3 BIT Computer Simulator

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3 BIT Computer Simulator

• Stewart Blakeway,
• FML208,
• blakews_at_hope.ac.uk
• 0151 291 3113 
• http//hopelive.hope.ac.uk/computing/

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3BIT

• 3BIT simulator was developed by Brian Farrimond
  when he was Pathway leader for Information
  Technology
• It was designed to teach students what goes on
  inside the computer.
• 3BIT progresses from simple processes to more
  complicated and has various components attached
  to its BUS

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What is 3BIT?

• 3Bit is a 3Bit computer simulator.
• It has various instructions that will manipulate
  how the data stored in memory is processed
• If we are only using 3BITs of memory for the
  processor instructions. How many instructions do
  we have?

23 or 8
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8 Instructions

• Which is a good thing. Because you have to
  memorise all 8 instructions to be able to get the
  most out of the simulator!
• LDI
• LDD
• STD
• ADD
• SUB
• STP
• JMP
• JEZ

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Lets look at the more obvious ones first!

• ADD Will add the contents of the memory address
  specified to whatever is in the ACCUMULATOR
• SUB - Will subtract the contents of the memory
  address specified from the ACCUMULATOR
• STP Will stop the execution of the instructions

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The CPU

• Every CPU has at least 3 registers
• Accumulator
• Instruction Register
• Program Counter

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ACUMULATOR

• What is an accumulator?
• a register or electric device on an arithmetic
  machine (eg adding machine, cash register, or
  digital computer)
• that receives a number and produces and stores
  the results of arithmetic operations of the given
  number with other numbers.

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INSTRUCTION REGISTER

• What is an Instruction Register?
• part of a CPUs control unit that stores the
  instruction currently being executed
• in simple processors each instruction to be
  executed is loaded into the instruction register
• which holds it while it is decoded, prepared and
  ultimately executed

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PROGRAM COUNTER

• What is a Program Counter?
• A register in a computer which indicates where
  the computer is in its instruction sequence.
• Depending on the details of the particular
  machine, it holds either the address of the
  instruction being executed, or the address of the
  next instruction to be executed

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3BIT CPU

• IR (INSTRUCTION REGISTER)
• AC (ACCUMULATOR)
• PC (PROGRAM COUNTER)
• In the program these labels also act as reset
  buttons. Click them to reset the values in the
  registers.
• Run will run through the program executing one
  instruction after the other
• Step will execute one instruction and wait for
  you to click step before executing the next.

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Where are the Instructions Stored?

• A CPU can only execute one instruction at a time.
• Where are the other instructions stored?

• The Instructions are stored in RAM. They are
  fetched and then executed.
• This is known as the fetch and execute cycle

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3BIT and Memory

• Our simulator has 32 Memory Locations
• Each location can store one byte.
• Enough memory in each location to store a
  character
• Real Memory has billions of locations.

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Question

• How Many Bits in a Byte?
• Why is it called 3BIT?

8 Bits in a byte
3 BITS for the INSTRUCTION 5 BITS for the DATA
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OP CODE and OPERAND

• The OP CODE is the instruction. The Operation
  Code. It uses 3 Bits so there are 8 possible
  instructions.
• The OPERAND is the data that is related to the
  instruction. 5 Bits are used for the OPERAND

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3BIT Memory

• Notice that some of the memory locations have red
  lines coming from them.
• This is to signify that that section of the
  memory has been reserved for use with the Video
  Card

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Back to the Instructions

• ADD Will add the contents of the memory address
  specified to the accumulator
• SUB - Will subtract the contents of the memory
  address specified from the accumulator
• It is very important to realise now that when you
  specify ADD x. The x refers to a memory
  location. Not a value!

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For Example
Answer 10 plus 2 equals 12

• Question What will the AC become once the
  program has finished executing?
• Address 16 holds value 10.
• Address 10 holds value 2

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Lets take a look at the other Instructions?

• LDD
• Load contents of location x into the AC
• STD
• Transfer the contents of the AC to location x

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LDD

• Given the program in memory, what will the
  accumulators state be once the program has
  executed?
• Answer 75

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STD

• Given the following program.
• What will happen?

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5/8

• We have covered 5 Instructions and how they work
• LDD, STD, ADD, SUB, STP
• What about the other 3?
• LDI, JMP, JEZ

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LDI

• LDI is similar to LDD with one key difference.
• LDI 27 will load 27 into the AC
• LDD 27 what does this do???

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JMP

• JMP is a Jump command. IT CHANGES THE EXECUTION
  OF THE PROGRAM BY CHANGING THE CONTENTS OF THE
  PROGRAM COUNTER TO THE ADDRESS SPECIFIED
• JMP 10, will start executing the program from
  memory position 10

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JEZ

• JEZ is similar to JMP but it will only jump if
  the AC is equal to zero
• Jump if Equal to Zero

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JEZ

• Given the following program
• What will happen?

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Thats eight instructions

• All eight instructions.
• You have to learn and memorise the instructions
• You will start programming in 3BIT soon.

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CPU

• The CPUs processing power is measured with a
  clock speed.
• The higher the speed the quicker the CPU will
  process instructions
• MHz
• GHz

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3BIT and the clock speed

• 3 BIT has a clock speed too.
• By default it is set to 0.3 Hz
• Very slow, this is deliberate so that you can see
  the instructions being executed
• The clock speed can be changed to
• 0.2 Hz
• 10 Hz
• 50 Hz

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Snippets

• So far you have only seen snippets of 3 BIT.
• This is so as not to confuse you
• What have we covered?
• The CPU
• The Memory
• The CPU Clock Speed

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What is left?

• The Hard Disk
• The Display (pixel and character)
• The Printer
• The Keyboard
• The Network Card (frame buffers,
• Computers 1 and 2

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What do you need to do next?

• You need to learn all 8 instructions
• Your tutor will expect you to know these
  instructions
• You will be given programming exercises to
  complete in 3BIT they progressively get harder
  to very hard!