RAM (RANDOM ACCESS MEMORY)

A memory unit stores binary information in groups of bits called words. It is an one unit. A group of 8-bits is called a byte.
1 byte = 8-bit word





Ex: memory unit 1k words of 16-bit each.
1k = 1024 = 210
16-bits = 2 bytes
Memory 2048 = 2k bytes





Relationship between words and address bits
2k≥m
m = total number of words
k = number of address bits

Types of memories

The mode of access of a memory system is determined by the type of components used.

SRAM (Static RAM)

Consists essentially of internal latches that store the binary information. The stored information remains valid as long as power is applied to the unit.

DRAM (Dynamic RAM)

Stores the binary information in the form of electric charges on capacitors. The capacitors are provided inside in the chip by MOS transistors. Capacitors must be periodically recharged by refreshing the dynamic memory.

SRAM vs. DRAM

DRAM
• Reduced power consumption
• Larger storage capacity in a single memory chip
• Less expensive
• Used for Main Memory
SRAM
• Easier to use
• Has shorter read/write cycle (Faster than DRAM)
• Used for Cache Memory

Internal construction

The internal construction of RAM of m words and n bits per word consists of m×n binary storage cells.









Select bit = ‘1’ – read
‘0’ – write




RAM

Coincident decoding

Basic idea in 2-Dimensional decoding is to arrange the memory cells in an array that is close as possible to square.