46. Introduction to Semiconductor Memories (RAM vs ROM)

Memory components are essential sequential circuits designed to store large amounts of data. They are typically organized into an array of storage cells, accessed via address decoding.

Key Concepts

• Address Lines (A): Selects a specific location (word) in the memory array. $2^A$ is the total memory capacity.
• Data Lines (D): The width of the data being read or written (e.g., 8 bits).
• Control Lines: Read/Write (R/W), Chip Select (CS).

1. RAM (Random Access Memory)

RAM is volatile memory; data is lost when power is removed. It allows both reading and writing.

• SRAM (Static RAM): Uses flip-flops (6 transistors per cell). Very fast but physically large and expensive.
• DRAM (Dynamic RAM): Uses capacitors to store charge (1 transistor per cell). High density but requires periodic refresh cycles to prevent charge leakage.

2. ROM (Read-Only Memory)

ROM is non-volatile; data is retained without power. It is typically used to store permanent instructions (like BIOS/firmware).

• Types: PROM, EPROM, EEPROM, Flash Memory (used in SSDs).