Dr. S. K. Verma Computer Organization vs Computer Architecture System And Computer System Computer Types ◦ Embedded computers used for a specific purpose ◦ Personal computers Desktop computers Workstation computers Portable and Notebook computers ◦ Servers and Enterprise systems ◦ Supercomputers and Grid computers cloud computing Design for Moore’s Law Use Abstraction to Simplify Design ◦ abstractions to represent the design at different levels of representation; Make the Common Case Fast Performance via Parallelism Performance via Pipelining Performance via Prediction Hierarchy of Memories Dependability via Redundancy Instructions, or machine instructions, are explicit commands ◦ transfer of information within a computer or between the computer and its I/O devices ◦ arithmetic and logic operations to be performed program is a list of instructions which performs a Data are numbers and characters used as task. operands by the instructions. Each instruction, number, or character is encoded as a string of binary digits called bits Computers accept coded information through input units Keyboard touchpad, mouse, joystick, and trackball. Microphones Cameras Digital communication facilities There are two classes of storage: Primary Memory ◦ main memory ◦ a fast memory that operates at electronic speeds ◦ memory consists of a large number of semiconductor storage cells ◦ cells are rarely read or written individually ◦ handled in groups of fixed size called words ◦ one word can be stored or retrieved in one basic operation ◦ word length of the computer ◦ distinct address is associated with each word location ◦ A memory in which any location can be accessed in a short and fixed amount of time after specifying its address is called a random-access memory (RAM). ◦ memory access time Cache Memory Secondary Storage ◦ adjunct to the main memory, a smaller, faster RAM unit, called a cache. ◦ Used to hold sections of a program that are currently being executed. ◦ cache is tightly coupled with the processor and is usually contained on the same integrated-circuit chip. ◦ additional, less expensive, permanent secondary storage is used. ◦ Access times for secondary storage are longer than for primary memory ◦ magnetic disks, optical disks (DVD and CD), and flash memory devices. operations are executed in the arithmetic and logic unit (ALU) When operands are brought into the processor they are stored in high-speed storage elements called registers. register can store one word of data Access times to registers are even shorter than access times to the cache unit output unit is the counterpart of the input unit. function is to send processed results to the outside world. Coordination among different units done by CU. Control circuits are responsible for generating the timing signals. large set of control lines (wires) carries the signals used for timing and synchronization of events in all units. The operation of a computer can be summarized as follows: ◦ The computer accepts information in the form of programs and data through an input unit and stores it in the memory. ◦ Information stored in the memory is fetched under program control into an arithmetic and logic unit, where it is processed. ◦ Processed information leaves the computer through an output unit. ◦ All activities in the computer are directed by the control unit. Load R2, LOC Add R4, R2, R3 Store R4, LOC the processor contains a number of registers used for several different purposes. instruction register (IR) holds the instruction that is currently being executed. program counter (PC) is another specialized register PC points to the next instruction that is to be fetched from the memory. general-purpose registers R0 through Rn−1, often called processor registers typical operating steps: ◦ A program must be in the main memory in order for it to be executed. ◦ Execution of the program begins when the PC is set to point to the first instruction of the program. ◦ contents of the PC are transferred to the memory along with a Read control signal. ◦ the addressed word has been fetched from the memory and loaded into register IR. ◦ the contents of the PC are incremented so that the PC points to the next instruction to be executed ◦ Normal execution of a program may be preempted if some device requires urgent service. processing required for a single instruction is called an instruction cycle. fetched instruction is loaded into a register in the processor known as the instruction register (IR). instruction contains bits that specify the action the processor is to take. Actions category: ◦ ◦ ◦ ◦ Processor-memory Processor-I/O Data processing Control a group of n bits can be stored or retrieved in a single, basic operation. n bits is called the word length Byte Addressability ◦ byte-addressable memory is used ◦ if the word length of the machine is 32 bits, successive words are located at addresses 0, 4, 8, . . . , with each word consisting of four bytes. Big-Endian and Little-Endian Assignments layout of the instruction is called the instruction format the numeric version of instructions machine language and a sequence of such instructions machine code. op: Basic operation of the instruction, traditionally called the opcode. rs: The first register source operand. rt: The second register source operand rd: The register destination operand. It gets the result of the operation. shamt: Shift amount. funct: Function. This field, often called the function code, selects the specific variant of the operation in the op field. MIPS (Microprocessor without Interlocked Pipelined Stages) computer must have instructions capable of performing four types of operations: ◦ Data transfers between the memory and the processor registers ◦ Arithmetic and logic operations on data ◦ Program sequencing and control ◦ I/O transfers Register Transfer Notation ◦ the contents of any location are denoted by placing square brackets around its name R2 ← [LOC] R4 ← [R2] + [R3] ◦ the right hand side of an RTN expression always denotes a value ◦ left-hand side is the name of a location. Assembly-Language Notation ◦ the transfer from memory location LOC to processor register R2, is specified by Load R2, LOC Add R4, R2, R3 RISC and CISC Instruction Sets Immediate Direct Indirect Register Register indirect Displacement Stack A = contents of an address field in the instruction R = contents of an address field in the instruction that refers to a register EA = actual (effective) address of the location containing the referenced operand (X) = contents of memory location X or register X Virtually all computer architectures provide more than one of addressing modes. (mode field) the effective address will be either a main memory address or a register. Immediate Addressing ◦ the operand value is present in the instruction Operand = A ◦ advantage of immediate addressing is that no memory reference other than the instruction fetch is required to obtain the operand ◦ disadvantage is that the size of the number is restricted to the size of the address field Direct Addressing ◦ the address field contains the effective address of the operand: EA = A ◦ It requires only one memory reference and no special calculation. ◦ limitation is that it provides only a limited address space. Indirect Addressing ◦ the address field refer to the address of a word in memory EA = (A) ◦ advantage of approach is that for a word length of N, an address space of 2N is now available. ◦ disadvantage is that three or more memory references could be required to fetch an operand. Register Addressing ◦ It is similar to direct addressing ◦ address field refers to a register rather than a main memory address: EA = R ◦ if the contents of a register address field in an instruction is 5, then register R5 is the intended address, and the operand value is contained in R5. ◦ advantages of register addressing are a small address field is needed in the instruction no time- consuming memory references are required ◦ modern processors employ multiple general-purpose registers, placing a burden for efficient execution on the assembly- language programmer Register Indirect Addressing ◦ register indirect addressing is analogous to indirect addressing. EA = (R) ◦ advantages and limitations of register indirect addressing are the same as for indirect addressing. ◦ register indirect addressing uses one less memory reference than indirect addressing. Displacement Addressing ◦ combines the capabilities of direct addressing and register indirect addressing EA = A + (R) Relative addressing Base-register addressing Indexing relative addressing ◦ also called PC-relative addressing ◦ implicitly referenced register is the program counter (PC) ◦ Relative addressing exploits the concept of locality ◦ If most memory references are relatively near to the instruction being executed, then the use of relative addressing saves address bits in the instruction. base-register addressing Indexing ◦ The referenced register contains a main memory address, and the address field contains a displacement ◦ The address field references a main memory address, and the referenced register contains a positive displacement from that address ◦ use of indexing is to provide an efficient mechanism for performing iterative operations. ◦ A, A + 1, A + 2, . . . , ◦ The value A is stored in the instruction’s address field, and the chosen register, called an index register, is initialized to 0. After each operation, the index register is incremented by 1. Stack Addressing ◦ a stack is a linear array of locations ◦ Also known as pushdown list or last-in-first-out queue Integers ◦ B = bn−1 . . . B1b0 where b = 0 or 1 for 0 ≤ i ≤ n − 1. This vector can represent an unsigned integer value V(B) in the range 0 to 2 − 1. V(B) = bn−1 × 2n−1 +・ ・ ・+b1 × 21 + b0 × 20 i n Three systems are used for representing numbers: ◦ Sign-and-magnitude ◦ 1’s-complement ◦ 2’s-complement The rules for addition and subtraction of n-bit signed numbers using the 2’scomplement representation ◦ To add two numbers, add their n-bit representations, ignoring the carry-out bit from the most significant bit (MSB) position. The sum will be the algebraically correct value in 2’s-complement representation if the actual result is in the range−2n−1 through+2n−1 − 1. ◦ To subtract two numbers X and Y , that is, to perform X − Y , form the 2’s-complement of Y , then add it to X using the add rule. Again, the result will be the algebraically correct value in 2’s-complement representation if the actual result is in the range −2n−1 through+2n−1 − 1 Sign Extension Overflow in Integer Arithmetic ◦ When the actual result of an arithmetic operation is outside the representable range, an arithmetic overflow has occurred. ◦ an overflow has occurred when the sign of the sum is not the same as the signs of the summands. Floating-Point Numbers ◦ binary floating-point number can be represented by: a sign for the number some significant bits a signed scale factor exponent for an implied base of 2 ◦ IEEE (Institute of Electrical and Electronics Engineers) standard for 32-bit floating-point number representation uses ◦ a sign bit ◦ 23 significant bits ◦ 8 bits for a signed exponent of the scale factor Character Representation characters is ASCII (American Standard Code for Information Interchange). Half Adder Full Adder Four Bit Adder Carry Look Ahead Adder Binary Adder and Subtractor unsigned integers ◦ Multiplication involves the generation of partial products, one for each digit in the multiplier. These partial products are then summed to produce the final product. ◦ When the multiplier bit is 0, the partial product is 0. When the multiplier is 1, the partial product is the multiplicand. ◦ each successive partial product is shifted one position to the left relative to the preceding partial product. ◦ The multiplication of two n-bit binary integers results in a product of up to 2n bits in length twos complement multiplication ◦ multiplication of a binary number by 2n is accomplished by shifting that number to the left n bits. ◦ the partial products should be viewed as 2n-bit numbers generated from the n-bit multiplicand. Booth’s algorithm D=Q*V+R Carry Save Multiplier A floating-point operation may produce one of these conditions: ◦ Exponent overflow: A positive exponent exceeds the maximum possible exponent value. ◦ Exponent underflow: A negative exponent is less than the minimum possible exponent value (e.g., - 200 is less than -127). ◦ Significand underflow: In the process of aligning significands, digits may flow off the right end of the significand. ◦ Significand overflow: The addition of two significands of the same sign may result in a carry out of the most significant bit. There are four basic phases of the algorithm for addition and subtraction: ◦ ◦ ◦ ◦ Check for zeros. Align the significands. Add or subtract the significands. Normalize the result.
