data that was read from or written to main memory the last time that this read/write operation was carried out.)
CPU REGISTERS (4) The Status Register (SR) contains bits that are set or cleared based on the result of an instruction. Different bits within this register represent different flags or status bits (sometimes also known as condition codes) such as: Zero (Z) is set to 1 if the output from the current operation is 0 Negative (N) is set to 1 if the output from the current instruction is negative. Carry (C) is set to 1 if there is a carry to the MSB during addition or shift Overflow (O) is set to 1 if there is an overflow when an arithmetic operation is carried out The values of these flags are used to control the program. The Accumulator is the principal ‗working area‘ of the computer. It stores the data item currently being processed.
CPU REGISTERS (5) The general-purpose registers are used for performing arithmetic functions. In some computers there is only one general-purpose register i.e. the accumulator. Other computers have a number of general-purpose registers. In some processors the program counter, instruction register, are not dedicated registers but general purpose registers in the CPU and each may be used as PC, CIR,Other special purpose registers found within the CPU: The Stack Pointer (SP) stores the current address of the top of (system) stack. Some uses of stack are: When execution of a program is interrupted, the status of the current program and the current contents of all the registers are saved on the stack and the stack pointer updated. Stores intermediate results of arithmetic operations. Holds return address (contents of program counter) and parameter information when subroutines are called. Index Register is used to implement a particular mode of memory addressing called indexed addressing. The index register holds the base address of an array of locations. In order to access a specific location of the array the offset is added to the base address. This is ideal for handling array structures.
ARITHMETIC AND LOGIC CIRCUITS (1)The ALU holds a number of arithmetic and logic circuits. These circuitscarry out various operations on one or two data items.ADD a pair of data items / increment by one a single data item A NOT A NOT operates on a single operand 0 1 Hence, NOT(1010 0110) 0101 1001 1 0 AND operates on two operands A B A AND B Hence, 0 0 0 AND (1010 0110, 1111 0000) 1010 0000 0 1 0 1 0 0 1 1 1
ARITHMETIC AND LOGIC CIRCUITS (2) A B A OR BOR operates on two operands 0 0 0Hence, OR(1010 0110, 1111 0000) 1111 0110 0 1 1 1 0 1 1 1 1 XOR (Exclusive OR) operates on two operands A B A XOR B Hence, XOR(1010 0110, 1111 0000) 0101 0110 0 0 0 0 1 1 1 0 1 1 1 0
ARITHMETIC AND LOGIC CIRCUITS (3) LOGICAL SHIFT operates on a single operand Logical SHIFT LEFT moves the individual bits of the operand one place to the left. The leftmost bit is lost and ‗0‘ is added as the rightmost bit Hence, Logical Shift Left(1010 0110) 0100 1100 Similarly Logical Shift Right(1010 0110) 0101 0011 ARITHMETIC SHIFT operates on a single operand Arithmetic SHIFT LEFT moves the individual bits of the operand one place to the left. The leftmost bit is lost and ‗0‘ is added as the second rightmost bit. Note that in arithmetic shifts the sign bit(MSB) must be preserved. Hence, Arithmetic Shift Left(1010 0110) 1100 1100 lost / Similarly Logical Shift Right(1010 0110) 1001 0011 lost If the shift operations are through the carry bit then the ‗lost bit‘ is copied into the carry bit.
THE FETCH-EXECUTE CYCLE (1) The Fetch-Execute cycle is also known as the Instruction Cycle. The sequence of operations involved in executing an instruction can be subdivided into two phases – the fetch cycle and the execution cycle. The fetch-execute cycle involves the following steps:• (Fetch phase) The address of the next instruction is copied from the PC to the MAR The instruction held at the address is copied to the MDR. At the same time the content of the PC is incremented so that it holds the address of the next instruction. The contents of the MDR are copied to the CIR• (Execute phase) The instruction held in the CIR is decoded The instruction is executed
THE FETCH-EXECUTE CYCLE (2) START Any instructions to N execute? Y Fetch next instruction Decode instruction Execute instruction N Any interrupts to be processed? YTransfer control to interrupt handling program
OTHER PROCESSOR FUNCTIONS In addition to fetching and executing program sequence instructions, the CPU has to supervise other operations such as data transfers between input/output devices and main memory. When an I/O device needs to transfer data, it generates an interrupt and the CPU suspends execution of the program and transfers control to an appropriate interrupt handling program. A test for the presence of interrupts is carried out at the end of each instruction cycle.
INTERRUPTS An interrupt is a signal from some device or source that causes the running program to be suspended. The interrupt signal is sent along one or more interrupt lines (part of the control bus) to the processor. Common causes of interrupt: input and output of data e.g. to signal to the processor that input required is complete timed interrupts e.g.: in a time-sharing system, user process is interrupted its time-slice. Error detection in a program e.g. division by zero, type mismatch. Malfunctioning of hardware e.g. memory violation error
INTERRUPT HANDLINGWhen an interrupt occurs: The instruction cycle is completed The current contents of all the processor registers are saved The source of interrupt is identified Interrupts of lower priority are disabled Initiation and execution of relevant interrupt servicing routine Interrupts are enabled The interrupted program is resumed from the point at which it was interrupted.
DETERMINATION OF SOURCE OF INTERRUPT (1)Determination of the source of interrupt may be implemented invarious ways:Directly by hardwarei.e. n interrupt lines required to identify 2n different sources ofinterruptSoftware identification method by pollingA skip chain as follows polls each different source Service routine for Source 1? source 1 Source 2? Service routine for source 2
DETERMINATION OF SOURCE OF INTERRUPT (2) Most often a combination of hardware and software is used in order to identify different sources of interrupt. In the CPU there is an interrupt register. Each bit of this register represents a different type of interrupt. When an interrupt occurs one of the bits is set to 1. The processor regularly checks (after each instruction cycle) the interrupt register to see if any interrupts have occurred. If a bit is set, the interrupt is identified and serviced.
PRIORITY OF INTERRUPTS Some interrupts such as hardware failure must be dealt with immediately while others may be temporarily ignored. Interrupts are assigned priorities so that when two interrupts are received simultaneously, the one with higher priority is dealt with first. Only an interrupt of higher priority may interrupt the servicing of another interrupt.
SOME FEATURES TO IMPROVE PERFORMANCESome machine architectural features which are intended to improve the performance of the computer are: Pipelining – single-pipeline architecture…multi-pipeline architecture Multi-processor architecture Bus systems and bandwidth (refer to earlier slides) Cache Memory (refer to OS course module) Direct Memory Access (DMA)
PIPE-LINING Pictorial illustration of single pipe-line architecture If the next instruction in the fetch decode execute queue is not the required one instruction 1 then the pipe-line is ‗flushed‘ and the process started again instruction 2 instruction 1 with the required instruction. instruction 3 instruction 2 instruction 1 Multiple pipe-line architecture signifies a number of pipe- instruction 4 instruction 3 instruction 2 lines working in parallel. If IN OUT the instructions being carried wrong instruction – flush pipeline! out in parallel are not connected in any way, this instruction 17 will result in a much faster execution rate. instruction 18 instruction 17 instruction 19 instruction 18 instruction 17
MULTI-PROCESSOR ARCHITECTURE Multiple-processor architectures employ a number of processors to work together ‗as a team‘. Different processors may be housed inside the same chip, say dual-core (quad core …) technology. There exists a whole spectrum of multi-processor architectures ranging from very tightly coupled systems sharing the same buses and memory, to massive parallel architectures where each processor uses multiple communication lines to connect to other processors in the system. CPU 1 CPU 2 CPU 3 CPU 4 Main memory
DIRECT MEMORY ACCESS The Direct Memory Access (DMA) technique is used for servicing high-speed peripherals such as the hard-disk, hence avoiding continuous intervention by the processor. DMA transfers are performed by a control circuit – the DMA controller, associated with the I/O device. For each word transferred, the DMA controller performs the functions normally performed by the processor when accessing memory, DMA i.e. for each word transferred it Controller must provide the memory Disk address and all the bus signals which control the transfer. Bus CPU Main memory
DMA CONTROLLER <starting address> <status and control> IRQ – Interrupt Request flag IE- Interrupt enable flag R/W – Read/Write flag <word count> IRQ done Done – Ready flag IE R/W DMA controller Although the DMA controller can perform data transfer without intervention from the processor, its operation is under processor control. To initiate block transfer under DMA control the processor sends the following data to the controller: the starting memory address, the number of words in the block and the direction of transfer. The DMA controller then handles the transfer. Memory accesses by the processor and the DMA controller are interwoven. Since the processor originates most memory access cycles, the DMA controller is said to ―steal‖ memory cycles from the processor and hence this interleaving technique is known as cycle stealing. When the entire block is transferred the DMA controller informs the processor by raising an interrupt signal.
MACHINE OPERATION The instructions, which control the step-by-step workings of a processor, (the Instruction Set) are in a language called Machine Language. The machine language instructions are closely related to the architecture of a computer. There is one instruction for each operation directly performed by the hardware of the computer. Different processors in general have different instruction sets.
INSTRUCTION TYPESIn a typical instruction set, you would find the following types of instructions: Data transfer instructions e.g MOV AX, BX; move contents of register BX to register AX Arithmetic operations e.g. SUB AX, BX; subtract the contents of BX from the contents of register AX Logical operations e.g. AND AX, 0001H; bit-wise AND between register AX and 000116 Test and branch instructions e.g. JG label1; branch to instruction labelled ‗label1‘ if the (preceding) comparison result is greater
INSTRUCTION FORMATS A machine instruction consists of a binary bit pattern. The length of the instruction (binary bit pattern) may vary from one instruction to another. The way the bits are interpreted by the processor gives the format of the instruction. Different instruction types within the same set usually have a different format. Say, data transfer instructions are to be interpreted as follows: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 Op code Mode Operand Address
MEMORY ADDRESSING Some instructions do not specify any memory address i.e. they do not require anything from main store. Such instructions are said to be ZERO ADDRESS INSTRUCTIONS e.g. HALT; stop execution Some instructions specify a single address. These are called ONE ADDRESS INSTRUCTIONS. e.g. INC AX; increment the contents of register AX by 1 Other instructions specify two addresses. Such instructions usually involve two operands. These are called TWO ADDRESS INSTRUCTIONS. e.g. AND AX,0001H; ‗mask‘ the least significant bit of AX
ADDRESSING MODESThere are several ways in which one may refer to a memory location. The different modes in which memory locations may be identified are called addressing modes. Register Mode: The operands specified in the instruction are held in CPU registers e.g. SUB AX, BX Immediate Operand Mode. In this mode, the operand is specified in the instruction itself. e.g. MOV AX, 25; load the number 25 to the register AX Direct Addressing Mode (Absolute addressing). The number in the address part of the instruction is to be interpreted as the address of the memory location holding the data item. e.g. MOV AX, C; load the contents of location C to the register AX; Indirect Addressing Mode. The address in the instruction locates the address of the required data item. e.g. MOX AX, [C]; load the data item whose address is in register C to the register AX
ASSEMBLY LANGUAGE In assembly language, mnemonics are used for all machine instructions. Mnemonics are typically two-, three- or four- lettered words such as ADD, MOV, INC,. There is usually a one-to-one mapping between assembly instructions and machine code instructions. Hence, in assembly language, we have instruction types: Data transfer Arithmetic instructions Logical instructions Test and branch instructions
DATA TRANSFER INSTRUCTIONS Move data from memory to a register, or from register to register Move data from a register to memory or some output unit Move data from an input unit to a registerE.g: MOV AX,BX; copies contents of register BX to register AX MOV AX,25; load register AX with the number 25 MOV C,AX; copies contents of AX to memory location C
ARITHMETIC INSTRUCTIONS Some processors offer only addition and subtraction operations. Others offer a whole range of operations: ADD -addition SUB - subtraction MUL - multiplication DIV - division INC - increment DEC - decrement ABS – absolute value NEG – change sign SHL – arithmetic shift left SHR – arithmetic shift right When arithmetic operations are carried out the status bits (condition codes) N(negative), Z(zero), O(overflow), C(carry) may be set or reset depending on the result of the instruction
LOGICAL INSTRUCTIONS The most typical logical instructions are: • NOT • AND • OR • XOR
TEST AND BRANCH INSTRUCTIONS Conditional branching instructions usually transfer control to some program address relative to the current depending on the status of some bit in the status registerExamples: SUB AX, N; compare contents of accumulator with contents of memory location N JNE dwn; transfer program control to label ‘dwn‘ if result is not negative i.e. contents of AX >= contents of N MOV AX, N; load contents of N to register AXdwn PUSH AX; push contents of accumulator to stack
LABEL vs SYMBOLIC ADDRESSRemarks re preceding example: ‗dwn‘ in 4th instruction specified, is said to be a label because it gives a name to that particular program address. A label must be unique i.e. you may only label one program address with a given label. ‗dwn‘ in 2nd instruction specified, is said to be a symbolic address. A symbolic address is a group of characters that represent the address of an instruction or a data item. Within a program, symbolic addresses are not unique i.e. you may have more than one reference to the same label.
UNCONDITIONAL BRANCHING INSTRUCTIONS Unconditional branching instructions transfer control to some other part of the program unconditionally.Examples: JMP 100; causes 100 to be stored in the PC so that the next instruction is retrieved from that location RTN; a return instruction from some subroutine; causes the top of stack to be popped into the PC.
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