Memory Technology and Interfacing

Stored Program Model

Memory

Output

Input

CPU

Stored-program computer model

Types of Memory

Random-Access Memory (RAM)VolatileStore data or programs that can be

conveniently reloaded Read-Only Memory (ROM)

Non-volatileProgramming once store essential

programs

Random-Access Memory (RAM) Any of its contents may be accessed at

any time. (Why this notation is used?) Two technologies are used:

Static RAM (SRAM)Dynamic RAM (DRAM)

Static RAM

A static RAM stores a bit of information within a flip-flop.

6-Transistor SRAM Cell

bit bit

word

Dynamic RAM

A DRAM stores a bit of information as a charge.

row select

bit

DRAM vs SRAM

DRAM is much higher density (less transistor is used for a cell).

DRAM needs to be refreshed every now and then (to prevent memory loss).Must incorporate with extra (complicated)

circuit for refreshing.

ROM Technologies

Pure ROM PROM (User-Programmable ROM) EPROM (Erasable PROM) EAROM (Electrically-Alterable ROM)

Pure ROM

A mask-programmed ROM made from manufacturer.

High cost – need to be produced for large volumes.

PROM

Can be programmed (once) by user. Using PROM programmer.

It’s a fusible-link PROM each cell is fuse (nichrome or polysilicon). Originally, all rows and columns are connected

(logical 0s). Once a cell is ‘burnt’, it becomes logical 1. “Grow back” problem reliability problem.

Once programmed, it can not be altered ever again.

EPROM

Can be reprogrammed a number of times using a programmer machine.

Two main types:UV-erasableElectrically-erasable

EAROM

Can be read from and written to. It actually is non-volatile RAM.

Now, we use Flash Memory

Accessing Memory

There must be a control bus for the following operations:Read data from memoryWrite data to memoryDo not access memory

ROM Interfacing

ROM Read

RAM Interfacing

Reading from the Memory (RAM)

Writing to the Memory

Memory Interfacing Design & Memory Map

4K

4K

20=121=222=423=824=1625=3226=6427=12828=25629=512210=1024211=2048212=4096213=8192214=16384215=32768216=65536

0000h

0FFFh1000h

1FFFh

4K2000h

2FFFh

4K

4K

E000h

EFFFhF000h

EFFFh

Use Decoder (i.e. 74138)