Memory: RAM, ROM, SRAM, DRAM, EPROM, EEPROM

What you need to be able to do

RAM and ROM

Both are primary memory: the processor can access them directly. They differ in whether the contents can change and whether they survive power-off.

SRAM and DRAM

PROM, EPROM and EEPROM

Why EEPROM in an embedded device: the firmware can be updated (new features, bug fixes) without removing the chip, without special equipment, even by a non-technical user, and only the bytes that change need erasing.

Mark-scheme focus

Two differences between EPROM and EEPROM (2 marks)

Credited

• EPROM uses ultraviolet light to erase data whilst EEPROM uses an electrical signal B1
• EPROM has to be removed from the circuit board to change the data, EEPROM remains in the circuit // EPROM erases all the data, EEPROM can erase parts B1

Not credited

• “EEPROM is electrically erasable” (repeats the name)
• one-sided statements

From: 9618/12/O/N/24 Q2(c)

Three differences between DRAM and SRAM (3 marks)

Credited

• DRAM requires refreshing, SRAM does not; DRAM stores each bit as charge, SRAM uses flip-flops; DRAM is less expensive to manufacture;
• DRAM has slower access speeds; DRAM has higher storage density; DRAM is used in main memory, SRAM in cache B3

Not credited

• “DRAM is dynamic, SRAM is static”
• “SRAM is better”

From: 9618/11/O/N/25 Q6(c)

Why ROM is used in an embedded system (2 marks)

Credited

• To store data that does not change B1
• Data must be stored even when the device has no power // to store boot-up instructions / firmware / BIOS B1

Not credited

• “ROM is read only”
• a description of embedded systems with nothing about ROM

From: 9618/12/O/N/22 Q3(c)

Worked examples

Example 1 · Complete the memory description; three ROM types (Paper 1, 8 marks) ★★ Medium

1 Read it with me what is actually happening?

(i) five gaps. (ii) three distinct comparisons across the three ROM types.

• … memorya type of memory, not “computer”

2 What is it asking for? the target, part by part

• (a)(i)5 gaps5
• (a)(ii)3 differences3

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

What was the commonest wrong first answer?

“computer” - the paragraph is already about computer memory.

5 Now solve it have a go on paper first, then open the worked solution

1. (i) primary; RAM: currently running software / data / part of the OS; ROM: start-up / boot-up instructions / BIOS; SRAM: flip-flops / latches; DRAM: capacitors B5
2. (ii) PROM can be set once; EPROM and EEPROM can be overwritten many times B1
3. EPROM must be removed from the device; EEPROM can be erased in situ B1
4. EPROM is erased with UV light, EEPROM with a voltage // EPROM must be entirely erased, EEPROM need not be B1

6 Check it does the answer make sense?

✓

Example 2 · SRAM in a laptop; EEPROM in a VR headset (Paper 1, 4 marks) ★★ Medium

1 Read it with me what is actually happening?

(a) reason + why; (b) two reasons tied to updating the headset's firmware.

2 What is it asking for? the target, part by part

• (a)22
• (b)22

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

1. (a) SRAM has a faster access time B1 … because it does not need to be refreshed // used for CPU cache B1
2. (b) EEPROM allows frequent / multiple read / write / erase operations B1 … so the headset's firmware can be changed easily, without fully erasing it first B1

6 Check it does the answer make sense?

✓

Example 3 · Two differences between DRAM and SRAM (Paper 1, 2 marks) ★ Easy

1 Read it with me what is actually happening?

Two different properties, each comparing both.

2 What is it asking for? the target, part by part

• (a)22

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

1. DRAM requires constant refreshing, SRAM does not B1
2. DRAM generally has greater capacity per chip // DRAM has slower access times than SRAM B1

6 Check it does the answer make sense?

✓

Example 4 · ROM contents and DRAM's advantages in a CCTV computer (Paper 1, 4 marks) ★★ Medium

1 Read it with me what is actually happening?

(i) two different things in ROM (computer start-up, CCTV firmware, OS kernel). (ii) precise advantages.

2 What is it asking for? the target, part by part

• (a)(i)22
• (a)(ii)22

3 What do we know? everything the question gives us

4 Plan the route work backwards from what we want

FindKey ideaAlready have itWork out firstTrap

5 Now solve it have a go on paper first, then open the worked solution

1. (i) Bootstrap program / start-up instructions / BIOS B1; start-up instructions / firmware for the CCTV cameras // parts of the OS kernel B1
2. (ii) Costs less per unit B1; higher storage density // simple design, fewer transistors B1

6 Check it does the answer make sense?

The report: vague “DRAM is cheaper” was not credited ✓.

Summary

• RAM: volatile, read / write, data in use. ROM: non-volatile, read only, boot instructions / firmware.
• SRAM: flip-flops, no refresh, fast, expensive, low density → cache. DRAM: capacitor + transistor, refreshed, slower, cheap, dense → main memory.
• PROM once; EPROM UV, removed, whole chip; EEPROM electrical, in situ, byte or whole → updatable firmware.

Can I do this?

Quiz

Concept checks

Answer each one in your head first, then open it.

Exam-style practice

New questions, not used above. Do them on paper, then mark your work against the official mark scheme.