RAM (Random Access Memory) and ROM (Read Only Memory) are two important types of computer memory that serve different purposes. Both have their own advantages and limitations. In general, RAM is faster, temporary data storage while ROM is slower permanent storage. Understanding the differences between the two is key to building an efficient computer system.
What is RAM?
RAM stands for Random Access Memory. It is a volatile type of memory which means it requires power to retain data. As soon as the power is turned off, all data stored in RAM is erased. RAM chips are made up of tiny capacitors and transistors that can be charged to store a 1 or 0 binary value. These values make up the data being stored.
Some key characteristics of RAM include:
- Volatile memory – Requires power to retain data
- Much faster than ROM and hard drives
- Used as short term data storage
- Data can be read and written frequently and quickly
- Common types are DRAM and SRAM
When a computer is running, it uses RAM to temporarily store the data it needs to access right away such as application data, program code being executed, and files being accessed/edited. Having enough RAM capacity and speed is vital for a computer to function smoothly and quickly.
What is ROM?
ROM stands for Read Only Memory. It is a non-volatile type of memory meaning it can retain data even without power. ROM chips are programmed with data during manufacturing and that data cannot be modified or erased by the end user. The data on ROM is permanent.
Key characteristics of ROM include:
- Non-volatile memory – Retains data without power
- Much slower than RAM
- Used for permanent storage of boot instructions
- Data cannot be modified or written to by end user
- Common types are masked ROM, PROM, EPROM, EEPROM
ROM is primarily used to store firmware bootstrap instructions that run when a computer is powered on. This firmware initializes components and loads the operating system from a hard drive into RAM. ROM is also used to store microcode in microcontrollers and other embedded systems.
Differences Between RAM and ROM
| Parameter | RAM | ROM |
|---|---|---|
| Volatility | Volatile (requires power) | Non-volatile |
| Speed | Much faster | Slower |
| Use | Short term data storage | Permanent storage of boot data |
| Writability | Data can be written and erased | Read-only, cannot write data |
| Manufacturing | Blank memory chips | Chips programmed during manufacturing |
| Cost per GB | Higher | Lower |
| Capacity | Less (GBs) | More (MBs) |
How does RAM work?
RAM works by storing data in tiny capacitors located on memory chips. Each capacitor can be charged or discharged to represent a 1 or 0 in binary. An external memory controller chip on the motherboard sends signals to the RAM chips telling them when to charge and discharge these capacitors to write data, and when to read the voltage levels to retrieve data.
DRAM and SRAM are two common types of RAM.
- Dynamic RAM (DRAM) – Each memory cell is made up of a capacitor and transistor. The capacitor holds one bit of data as charge. Must be refreshed thousands of times per second to maintain data. Slower but less complex and cheaper than SRAM.
- Static RAM (SRAM) – Uses a flip-flop circuit rather than capacitor to store each bit. Faster and more reliable than DRAM but more complex and expensive. Used for CPU caches.
RAM chips have an external address bus and data bus that is controlled by a memory controller. To write data, the controller sends address signals via the address bus to select the memory locations. It then sends data signals via the data bus to charge those capacitors to the desired 1 or 0 value. To read data, the controller selects the addresses to read from which causes those capacitors to discharge onto the data bus, allowing the controller to measure their voltage levels.
Advantages of RAM
- Much faster to read and write than other forms of storage
- Allows random access to any location in memory
- Used for short term temporary storage while computer is running
- Necessary for applications and programs to function
- Data can be changed and overwritten frequently
Disadvantages of RAM
- Volatile – Data is lost when power is turned off
- More expensive per GB than other forms of storage
- Capacity is generally less than ROM and disk storage
- Requires constant power to maintain stored data
How does ROM work?
ROM stores data by programming permanent or semi-permanent digital memory circuits within the chip during manufacturing. This data cannot be modified by end users and is retained even when the device is powered off.
Some common types of ROM include:
- Masked ROM – Programmed during manufacturing by masking certain connections on the memory chip. Cannot be reprogrammed. Used for high volume production.
- PROM (Programmable ROM) – Can be programmed once after manufacturing using a special PROM programming device.
- EPROM (Erasable PROM) – Can be erased by exposing the chip to UV light and reprogrammed using a programmer.
- EEPROM (Electrically Erasable PROM) – Can be electrically erased and reprogrammed repeatedly through erase cycles.
ROM chips have an internal hard-wired memory structure that does not require power to maintain the stored bits. Data is read out by selecting addresses on the address bus. The data at those internal memory locations gets output on the data bus. ROM cannot be written to during normal operation.
Advantages of ROM
- Non-volatile data retention
- Lower cost per GB than RAM
- Large storage capacity possible
- Permanent storage of boot instructions and critical programs
- Simpler internal structure than RAM
Disadvantages of ROM
- Slower access times and data rates than RAM
- Data cannot be modified by end user after programming
- Requires programming before use
- Not as flexible as writable memories like RAM
Why is RAM Faster than ROM?
There are several key reasons why RAM offers much higher data access speeds compared to ROM:
- Simple internal structure – RAM chips have a simple grid-like structure of rows and columns. Any memory location can be accessed randomly by selecting rows/columns.
- No programming necessary – RAM does not require any programming and has no permanent data stored. ROM requires programming which slows access.
- Direct electrical access – RAM bits are stored as direct electrical charges in capacitors. Reading just requires sensing voltage levels. ROM requires more complex circuitry.
- Optimized for speed – RAM is designed for fast temporary data storage. ROM focuses on permanent data storage at lower cost.
- Manufacturing process – SRAM uses faster transistor-based memory cells while ROM chips use slower diode matrix structures.
In summary, RAM uses simpler electrical memory circuits, has an optimized internal architecture for speed, and does not require programming. This allows much faster random reads and writes. ROM relies on more permanent data structures and suffers speed disadvantages as a result.
Typical RAM vs ROM Speeds
Some typical access speeds for RAM and ROM technologies include:
| Memory Type | Access Time |
| SRAM | 10 – 20 ns |
| DRAM | 20 – 40 ns |
| NOR Flash (ROM) | 50 – 150 ns |
| NAND Flash (ROM) | 25 – 50 us |
| HDD/SSD | 50 – 100 us |
As the table shows, SRAM offers access times in 10s of nanoseconds, while DRAM is in the 10s of nanoseconds range. ROM technologies are much slower with access times in the microseconds for NOR Flash and tens of microseconds for NAND Flash. Mechanical hard drives and solid state drives have access times in the range of tens to hundreds of microseconds.
The combination of SRAM caches and DRAM main memory provides computers with nanosecond access speeds that allow for smooth responsiveness and fast program execution. The much slower ROM and disk storage is acceptable for permanent data storage where access speed is less critical.
Conclusion
In summary, RAM offers much faster access speeds compared to ROM primarily due to its simple volatile memory architecture, lack of programming requirements, highly optimized electrical circuits, and focus on temporary data storage. ROM chips trade speed for permanent data retention capability and lower cost per GB. All computers require a combination of speedy RAM and slower permanent storage like ROM and disk drives to function effectively.