1. Riyadh Philanthropic Society For Science Prince Sultan College For Woman Dept. of Computer & Information Sciences CS 251 Introduction to Computer Organization & Assembly Language Lecture 5 (Computer System Organization) Main Memory - I

2.  From text Book: 2.2.1, 2.2.2, 2.2.3  Memory Organization  RAMs  ROMS  Cache memories  Memory types comparison  Memory packaging and types 2Main Memory

3. 3  The memory is the place where data and programs are stored  The bit, a binary digit, is the main unit of memory  The collection of bits are organized into cells ( or locations)  Each of these cells will store a piece of information

4. 4Main Memory  Memories consists of a number of cells  Each cell has a number called its address by which programs can refer to it  If a memory has k cells, they will have addresses from 0 to k-1  All cells in memory contain the same number of bits  If the memory holds n bits, it can hold 2 k bit combinations  Adjacent cells have consecutive addresses

5. 5Main Memory  Three different ways to represent a 96-bit memory Address Address Address 0 1 2 3 4 5 6 7 8 9 10 11 0123456701234567 012345012345 8 bits 12 bits 16 bits Cell

6.  Memory addresses are expressed as binary numbers  An n bit memory address allows for a maximum of 2 n cells access  Ex. How many bits would you need to address 12 cells  4 bits are needed to be able to reference the addresses from 0 to 11  The number of bits in the address is independent than the number of bits per cell  Ex. A memory with 2 12 cells of 8 bits each and a memory with 2 12 cells of 64 bits each need 12-bit address. 6Main Memory

7.  Nearly all computer manufacturers have standardized on an 8-bit cell, called byte  Bytes are grouped into words  A computer with a 32-bit word has 4 bytes/word. Whereas a computer with a 64-bit word has 8 bytes/word.  Most instructions operate on entire words (e. adding two words)  Thus, a 32-bit machine will have 32-bit registers and instructions for manipulating 32-bit words.  The same applies for a 64-bit machine 7Main Memory

8.  There are two conventions for storing data in memory 8Main Memory 3210 0 7654 4 111098 8 15141312 Byte 32 – bit word 0 0123 4 4567 8 891011 12 131415 32 – bit word Byte

9.  Problems occur when the data is a collection of integers, character strings, and other data types  When computers with different data representation communicate, problems occur !!! 9Main Memory MIJ 0 TIMS 4 000H 8 21000 12 4100 16 Byte 32 – bit word 0 JIM 4 SMIT 8 H000 12 21000 16 4100 32 – bit word Byte

10.  Transfer has reversed the order of the bytes  Swapping doesn’t solve the problem  A protocol including a header with each transfer can be an inefficient solution 10Main Memory JIM 0 SMIT 4 H000 8 00021 12 0014 16 Byte 32 – bit word 0 MIJ 4 TIMS 8 000H 12 21000 16 4100 32 – bit word Byte

11. The memory is specified by the number of words it has and the number of bits per word. Ex. A memory of 2048*10 memory width = word size = 10 bits address space = memory size = 2048 = 2 11 Address consists of 11 bits 11Main Memory M bits Memory Words Address 00...0 00...1 2 n -1 2 n memory size