Computer System Architecture - Learn Computer Architecture Tutorials

Introduction to Computer Architecture

Computer architecture is the conceptual design and fundamental operational structure of a computer system. It is a blueprint and functional description of requirements and design implementations for the various parts of a computer, focusing largely on the way by which the central processing unit (CPU) performs internally and accesses addresses in memory. It may also be defined as the science and art of selecting and interconnecting hardware components to create computers that meet functional, performance and cost goals.

Computer architecture can be classified into three main categories:

Instruction Set Architecture, or ISA, is the image of a computing system that is seen by a machine language programmer. It includes the instruction set, word size, memory address modes, processor registers, and address and data formats.

·         Computer Organization is a lower level and detailed description of the system that involves how the different parts of the system are interconnected and how they interoperate in order to implement the ISA.

·         System Design which includes all of the other hardware components within a computing system such as:

·         Computer buses and switches

·         Memory controllers

·         Direct Memory Access (DMA)

·         Issues like multi-processing

Central Processing Unit

Central Processing Unit (CPU) performs all the arithmetic and logical calculations in a computer. The CPU is said to be the brain of the computer system. It reads and executes the program instructions, perform calculations and makes decisions. The CPU is responsible for storing and retrieving information on disks and other media.

Figure: Meaning of CPU

The CPU consists of Control Unit, Arithmetic and Logic Unit (ALU) and  register set.

Control Unit: The control unit issue control signals to perform specific operation and it directs the entire computer system to carry out stored program instructions

Arithmetic and Logic Unit: The ALU is the ‘core’ of any processor. It executes all arithmetic operations (addition, subtraction, multiplication and division), logical operations (compare numbers, letters, special characters etc.) and comparison operators (equal to, less than, greater than etc.).

Register Set:  Register set is used to store immediate data during the execution of instruction. This area of processor consists of various registers.

Computer Registers

A register is a very small amount of very fast memory that is built into the CPU (central processing unit) in order to speed up its operations by providing quick access to commonly used values. Registers refers to semiconductor devices whose contents can be accessed (i.e., read and written to) at extremely high speeds but which are held there only temporarily (i.e., while in use or only as long as the power supply remains on).

Registers are the top of the memory hierarchy and are the fastest way for the system to manipulate data. Registers are normally measured by the number of bits they can hold, for example, an 8-bit register means it can store 8 bits of data or a 32-bit register means it can store 32 bit of data.

Registers are used to store data temporarily during the execution of a program. Some of the registers are accessible to the user through instructions. Data and instructions must be put into the system. So we need registers for this.

The basic computer registers with their names, size and functions are listed below

Register Symbol	Register Name	Number of Bits	Description
AC	Accumulator	16	Processor Register
DR	Data Register	16	Hold memory data
TR	Temporary Register	16	Holds temporary Data
IR	Instruction Register	16	Holds Instruction Code
AR	Address Register	12	Holds memory address
PC	Program Counter	12	Holds address of next instruction
INPR	Input Register	8	Holds Input data
OUTR	Output Register	8	Holds Output data

STACK ORGANIZATION

Stack is a storage structure that stores information in such a way that the last item stored is the first item retrieved. It is based on the principle of LIFO (Last-in-first-out). The stack in digital computers is a group of memory locations with a register that holds the address of top of element. This register that holds the address of top of element of the stack is called Stack Pointer.

Stack Operations
The two operations of a stack are:

1.      Push:        Inserts an item on top of stack.

2.      Pop:          Deletes an item from top of stack.

Implementation of Stack
In digital computers, stack can be implemented in two ways:

1.      Register Stack

A stack can be organized as a collection of finite number of registers that are used to store temporary information during the execution of a program. The stack pointer (SP) is a register that holds the address of top of element of the stack.

2.      Memory Stack

A stack can be implemented in a random access memory (RAM) attached to a CPU. The implementation of a stack in the CPU is done by assigning a portion of memory to a stack operation and using a processor register as a stack pointer. The starting memory location of the stack is specified by the processor register as stack pointer.

RISC and CISC
RISC (Reduced Instruction Set Computer)
RISC stands for Reduced Instruction Set Computer. To execute each instruction, if there is separate electronic circuitry in the control unit, which produces all the necessary signals, this approach of the design of the control section of the processor is called RISC design. It is also called hard-wired approach.
Examples of RISC processors:

·         IBM RS6000, MC88100

·         DEC’s Alpha 21064, 21164 and 21264 processors

·         Features of RISC Processors:

·         The standard features of RISC processors are listed below:

·         RISC processors use a small and limited number of instructions.

·         RISC machines mostly uses hardwired control unit.

·         RISC processors consume less power and are having high performance.

·          Each instruction is very simple and consistent.

·         RISC processors uses simple addressing modes.

·         RISC instruction is of uniform fixed length.

CISC (Complex Instruction Set Computer)
CISC stands for Complex Instruction Set Computer. If the control unit contains a number of micro-electronic circuitry to generate a set of control signals and each micro-circuitry is activated by a micro-code, this design approach is called CISC design.

Examples of CISC processors are:

·         Intel 386, 486, Pentium, Pentium Pro, Pentium II, Pentium III

·         Motorola’s 68000, 68020, 68040, etc.

Features of CISC Processors:

The standard features of CISC processors are listed below:

·         CISC chips have a large amount of different and complex instructions.

·         CISC machines generally make use of complex addressing modes.

·         Different machine programs can be executed on CISC machine.

·         CISC machines uses micro-program control unit.

·         CISC processors are having limited number of registers.