Little Man Computer (LMC)

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Joshua Carpenter
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Little Man Computer (LMC) A-level Computing Independent Study Project Part Two The Little Man Computer (LMC) is a simulator which models the basic features of a modern computer.

1 Little Man Computer (LMC) A-level Computing Independent Study Project Part Two The Little Man Computer (LMC) is a simulator which models the basic features of a modern computer. It features a central processing unit (CPU) consisting of an arithmetic logic unit (ALU) and registers, a control unit containing an instruction register and program counter, input and output mechanisms and memory (RAM) to store both data and instructions. The LMC is based on the concept of a little man shut inside a computer acting like the control unit of a CPU, i.e. fetching instructions from memory, decoding and executing the instructions, and managing the input and output. The LMC can be programmed using assembly language which is then converted ( assembled ) into machine code (appearing in denary rather than binary). Each assembly code instruction is converted into a 3 digit machine code instruction (1 digit for the instruction and 2 for the memory address). The 3 digit machine code instructions are then loaded into RAM, starting at memory address 0. Any data is also loaded into memory at a particular memory address. The 'Little Man' can then begin execution, starting with the instruction in RAM at memory address 0. CPU (Central Processing Unit) This is the part that executes the program and tells the other components what they need to do. the main function of the CPU is to fetch and execute the instructions. ALU (Arithmetic/Logic Unit) & (RAM) memory The ALU is the part of the computer that performs all the math and logic operations. The memory keeps track of information so that it can be recalled later.

2 The LMC performs the following steps to execute a program: Checks the Program Counter so it knows the memory address to look at next Fetches the instruction from the memory address that matches the program counter. Increments the Program Counter (so that it contains the memory address of the next instruction). Decodes the instruction (includes finding the memory address for any data it refers to). If required by the instruction code, fetches the data from the memory address found in the previous step. Executes the instruction and if necessary sets the Program Counter to match any branch instructions. The LMC Instruction Set A real CPU would have hundreds of instructions but the Little Man Computer has a limited instruction set containing only the following commands: Instruction Mnemonic Machine Code Description Example Load LDA 5xx Store STA 3xx Add ADD 1xx Subtract SUB 2xx Input 901 Output 902 Loads the contents of address xx into the accumulator. (Note: the contents of the address are not changed.) Stores the contents of the accumulator to address xx. (Note: contents of the accumulator are not changed.) Adds the contents of address xx to the accumulator. (Note: the contents of the address are not changed and the total cannot exceed 999.) Subtracts contents of address xx from the accumulator. (Note: the contents of the address are not changed.) (Note: sets a negative flag when result is less than zero) Copies the value from the "input box" into the accumulator. Copies the value from the accumulator to the "output box". (Note: contents of accumulator are not changed.) End HLT 000 Halts (stops) the LMC simulator executing the program. Data storage Branch if positive (or zero) Branch if zero Branch always DAT BRP BRZ 8xx 7xx Reserves the memory address for data storage. The address used is wherever is next when this instruction is compiled. Note: a value can also be stored at the memory address If the accumulator is positive or zero (i.e. the negative flag is not set) then the program counter jumps to address xx. If the accumulator is zero then the program counter jumps to address xx. BRA 6xx Program counter jumps to address xx. NOTE: xx represents a memory address between 0 and 99.

3 Recap of previous tasks 1. You have a presentation that explains the basic concepts and features of the Little Man Computer (LMC). 2. UT and PUT instructions of the LMC. # the input value is copied into the accumulator STA data1 # contents of accumulator stored at the memory address allocated for data1 LDA data1 # data stored at the memory address for data1 is loaded into the accumulator. # the value in the accumulator is sent to the PUT HLT # the program halts. data1 DAT # this memory address will have data stored as data1 3. Load and Save UT a set of two numbers. PUT them in the same order that they were entered. first # input value is copied into the accumulator STA first # accumulator contents are stored in memory address labelled first # second input value is copied into the accumulator STA second # accumulator contents are stored memory address labelled second LDA first # contents of memory address labelled first are loaded into accumulator # value in the accumulator is sent to the PUT LDA second # contents of memory address labelled second are loaded into the accumulator # value in the accumulator is sent to the PUT HLT # the program halts first DAT # this memory address will have data stored as first second DAT # this memory address will have data stored as second 4. Addition and subtraction UT a set of three numbers. The program adds the first two numbers and outputs the answer, then subtracts the first number from the third and outputs the answer. # input value is copied into the accumulator STA num # accumulator contents are stored in memory address labelled first # second input value is copied into the accumulator ADD num # adds the contents of memory address labelled first to the accumulator and stores the result in the accumulator. # value in the accumulator is sent to the PUT # input value is copied into the accumulator SUB num # subtracts the contents of memory address labelled first from the accumulator and stores the result in the accumulator. # value in the accumulator is sent to the PUT HLT # the program halts num DAT # this memory address will have data stored as num 5. You also wrote other programs including: Write a program to add 3 numbers Write a program that correctly calculates x + y + z a b Write a program that will prompt for 2 numbers, subtract the first from the second and output the answer, then subtract the second from the first and output the answer.

4 TASK ONE: SELECTION (BRANCHING) High level language code #high-level program to input a number and display it but limit the displayed value to 100 LMC assembly code For branching, LMC has BRP (branch if positive) and BRZ (branch if zero) No operators such as < and > so how can we do it? LMC assembly code (with comments):

5 Variations to practice: #high-level program to input a number and display it only if it is above 10, otherwise 10 is displayed input num1 if num1 > 10 then print num1 else print 10 endif LMC assembly code (with comments): #high-level program to input a number and display it only if it is above 10, otherwise blank input num1 if num1 > 10 then print num1 endif LMC assembly code (with comments):

6 PROGRAM 1: Write a program that will prompt for 2 numbers and check if they are the same. If they are then the program should output the number 1. If they are not then the program should output the number 0. PROGRAM 2: Write a program to input 3 numbers and then output the highest.

7 PROGRAM 3: Guess the number: set a number, user guesses, if they get it right then output 1, otherwise output 0 Extension: add more features to your guess the number program, e.g. say if the guess is close, within 2 (2) or say if guess is too high (99) or low (88)

8 TASK TWO: ITERATION (LOOPING) High level language code #validation of an input to make sure it is less than 10 num1=input( Enter a number less than ten ) while num1 > 10 num1=input( Enter a number less than ten ) endwhile LMC assembly code No assembly commands for iteration itself so we need to use branching and labels In this case we can subtract 11 from num1 and see if the answer is positive or zero (i.e. num1 is above 10) loop STA num1 SUB eleven BRP loop LDA num1 HLT eleven DAT 10 num1 DAT #input value (and label this memory address as loop) #store as num1 #subtract 11 from acc (num1-10) #jump to label loop if (num1-10) positive #display num1 #create data location called hundred and store 11 in it #create data location called num1 High level language code #validation of an input to make sure it is less than 10 max=input( Enter a number to count up to ) for count = 1 to max print(count) LMC assembly code STA max loop LDA count LDA max SUB count BRZ endofloop LDA count ADD one STA count BRA loop endofloop HLT #input value #store as max # count loaded into accumulator (and label this memory address as loop) # display accumulator (current value of count) # max loaded into accumulator # (max count) will be zero if count has increased enough # in which case stop the loop # count loaded into accumulator # add one to count # store new value for count # loop back # loop stops max DAT #create data location called max count DAT 1 #create data location called count and store 1 in it (initially) one DAT 1 #create data location called one and store 1 in it

9 PROGRAM 4: Enter a number and then display a countdown to zero from that number PROGRAM 5: Rewrite the for loop example so that it only uses BRP and not BRZ or BRA

10 PROGRAM 6: Write a program that will input two numbers and multiply them. PROGRAM 7: Write a program that will input an number, then output the square of the number. Extension: Write a program to divide one number by another.

Teaching London Computing

Teaching London Computing CAS London CPD Day 2016 Little Man Computer William Marsh School of Electronic Engineering and Computer Science Queen Mary University of London Overview and Aims LMC is a computer