SRAM, DRAM, EPROM, MRAM, and DDRAM Explained

Computer Fundamentals

Types of Memory and Their Technologies

Understand the fundamental relationship between different types of computer memory and the technologies used to implement them.

Fundamental Matching

Memory type, technology, and key characteristics

Memory Type	Technology	Key Characteristic
Cache Memory	SRAM	Very fast; stores frequently accessed data
Main Memory	DRAM	Holds programs and data currently in use
Auxiliary Memory	HDD / SSD	Non-volatile, long-term data storage
Portable Storage	Flash Memory	Used in USB drives and memory cards

Important Memory Technologies

SRAM

Static Random Access Memory. Fast and does not require periodic refreshing while powered. Commonly used for CPU cache.

DRAM

Dynamic Random Access Memory. Requires periodic refreshing and is commonly used as main system memory.

EPROM

Erasable Programmable Read-Only Memory. Non-volatile memory that can be erased using ultraviolet light and reprogrammed.

MRAM

Magnetoresistive Random Access Memory. Non-volatile memory that stores data using magnetic states.

 

DDRAM means Display Data RAM.

In a character LCD, DDRAM is the LCD controller's small memory area that stores which characters should appear on the screen.

Think of it like this:

DDRAM

Address     Stored character
00H    →       H
01H    →       E
02H    →       L
03H    →       L
04H    →       O

The LCD reads those stored characters and displays:

HELLO

Why do we need DDRAM?

Suppose you tell the LCD:

Put A at address 05H.

The LCD stores:

05H → A

Then the LCD knows that the character at that display position should be A.

So remember:

$$\boxed{\text{DDRAM = memory used to store display characters}}$$

And:

$$\boxed{\text{Address = where the character is stored}}$$

$$\boxed{\text{Data = which character is stored}}$$

For a typical 40×2 LCD, the visible character positions are mapped to DDRAM addresses such as 00H–27H for the first row and 40H–67H for the second row.

DDRAM is not permanent memory.

It is temporary memory (RAM) inside the LCD controller.

Think of it like this:

DDRAM
   ↓
Power ON
   ↓
LCD stores "HELLO"
   ↓
Power OFF
   ↓
Data is lost

So:

• DDRAM → temporary → stores characters currently being displayed.

• ROM → permanent → stores fixed things, such as the LCD's built-in character patterns.

Example

When you send:

'A'

to the LCD, the LCD puts A into DDRAM.

When power is removed, that stored A is lost.

So:

$$\boxed{\text{DDRAM = temporary character storage}}$$

$$\boxed{\text{ROM = permanent/fixed storage}}$$

Also, DDRAM is different from the RAM in your computer—it's a small memory area specifically used by the LCD controller.