Sub-systems made of further sub-systems

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Quick Questions

Shade in the A or B circle for each question.

1. A CPU is a sub-system of a computer system. Can the CPU itself be split into smaller sub-systems?

A No, a sub-system can never be split again

B Yes, e.g. into the ALU, control unit and registers

2. Which choice shows the levels in the correct order, from top to bottom?

A Computer system, CPU, ALU

B ALU, CPU, computer system

3. The CPU is a sub-system of the computer. When we split the CPU into parts, how do we treat the CPU?

A As a system in its own right at the next level

B As a single part that cannot be looked at again

4. When should we stop splitting sub-systems into smaller ones?

A When each part does one simple job

B When we have drawn exactly three levels

5. Which part of the CPU carries out calculations and logic decisions?

A The registers

B The ALU

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Computer system → CPU → ALU		AThe levels are in the wrong order: the ALU is part of the CPU, so the CPU must be above the ALU in the tree.
2CPU → ALU, control unit, registers		BA system split level by level: the CPU is a sub-system of the computer, and the ALU is a sub-system of the CPU
3A sub-system at one level is a system at the next level down		CComputer system → CPU, then CPU → ALU, control unit, registers
4Keep splitting until each part does one simple job, then stop		DThe CPU is one system at this level, and it splits into three smaller sub-systems at the next level
5Computer system → ALU → CPU		EThe monitor is not part of the CPU. It is an output device, so it does not belong to this branch of the tree.
6CPU → ALU, control unit, registers, monitor		FALU: carries out arithmetic and logic (no further split)

Definitions

hierarchy of sub-systems

Fill in the Blanks

Complete each line of code by filling in the missing parts.

_________ Computer() CALL CPU() ENDPROCEDURE

PROCEDURE Computer() ____ CPU() ENDPROCEDURE

PROCEDURE CPU() CALL ALU() ____________

CALL ControlUnit() CALL _________() CALL ALU()

Computer -> CPU -> ___

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A PROCEDURE ComputerSystem()

B CPU()

C CALL Memory()

D ENDPROCEDURE

Question 2

A PROCEDURE CPU()

B RUN ALU()

C CALL ControlUnit()

D CALL Registers()

E ENDPROCEDURE

Question 3

A PROCEDURE ALU()

B CALL Add()

C CALL Compare()

D ENDFUNCTION

Question 4

A FUNCTION ControlUnit()

B CALL Fetch()

C CALL Decode()

D ENDPROCEDURE

Question 5

A WHILE IsSimple(part) DO

B CALL Split(part)

C part ← NextPart()

D ENDWHILE

Answer Key

Sub-systems made of further sub-systems

Section A: Quick Questions

B — Yes, e.g. into the ALU, control unit and registers

A — Computer system, CPU, ALU

A — As a system in its own right at the next level

A — When each part does one simple job

B — The ALU

Section B: Match the Pairs

Computer system → CPU → ALU → A system split level by level: the CPU is a sub-system of the computer, and the ALU is a sub-system of the CPU

CPU → ALU, control unit, registers → The CPU is one system at this level, and it splits into three smaller sub-systems at the next level

A sub-system at one level is a system at the next level down → Computer system → CPU, then CPU → ALU, control unit, registers

Keep splitting until each part does one simple job, then stop → ALU: carries out arithmetic and logic (no further split)

Computer system → ALU → CPU → The levels are in the wrong order: the ALU is part of the CPU, so the CPU must be above the ALU in the tree.

CPU → ALU, control unit, registers, monitor → The monitor is not part of the CPU. It is an output device, so it does not belong to this branch of the tree.

Section C: Definitions

hierarchy of sub-systems — Each sub-system can be split into smaller sub-systems, level by level, until every part does one simple job. A sub-system at one level is a system at the next.

Section D: Fill in the Blanks

PROCEDURE Computer()
  CALL CPU()
ENDPROCEDURE

PROCEDURE Computer()
  CALL CPU()
ENDPROCEDURE

PROCEDURE CPU()
  CALL ALU()
ENDPROCEDURE

CALL ControlUnit()
CALL Registers()
CALL ALU()

Computer -> CPU -> ALU

Section E: Spot the Error

B — A sub-system is run from its parent with the CALL keyword. The line should be CALL CPU().

B — Cambridge pseudocode uses CALL to use a sub-system, not RUN. The CPU splits into the ALU, control unit and registers, so the line should be CALL ALU().

D — A PROCEDURE must finish with ENDPROCEDURE. ENDFUNCTION closes a FUNCTION, so the last line should be ENDPROCEDURE.

A — The control unit is written as a PROCEDURE, which is closed by ENDPROCEDURE and uses CALL. FUNCTION does not match. The first line should be PROCEDURE ControlUnit().

A — We keep splitting while the part is NOT yet simple. The loop should stop once each part does one simple job, so the line should be WHILE NOT IsSimple(part) DO.