• Program Counter: Counts the number of instructions. Where a computer is in its execution
• Instruction register - Holds the instruction currently being executed
• MAR: Memory Address Register – Stores memory addresses
• MDR: Memory Data Register – Store data in memory
• ITS ALL ONES AND ZEROS XD

Memory

• K x M array stored bits (k is usually 2^n)

• Address
  • Unique n-bit identifier of location
• Contents
  • m-bit value stored in location
• Addressability
  • The size of the memory location referenced by a given address
  • A word is a basic unit of data used by the processing unit (usually multiple bytes)
• Basic operations
  • LOAD: read a value from a memory location

        1. Write the address (A) into the MAR  
        2. Send a read signal to the memory
        3. Read data from the MDR
    

  • STORE: write a value to a memory location

        1. Write the dara to the MDR
        2. Write the address into the MAR
        3. Send a write signal to memory
    

• LC3 : 16 bit addresses, 16 bit values

Processing Unit

Contains the ALU and the Register File

SR, D R

Source Register and Destination Register. You would add two registers, through the ALU which is process it and then store it in a destination register.

Functional units

• ALU; Has an adder and other components like multiply and square root
• LC-3 Performs ADD, AND, NOT

Registers

• Small storage
• Operands and results of functional units
• LC-3 has eight registers (R0..R7) each 16 bits wide

Word Size

• number of bit normally processed by the ALU in one instruction
• also width of registers
• LC-3 is 16 bits

SR2MUX

The multiplexer controls what ALU process is going to be used

Control Unit

• Conductor of an orchestrates

Instruction Register

Contains the current instruction.

Program Counter

Contains the address of the next instruction to be execution

Control Unit

• Reads instructions from memory
• Interprets the instruction, generate singals that tell the other components what to do.

Input and Output

• Devices for getting data into and out of computer memory
• Each device has its own interface, usually a set of registers like the memory’s MAR and MDR
• LC-3 Supports keyboard input and console output
  • Keyboard: data reg (KBDR) and status (KBSR)
  • Monitor: data reg (DDR) and status (DSR )
• Some devices provie both input and output
  • disk, network
• Program/Software that controls access to a device is usually called a driver

Instruction Processing

• There are limited sets of instructions in LC-3

Instructions are encoded as a sequence of bits as data in memory.

Format:

[4 bit opt code][12 bit operand]
operation       data to be used for operation

• Control unit interprets instruction: Generates a sequence of control ignal to carry out operation.

• Every operation is either executed completely or not at all.

• Instruction sets are known as ISA

• Kinds of instructions

  • Computational (ADD, AND)
  • Data movememnt (LD, ST)
  • Control instructions (JMP, BRnz)

Every operation that starts needs to finish

• Fetch operation: load an instrcution (at address stored in PC) from memory into IR.
  • Load contents of PC into MAR
  • Send “read” signals to memory
  • Copy the MDR content to IR
  • Increment PC so that it points to this instruction
    • PC <- PC+1
• Decoding:
  • Identify the op code (first four bits in LC-3)
  • Depending n the op code look at the other operations
  • Obtain source operands needed to perform operation
• Execute:
  • Perform the operation using the source operand
  • Executing the ADD operation of LC-3
• Store:
  • Writes results to destination (Register or memory)
  • Memory store: Write address to MAR, data to MDR, assert WRITE signal to memory

Fetch - Decoding - Execute - Store

Changing the sequence of execution

• In the FETCH phase, we incremented PC by 1.

• What if don’t want to always execute the instrcution that follows this one? - Loop/If then

• Control Instructions
  • Branches are conditional – they change the PC only if some condition is true (So you can run certain instructions in certain cases)
  • Jumps are unconditonal - they always change the PC

// Set the PC to value the contained in a register. This becomes the address of the next instruction to fetch
[JMP][000][BASE][00000]

The Clock

The clock is a signal that keeps the control unit moving. At each clock tick, control unit moves to the next machine cycle

Clock generator circuit.

• 0V (0) and 2.9V (1)