Assembly language programming (Little Man Computer)

Kofi is a software engineer and has been asked to write some code using the Little Man Computer (LMC) instruction set.

State the LMC command that would be used for each description given in the table.

Kofi has been asked to write a program that inputs two numbers and then outputs the largest of the two numbers.

Write the code for this program using the LMC instruction set.

You can assume that the two numbers will always be different.

* Kofi is working on a number of projects. These include:

• project 1 – Writing a mobile phone app to look at the news

• project 2 – Writing software to send temperature readings from an embedded heat sensor to other devices

• project 3 – Writing an image editing application for a desktop computer

• project 4 – Writing software to calculate the results of a physics experiment that produces terabytes of data

• project 5 – Writing a new device driver for a new piece of hardware.

Discuss which of these projects should be written in a high level programming language and which should be written in a low level language.

You should include the following in your answer:

• what is meant by high level programming language and low level language

• which projects are suitable for high level and which are suitable for low level languages

• a conclusion that justifies why these different languages are needed.

The stored program concept uses the Fetch-Decode-Execute cycle to get the next instruction from memory and then execute it.

Describe what happens during the fetch stage of the Fetch-Decode-Execute cycle.

You should state the different registers and buses that are used in your answer.

One of the instructions that may be fetched and executed as part of this cycle is a branch instruction.

State the name of the register that would be altered in the execute phase during a branch instruction.

Three ways of improving the performance of a CPU are increasing the clock speed, adding more cores and using pipelining.

Explain how pipelining improves the performance of a CPU.

The Fibonacci sequence is a series of numbers. It starts with the number 0 and then 1. Each number after that is a sum of the two numbers before it.

The first seven numbers in the series are: 0 1 1 (i.e. 1 + 0) 2 (i.e. 1 + 1) 3 (i.e. 1 + 2) 5 (i.e. 2 + 3) 8 (i.e. 3 + 5)

Orla has written some code to show the first five numbers in the Fibonacci sequence (0,1,1,2,3) using the Little Man Computer (LMC) instruction set.

The LMC code that Orla has written contains an error.

State the five outputs that Orla’s code would give.

In Orla’s LMC code, she used direct memory addressing.

Give three other modes of memory addressing.

Since the development of high level languages, the use of assembly languages has reduced.

Give two reasons why in some circumstances programmers will choose to write code in assembly language.

The performance of a computer system can be improved by adding more RAM.

Explain why adding more RAM will improve the performance of a computer system.

A charity uses a desktop computer to record financial donations that it receives. The computer contains a single core, 2.4GHz processor with 2MB cache.

The processor uses the Von Neumann architecture.

Describe what is meant by the term ‘Von Neumann architecture’.

Give one way that the Harvard architecture differs from the Von Neumann architecture.

The charity is concerned that the performance of the computer is not sufficient and wishes to replace the processor.

Give two features of a replacement processor that would increase the typical performance of the computer.

Fig. 1 shows assembly code written using the Little Man Computer (LMC). The program calculates and outputs the total amount that is donated to the charity in any particular day. Depending on the amount, an additional bonus may be added to each amount donated.

The program shown in Fig. 1 is run once using three different inputs. Therefore, while the program is running once, it will output the updated total three times.

Give the total values that are output when the values 10, 50 and 120 are input into this program.

Write LMC code that will reset the value of the memory location labelled total to zero and then stop the program.

This program is run on a processor that allows pipelining.

Define the term ‘pipelining’.

Explain one benefit to the charity of using a processor that allows pipelining.

The processor contains registers including the accumulator and the program counter. The contents of these registers are modified during the Fetch-Decode-Execute cycle.

Describe how the accumulator is used during the Fetch-Decode-Execute cycle.

Describe how the program counter is used during the Fetch-Decode-Execute cycle.

State the name of three other registers that are used during the Fetch-Decode-Execute cycle.

* The charity has several desktop computers in their office that use a CISC processor. They are considering buying mobile devices for their staff to use when they are not in the office.

Discuss whether these mobile devices should use the same CISC processors that are used in their desktop computers or if they should use a RISC processor instead.

You should include the following in your answer:

• the difference between each processor type

• the suitability of each processor type for mobile devices.

A program written using the Little Man Computer instruction set is shown in Fig. 1.

Various registers are used when the program above is executed.

State what is meant by the term ‘register’.

Explain how the accumulator is used when the line BRP pos is executed.

Complete the table below to show the output(s) from this program given the inputs.

Write an algorithm using pseudocode that has the same functionality as the code in Fig. 1

* In assembly language, different modes of addressing memory can be used.

Discuss the different modes used. You should include:

• How the operand value is determined

• What an operand of 27 would refer to in that mode

• The reasons for requiring multiple modes of addressing

The table below shows the Little Man Computer instruction set.

Complete the table above to show the missing mnemonics and instructions.

Write a program using the Little Man Computer instruction set that will allow a user to input two numbers and then output the larger of the two numbers. The program should loop continuously.

A program written in the Little Man Computer instruction set is given below.

State what the program outputs are for the following inputs.

State what the purpose of the program is.

Explain which registers are used and their values when the line STA count is executed and the accumulator is holding the value 9. The label count refers to memory location 16.

Whilst the line STA count is being executed, the CPU receives a signal from another process, requiring its attention.

State the name for the signal received by the CPU.

The code uses direct addressing. Describe one other mode of addressing.

A digital coffee making machine has a CPU that uses the Little Man Computer Instruction Set.

Little Man Computer operates on a computer system based on the Von Neumann Architecture.

State two features of the Von Neumann architecture.

2

Describe one feature, not part of the standard Von Neumann Architecture, which contemporary CPUs may have in order to improve performance.

Part of the coffee making machine’s code asks the user to press a button to select strength. The code outputs 1 which will switch on a green light to indicate a valid selection or outputs 0 to indicate an invalid selection.

The code is shown below:

Tick the appropriate boxes below to indicate which inputs will result in a green light (i.e. code outputs 1) and which with a red light.

Explain which registers and buses are used, and the values they store/carry, when the line LDA redLight is executed (after it has been fetched and decoded). You should assume the address redLight refers to memory location 11.

Write code in a high‑level language or pseudocode that has the same functionality as the code in Fig. 1.

* Discuss the differences between assembly code and high‑level languages. You should refer to:

• the advantages and disadvantages of writing programs in assembly code rather than a high-level language

• when each approach might be used

• why the coffee machine was programmed in assembly code.

Below is part of a program written using the Little Man Computer instruction set. This section of code can exit by either jumping to the code labelled pass or fail depending on what value is in the accumulator when the code is run.

Explain what the line ten DAT 10 does.

Complete the table below determining whether the program branches to pass or fail given the following values in the Accumulator when it is run.

The complete program is shown below:

Give one instruction in the program that when executed, changes the value in the Accumulator.

Give one instruction in the program that when executed, changes the value in the Program Counter.

State the value the code outputs for the input 18.

State the value the code outputs for the input 37.

Describe the purpose of the program.