3.1.3 Primary Memory Technologies
A processor needs fast access to instructions and data while a computer system is operating. Different memory technologies are used because no single design gives the greatest speed, capacity, flexibility, and lowest cost at the same time.
This page compares RAM with ROM, explains why DRAM and SRAM are chosen for different roles, and examines how PROM, EPROM, and EEPROM differ.
By the end of this section, you should be able to:
- Compare RAM and ROM using volatility, access, permitted operations, and typical use.
- Explain why DRAM requires refreshing and why SRAM does not.
- Relate the speed, cost, and storage density of DRAM and SRAM to their uses.
- Distinguish between PROM, EPROM, and EEPROM.
- Choose a suitable memory technology for a described device and justify the choice.
Why a system needs primary memory
The processor repeatedly fetches instructions and reads or changes data. Keeping all of this only in secondary storage would make the system too slow for normal operation. Primary memory therefore provides electronic storage that the processor can access directly while the system is working.
A restart at an environmental monitoring station
Imagine a remote station that records air quality. While it is running, current sensor readings and calculation results must be updated frequently. After a power cut, those temporary values do not need to remain, but the instructions that start and configure the station must still be available.
| Information | Required behaviour | Suitable broad category |
|---|---|---|
| Current readings and intermediate calculations | Changed frequently while power is on | RAM |
| Startup and device-control instructions | Must remain available after power is removed | ROM or another non-volatile firmware memory |
Common mistake
Primary memory is not automatically permanent memory. The category includes both volatile RAM and non-volatile ROM.
RAM and ROM serve different purposes
RAM and ROM are both electronic memory technologies that allow particular locations to be accessed directly. Their important difference is what happens to the stored data and whether the contents are normally changed during operation.
| Feature | RAM | ROM |
|---|---|---|
| Full name | Random-Access Memory | Read-Only Memory |
| Power removed | Contents are lost: volatile | Contents are retained: non-volatile |
| Normal operation | Read from and written to repeatedly | Mainly read; alteration requires a special method or is impossible |
| Typical role | Current programs, active data, and temporary results | Firmware or instructions that must survive a restart |
Exam tip
A comparison is stronger than two isolated definitions. State a paired difference, such as: “RAM is volatile, whereas ROM is non-volatile,” and then explain the consequence.
Common misconception
“Random access” does not mean that data is chosen unpredictably. It means the processor can access a required memory location directly rather than reading every earlier location first.
DRAM and SRAM: two ways to build RAM
Both technologies are volatile and both can be read and written repeatedly. Their internal designs create different trade-offs, so they are normally assigned different jobs.
| Comparison point | DRAM | SRAM |
|---|---|---|
| Storage mechanism | Stores each bit using a tiny electrical charge, commonly associated with a capacitor | Stores each bit in a stable circuit state formed from flip-flop circuitry |
| Refreshing | Charge leaks, so stored values must be refreshed repeatedly | No refresh cycle is needed while power remains available |
| Access speed | Generally slower | Generally faster |
| Cost per stored bit | Lower | Higher |
| Storage density | Higher: more bits can fit into a given chip area | Lower: each bit requires more circuitry |
| Common use | Main memory, where substantial capacity is required at an affordable cost | Cache memory, where a smaller amount of very fast memory is valuable |
Reasoning from requirements
A laptop may need many gigabytes of working memory, so DRAM provides a practical balance between capacity and cost. A processor cache is much smaller but must respond quickly, so the extra cost of SRAM can be justified.
Exam tip
Do not stop after naming the use. Connect the property to the decision: SRAM is selected for cache because its access is faster, while DRAM is selected for main memory because it offers greater density at lower cost.
Common misconception
SRAM is still volatile. “Static” means that it does not require periodic refreshing while powered; it does not mean that it keeps data without power.
Programmable ROM technologies
ROM technologies differ mainly in when the contents are written and whether the chip can later be erased and programmed again. The names describe this increasing level of flexibility.
| Technology | How the contents are established | Can the contents be replaced? | Practical implication |
|---|---|---|---|
| Factory-programmed ROM | Data is fixed as part of chip manufacture | No | Efficient for a final design produced in very large quantities |
| PROM | A blank chip is programmed once after manufacture | No | Suitable when the final contents are added by the equipment maker |
| EPROM | Existing contents are erased using ultraviolet light before reprogramming | Yes | The chip normally has to be removed and exposed during the update process |
| EEPROM | Existing contents are erased and rewritten using electrical signals | Yes | Updates can normally be completed while the chip remains in the device |
Common mistake
The letter E does not mean the same thing in EPROM and EEPROM. EPROM is erased by ultraviolet light; EEPROM is erased electrically.
Choosing a memory technology
A good choice begins with the system requirements rather than with the name of a device. Ask whether the data must survive a power loss, how often it changes, how quickly it must be accessed, how much capacity is needed, and whether future updates are expected.
| Requirement | Reasonable choice | Reason |
|---|---|---|
| Several gigabytes for active applications in a workstation | DRAM | High density and lower cost make a large capacity practical |
| A small, very fast store between the processor and main memory | SRAM | Low access time is more important than capacity |
| One final control program written after the chips have been manufactured | PROM | The contents are added once and are not expected to change |
| Device firmware that may receive updates without removing the chip | EEPROM | Electrical erasure allows in-circuit reprogramming |
Justification pattern
Use this structure: requirement → memory property → justified choice. For example, “The controller may need firmware updates, so EEPROM is appropriate because it can be erased electrically and rewritten without removing the chip.”
Interactive: Memory Technology Explorer
Use the existing widget to compare memory properties, test what happens when power is removed, and inspect the programmable ROM families.
Widget note
The Buffer Queue tab has been retained as requested. Treat it as a preview of the later page on buffers and data transfer rather than as a core part of this memory-technology lesson.
Exam support
Comparing RAM and ROM
Include at least two paired contrasts: volatility, ability to change contents during normal use, and the type of information normally stored.
Comparing DRAM and SRAM
Link construction to consequences: capacitors require refreshing; flip-flop circuitry uses more components but provides faster access.
Common mistakes and misconceptions
- Claiming that ROM can never be changed, despite programmable ROM technologies.
- Saying that SRAM keeps data when the power is off.
- Naming cache or main memory without explaining why the technology suits that use.
- Confusing EPROM erasure by ultraviolet light with EEPROM erasure by electrical signals.
- Using “faster” or “cheaper” without identifying which memory technology is being compared.
Practice
1. Power-loss reasoning
A digital ticket machine is calculating a fare while also storing the instructions needed to start the machine. Explain which information belongs in RAM and which belongs in non-volatile memory.
2. Select DRAM or SRAM
- A large working-memory module for a video-editing computer.
- A small, high-speed cache beside a processor core.
- A low-capacity memory area in a controller where rapid access is the priority.
For each case, choose a technology and justify the choice using at least two properties.
3. Select a ROM technology
- A manufacturer receives blank chips and writes one final program to each chip.
- A laboratory repeatedly clears a legacy chip using ultraviolet light before testing new firmware.
- A museum display controller receives firmware updates while the chip remains fitted.
Identify PROM, EPROM, or EEPROM for each situation and explain the deciding feature.
4. Extended comparison
Explain why a computer might use both DRAM and SRAM rather than constructing all of its primary memory from only one of these technologies.
Review
| Concept | Essential idea |
|---|---|
| RAM | Volatile, read-write memory for active instructions and data |
| ROM | Non-volatile memory mainly read during normal operation |
| DRAM | Requires refreshing; high density and lower cost suit main memory |
| SRAM | No refresh while powered; faster but more expensive, so often used for cache |
| PROM | Programmed once after manufacture |
| EPROM | Erased using ultraviolet light before reprogramming |
| EEPROM | Erased and rewritten electrically, usually without removing the chip |
Final check
Can you justify a choice rather than merely name a memory type? Strong answers connect the system requirement to a technical property and then to the selected technology.