Computer Science 101

Computer Science 101 Computer Systems Organization MEMORY

Pappaw, our puter broke.

What’s a man to do? Perfectly clear, huh?

I think I’m getting a headache.

Von Neumann Architecture • Basic Architecture of most computers • Four Major Subunits • Memory • Input-output • Arithmetic-logic unit (ALU) • Control Unit • Stored Programs

Von Neumann Architecture • Execution Cycle • Fetch Instruction • Decode • Execute HP Pavilion Media CenterProcessor speed: 2.8GHz giga- G 1000^3  1024^3 = 2^30 = 1,073,741,824Hz- cycle per second2.8 billion cycles per second

Bus Memory Input Output Units Control Unit ALU CPU Major Components HP Pavilion Media CenterBus Speed: 800MHz Front Side Bus

Memory • Stores: Numbers, text, programs, addresses, graphics, sound, video, etc. that are currently in use. • Divided into fixed size cells (fixed number of bits). • This size is commonly 8 bits, and this 8-bit unit is called a byte. • The simple hypothetical lab machine uses 16 bit cells. The lab machine is a simulated version of a very simple, but illustrative computer. HP Pavilion Media Center Memory: 1024MB PC2-4200 DDR2 SDRAM memory (expandable to 2GB)

A bit is what the dentist uses to fix your byte?

00100110 00111110 10100110 10101010 Memory Addresses • Each cell has an address, an unsigned integer. • All accesses to memory are via a specific address Address Cell Content 0 1 2 3

Basic Memory Operations • Memory Fetch • Given a specific memory address. • Retrieve the content stored at that address. • Memory Store • Given a specific memory address and • a specific value, • store the given value in the cell with the specified address.

00100110 00111110 10100110 10101010 Memory Fetch Address Cell Content 2002 2003 2004 2005 Given Address: 2004 Content:

00100110 00111110 10100110 10101010 Memory Fetch Address Cell Content 2002 2003 2004 2005 Given Address: 2004 Content: 00111110

Address Cell Content 2002 00100110 00111110 2003 10100110 10101010 2004 2005 Given Address: 2004 Given Value: 10101010 Memory Store

00100110 00111110 10100110 10101010 Memory Store Address Cell Content 2002 2003 2004 2005 Given Address: 2004 Given Value: 10101010

00100110 10101010 10100110 10101010 Memory Store Address Cell Content 2002 2003 2004 2005 Given Address: 2004 Given Value: 10101010

Memory Facts and Terminology • A cell is the minimum unit of access. • Access time is same for all cells - Random Access Memory or RAM (nanoseconds - billionths of second) • ROM - Read only Memory (fetch but not store) • Some data items require more than one cell. For example, an instruction might need four cells. Note: ints require 4 cells (bytes)

More on Memory • All addresses are of some fixed number of bits, say N. Addresses would be 0000…0  0 0000…1  1 Total of 2N cells … 1111…1  2N – 1 • Note: This is an application of unsigned numbers.

Terminology • Storage capacity: • K  210 = 1024  Kilo as in Kb • M  220 = 1,048,576  Mega as in Mb • G  230 = 1,073,741,824  Giga as in Gb • Speed • 1  = 1 microsecond = 1 millionth of second • 1 ms = 1 millisecond = 1 thousandth of second • 1 ns = 1 nanosecond = 1 billionth of second • Cycle rate: Hertz is cycle per second

Memory Registers • A register is an extremely fast cell - more expensive than RAM cells - fewer of them - for special purposes. • MAR - Memory Address Register- size is same as size of an address - holds the address of the cell to access. • MDR - Memory Data Register- size is multiple of RAM cell size - holds content of cell fetched or value to be stored.

HP Pavilion Media Center Memory: 1024MB PC2-4200 DDR2 SDRAM memory (expandable to 2GB) • This HP has maximum memory of 2GB or 231; so MAR must be 31 bits. MAR and Max Memory Size • MAR of size N bits gives 2N possible addresses; so up to 2N possible memory locations.

Memory Fetch- More Detail • Fetch (address) To obtain data at a given address: • Load address into MAR • Address is decoded and cell is selected • Contents of cell put into MDR

00100110 00111110 10100110 10101010 Fetch(10110) Address Cell Content 10100 10101 10110 10111 MAR 10110 MDR 0011110

Memory Store- More Detail • Store (address,value) To store a given value at a given address: • Load address into MAR • Load value into MDR • Address is decoded and cell is selected • Value from MDR is put into the selected cell

00100110 00111110 10100110 10101010 Store(10110,11111111) Address Cell Content 10100 10101 10110 11111111 10111 MAR 10110 MDR 11111111

00100110 00100110 00111110 10100110 10100110 10101010 MAR ---n--- 00111110 2n lines 10101010 n to 2n decoder Address Decoding • Recall: Decoder has n inputs, 2n outputs, inputs select one output to be 1, others 0.

Address decoding (cont.) • Note: If we have 26 bit addresses, this allows for 226 = 64M cells. • In the diagram before, the decoder would have 26 input lines and 67,108,864 output lines. • That’s a lot of lines.

Two-dimensional memory layout • Now picture the 64M memory laid out as a 2-dimensional grid. There would be 213 rows and 213 columns. • Suppose we use half of the address (13 bits) to choose a row and half to choose the column in order to locate the cell. • This would use two decoders - one for the row and one for the column.

Two-dimensional Memory Layout • Here we have 2* 213= 16384 lines as opposed to 67,108,864 as in 1-dimensional MAR 13 13 n to 213 n to 213

But what happened to those adders?

Computer Science 101