Computer Science 210 Computer Organization

1. Computer Science 210Computer Organization Machine Language Instructions: Control

2. Control Structures: Sequence Statement-1 begin statement-1 . . statement-n end . . Statement-n

3. Control Structures: Selection if condition then consequent sequence else alternative sequence Boolean expression False Alternative True Consequent

4. Control Structures: Loops (Entry Control) while condition do loop body sequence Boolean expression False True Loop body

5. Control Structures: Loops (Exit Control) do loop body sequence until condition Loop body False Boolean expression True

6. LC-3 Control Instructions • Conditional branch (BR) • Absolute branch (JMP) • Procedure call (JSR, JSRR, RET, RTI) • System call (TRAP)

7. Condition Codes • 3 single-bit registers named N, Z, and P • Exactly one will be set at all times • Set by any instructions that write data to a register (ADD, AND, NOT, LD, LDR, LDI, LEA) 0 1 0 N Z P

8. Example: Subtract 1 from R3 0 1 0 When R3 = 0 add R3, R3, -1 N Z P 0 0 1 When R3 > 0 Circuitry sets condition codes after add executes N Z P 1 0 0 When R3 < 0 N Z P

9. Conditional Branch (BR) • Alters a sequence of instructions by changing the PC • Branch is taken if the condition is true • Signed offset is added to PC if condition is true; otherwise, PC not changed

10. Conditional Branch 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 0 0 N Z PPC offset 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 0 0 0 1 0 0 1 1 0 1 1 0 0 1 BR n z p x0D9 Offset is sign-extended and added to the incremented PC Destination must be no more than +256 or -255 from the BR itself

11. Data Path for BR

12. Example: An Unconditional Branch 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 0 0 N Z PPC offset 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 0 0 0 0 1 1 1 0 1 1 0 1 1 0 0 1 BR n z p x0D9 At least one condition code is guaranteed to match the codes in this instruction

13. Example: Sum 10..1 sum = 0 number = 10 while number > 0: sum += number number -= 1

14. Example: Sum 10..1 sum <- sum & 0 number = 10 while number > 0: sum += number number -= 1

15. Example: Sum 10..1 sum <- sum & 0 number <- sum + 10 while number > 0: sum += number number -= 1

16. Example: Sum 10..1 sum <- sum & 0 number <- sum + 10 while number > 0: sum <- sum + number number -= 1

17. Example: Sum 10..1 sum <- sum & 0 number <- sum + 10 while number > 0: sum <- sum + number number <- number + (-1)

18. Example: Sum 10..1 sum <- sum & 0 number <- sum + 10 while: sum <- sum + number number <- number + (-1) BR p while while is a label for the address of an instruction

19. Example: Sum 10..1 R2 <- R2 & 0 R1 <- R2 + 10 while: R2 <- R2 + R1 R1 <- R1 + (-1) BR p while Replace variables with registers R2 = sum, R1 = number

20. Example: Sum 1..10 sum = 0 number = 1 limit = 10 while number <= limit: sum += number number += 1

21. Example: Sum 1..10 sum <- sum & 0 number <- sum + 1 limit <- sum + 10 while: diff <- limit – number BR n endwhile sum <- sum + number number <- number + 1 BR nzp while endwhile:

22. Jump Instruction (JMP) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 0 0 Base 0 0 0 0 0 0 0 0 0 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 0 0 0 1 0 0 0 0 0 0 0 0 0 0 Contents of the base register are copied to the PC Can go anywhere in memory!

23. Trap Instruction (TRAP) 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1 0 1 1 1 1 0 0 0 0 Trap vector (8 bits) OS service routine Operation coded in trap vector x23 = character input, x21 = character output, x25 = halt program R0 used for input and output After completion, PC is set to instruction following the TRAP