Computer Science 101

1. Computer Science 101 Computer Systems Organization ALU, Control Unit, Instruction Set

2. CPU Major Components Bus Memory Input Output Units Control Unit ALU

3. ALU ALU - Arithmetic Logic Unit • Consists of • Bank of registers • Operation Circuitry • ALU Control Circuitry • Connections Result L R R0 R1 R15

4. Multiplexor - Review • 2n input lines • 1 output line • n control lines • control lines “select” one of the inputs to be “passed” to the output

5. Zoom in on ALU R15 + - Mux Control

6. R + - Mux Control Simple Hypothetical Lab Machine • One Register • One Operand

7. Control Unit • Cycle • Fetch next instruction from memory • Decode instruction • Execute it by giving proper control signals to ALU and memory.

8. Control Unit Registers • The Program Counter(PC) is a register in the control unit which holds the address of the nextinstruction to execute. • The Instruction Register(IR) is a register in the control unit which holds the current instructionbeing executed.

9. Machine Language • Instruction Set - set of instructions (binary) that the specific type of machine can execute. • Instruction typically has • Opcode field - set of bits telling which instruction • Address fields - memory or register address where operands will be found.

10. Opcode Address 0110 001101010011 Instruction form for hypothetical machine • 4-bit opcode field - 16 instruction types • 12-bit address field- 4096 memory cells

11. Machine Language Groups • Data Transfer • Arithmetic • Compare • Branch

12. Data Transfer instructions • General machine • Memory cell  ALU register • ALU register  Memory cell • Memory cell Memory cell • ALU register  ALU register

13. 12-bit address Data Transfer instructions • Hypothetical machineOpcode Operation Meaning 0000 Load X Con(X)R 0001 Store X RCon(X) Content stored at address X

14. Arithmetic - ADD,SUB,AND,etc. • Hypothetical machineOpcode Operation Meaning 0010 Clear X 0Con(X) 0011 Add X R+Con(X)R 0100 Increment X Con(X)+1 Con(X) 0101 Subtract X R-Con(X) R 0110 Decrement X Con(X)-1 Con(X)

15. Compare instructions - Compare values and set an indicator. • Condition Code Register (CC): 3 bit register with bits GT, EQ, LT. • When two values are compared, the result of the comparison will be stored in the CC – was the result GT, EQ, or LT. • Later instructions can look at CC to see what happened with the last comparison.

16. Compare instructions - Compare values and set an indicator. • Hypothetical machineOpcode Operation Meaning 0111 Compare X If Con(X) > R CC: 100 else if Con(X)<R CC: 001 else CC: 010

17. Branch instructions - Alter flow of control based on CC. • Hypothetical machineOpcode Operation Meaning 1000 Jump X X  PC 1001 JumpGT X If GT=1, X  PC 1010 JumpEQ X If EQ=1, X  PC 1011 JumpLT X If LT=1, X  PC 1100 JumpNE X If EQ=0, X  PC

18. Input/Output and Halt • Hypothetical machineOpcode Operation Meaning . 1101 In X Input valueCon(X) 1110 Out X Con(X)Output value 1111 Halt Stop execution

19. Could you pass that by me again, please?