Memory

Consists of RAM and secondary memory.

In Von Neumann architecture, both data and instructions are stored in the same memory. In Howard Aiken’s architecture, data and instructions are stored in separate memories.

Memory cell

Basic element of a semiconductor memory. Carries two stable states: 1 and 0. State can be read or written.

Memory chip

Contains a 2D array of memory cells. A memory chip’s size is mentioned as "$M \times N\,\text{bit}$". $M$ is the number of words, and $N$ is the size of a word.

Word

Number of bits written to/read from a chip simultaneously.

Memory module

Collection of memory chips.

Memory types

RAM

Volatile. High-speed read and write. Can either be dynamic (built using capacitors) or static (built using flip flops).

• SRAM: built using flip flops. expensive. faster. used for caching.
• DRAM: built using capacitors. cheaper. slower. used for main memory. needs constant refreshing.

ROM

Non-volatile memory.

Memory Type	Category	Erasure	Write Mechanism
Read-only memory (ROM)	Read-only	Not possible	Masks
Programmable ROM (PROM)	Read-only	Not possible	Electrically
Erasable PROM (EPROM)	Read-mostly	UV light, chip-level	Electrically
Electrically Erasable PROM (EEPROM)	Read-mostly	Electrically, byte-level	Electrically
Flash memory	Read-mostly	Electrically, block-level	Electrically

Designing a memory module

For example, to build a $256\text{K}\times 8\,\text{bit}$ module with $256\text{K}\times 1\,\text{bit}$ chips:

• Total number of chips required: $(256 \times 2^{10} \times 8) / (256 \times 2^{10} \times 1) = 8$ chips
• Size of address space: $256\times 2^{10}$ addresses
• Number of bits for address bus: $18$ bits