Memory Basics Chapter 8

8-1 Memory definitions Memory is a collection of cells capable of storing binary information. Two types of memory: Random-Access Memory (RAM) Read-Only Memory (ROM)

8-2 Random-Access Memory Memory cell can be accessed to transfer information to or from any desired location Access will take the same time regardless of the location compare to serial memory such as hard drive, tape etc.

Block diagram of memory

Example of memory content

Write and Read Operation Apply the address on address lines (bus) Apply data on data lines (bus) Activate the Write input Read Activate the Read input

Timing waveforms Operation of memory is controlled by an external device, such as CPU Memory doesn’t employ CPU clock Operations are timed by changes in values on control input Access time for read: max. time from application of address to appearance of data on Data Output Write cycle time: max. time form application of address to completion of all operation required to store a word

Timing waveforms 4 clocks is needed for both Write cycle and Access time

Properties of memory Volatile: data disappeared after turning off the power Static RAM (SRAM): faster, less capacity Dynamic RAM (DRAM): slower, need refresh, more capacity Non-volatile: date kept after turning off the power Magnetic disk

8-3 SRAM Integrated Circuits Static RAM cell

RAM bit slice model

16-word by one-bit RAM chip

Coincident selection (16x1 RAM using 4x4 cell array)

Coincident selection (8x2 RAM using 4x4 cell array)

8-4 Array of SRAM ICs Symbol for 64K x 8 RAM

Block diagram of a 256K x 8 RAM Extension from 64K x 8 RAM Fig. 8-10

Block diagram of a 64K x 16 RAM