Computer systems, embedded systems and buffers

What you need to be able to do

Input, output, memory and storage

Primary or secondary? Ask whether the data is needed now (being processed: primary) or must survive switching off (secondary). In a car's trip computer, the miles for the current journey are in primary memory while the engine runs; the total miles since the car was built are in secondary storage.

Embedded systems

Examples: washing machines, microwave ovens, TVs, car engine management and lane-detection systems, video doorbells, alarm systems, 3D printers, traffic lights, smart watches.

Buffers

1. The sender (e.g. the computer) writes data into the buffer at its own, faster speed.
2. The computer is then free to carry on with other tasks instead of waiting for the slower device.
3. The receiver (e.g. the printer) takes data out of the buffer at its own, slower speed.
4. When the buffer is empty, the device sends an interrupt to the processor requesting more data (or, if full, signals to stop sending).

Mark-scheme focus

Why is the lane detection system an embedded system? (2 marks)

Credited

• The lane detection system is built into / integrated into the car B1
• It only performs one task // it is not easily changed / updated by the car owner B1

Not credited

• “It is embedded in the car” (repeats the term)
• “It is a control system” (a different idea)
• Repeating how the system works (given in the question)

From: 9618/12/M/J/22 Q2(a)

Explain the purpose of a buffer, with one other example (3 marks)

Credited

• Purpose: to act as temporary storage … before data is used by the receiving device … to allow processes / devices to operate at different speeds / independently B2
• Example: printer buffer // video buffer when streaming // keyboard buffer B1

Not credited

• Only describing how the buffer works, with no reason (speed difference)
• Using audio again (the question wants another example)

From: 9618/13/O/N/22 Q1(c)

Worked examples

Example 1 · An embedded system in a washing machine, with its RAM and ROM (Paper 1, 4 marks) ★ Easy

1 Read it with me what is actually happening?

Every answer must be about the washing machine: the report said many were generic.

• using … as an exampleapply each point to the washing machine

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

• (a)define + apply2
• (b)RAM, ROM in the washer2

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

• (b)
  • RAMdata in use now: the chosen program, sensor readings
  • ROMstart-up instructions / the wash programs

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

1. (a) A microprocessor within a larger system that performs one specific task B1; e.g. it only controls the programs for the washing cycle B1
2. (b) RAM: stores the wash program the user chose / the data read from the sensors / the time left B1; ROM: stores the start-up instructions (for the washing cycles) B1

6 Check it does the answer make sense?

Each point names something the washing machine actually does ✓.

Example 2 · Characteristics of an embedded system (Paper 1, 3 marks) ★ Easy

1 Read it with me what is actually happening?

Three separate characteristics.

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

• (b)(ii)3 characteristics3

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

Is “embedded in another system” enough?

No: say “integrated into a larger system” (the report).

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

1. Built into / integrated into a larger system (the car alarm) B1
2. Combination of hardware and software designed for a specific function B1
3. Must have a processor, memory and input / output // not easily changed / updated by the owner B1

6 Check it does the answer make sense?

✓

Example 3 · Drawbacks of embedded systems (Paper 1, 3 marks) ★ Easy

1 Read it with me what is actually happening?

Drawbacks of being dedicated, built-in and hard to change.

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

• (a)3 points3

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. It is difficult for the user to change / update the firmware B1
2. Errors cannot be fixed easily: repair is a specialist / expensive task B1
3. Faulty / outdated devices are often thrown away rather than repaired … leading to e-waste B1

6 Check it does the answer make sense?

Also credited: functionality can't easily be changed / extended for another task ✓.

Example 4 · A buffer for a hard disk (Paper 1, 4 marks) ★★ Medium

1 Read it with me what is actually happening?

Speed mismatch → temporary store → computer fast in → drive slow out.

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

• (b)4 points4

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

• Buffer
  • Whycomputer faster than the HDD
  • Incomputer writes at high speed, not held up
  • OutHDD reads at its slower rate

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

1. The computer transfers data faster than the hard disk can receive it B1
2. The buffer is used for temporary storage B1
3. The computer transfers data to the buffer at the higher speed and is not held up waiting B1
4. Data is transferred from the buffer to the hard disk at the slower rate B1

6 Check it does the answer make sense?

The order follows the data: computer → buffer → disk ✓.

Example 5 · How a laser printer uses a buffer (Paper 1, 4 marks) ★★ Medium

1 Read it with me what is actually happening?

The same pattern, now with the interrupt when the buffer empties.

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

• (c)4 points4

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. The print data is sent by the laptop to the buffer (at laptop speed) B1
2. The data is transferred from the buffer to the printer (at printer speed) B1
3. … allowing the user / processor to carry on instead of waiting for the slower printer B1
4. When the buffer is empty an interrupt is sent to the laptop requesting more data B1

6 Check it does the answer make sense?

✓

Summary

• Input in, output out; primary memory holds what is in use (processor-accessible); secondary storage keeps data with the power off; removable storage moves it.
• Embedded: dedicated microprocessor system built into a larger device, one task, hard to update. Drawbacks: hard to update / repair, can't be adapted, e-waste.
• Buffer: temporary store between devices of different speeds, so the processor isn't held up; interrupt when empty.

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.