1. Computer Science 210 Computer 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) 1
N Z P

8. Example: Subtract 1 from R31
When R3 = 0
add R3, R3, -1
N Z P 1
When R3 > 0
Circuitry sets condition codes after add executes
N Z P 1
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
N Z P PC offset
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
N Z P PC offset
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:

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

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)
Base
Contents of the base register are copied to the PC
Can go anywhere in memory!

23. Trap Instruction (TRAP)
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

24. Assembly Language Chapter 7
For Monday
Assembly Language
Chapter 7