1. ARM Control Structures

2. Branch
B identifier Branch (jump) to given identifier

3. Branch Instruction Format
24 bit immediate
Shifted left 2 bits to make 26 bit value
Sign extended to 32 bits

4. Branch Instruction Format
24 bit immediate
Shifted left 2 bits to make 26 bit value
Sign extended to 32 bits
Signed 26 bit value to be added to PC
+/- 32Mbytes

5. Status Register
CPSR register Tracks state of processor, recent instructions

6. Setting Status
CMP : Compare CMP rn, #___ CMP rn, rm Set status register based on op1 – op2

7. Other Comparisons
CMN : Compare Negative CMN op1, op2 Set status register based on op1 + op2
TST : Test bits TST op1, op2 Set status register based on op1 AND op2 Does not set Carry or oVerflow bits
TEQ : Test equal TEQ op1, op2 Set status register based on op1 XOR op2 Does not set Carry or oVerflow bits

8. Conditional Branches BEQ identifier : Branch on equal
Status register determines if branch taken

9. Conditional Branches
Branch options
Most
Common

10. If Assembly if’s are “backwards” If: Tests when to run code
Assembly: Tests when to skip code
High Level
Assembly
if(i == j) { k = 1; } …
Branch to else if r1 != r2 r3 = 1 else: ….
.data
i: .word 0x2
j: .word 0x2
k: .word 0x0
.text
main:
lw $8, i #load value at i in memory into $8
lw $9, j #load value at i in memory into $8
bne $8, $9 endif #if NOT same, skip ahead
nop #branch delay!!
ori $10, $0, 1 #store 1 into $10
sw $10, k #store the 1 into k
endif:
ori $30, $0, 0 #random instruction = rest of program...

11. If If implemented with branch: Skip ahead if NOT doing if body .data
i: .word 0x2
j: .word 0x2
k: .word 0x0
.text
main:
lw $8, i #load value at i in memory into $8
lw $9, j #load value at i in memory into $8
bne $8, $9 endif #if NOT same, skip ahead
nop #branch delay!!
ori $10, $0, 1 #store 1 into $10
sw $10, k #store the 1 into k
endif:
ori $30, $0, 0 #random instruction = rest of program...

12. If / Else If/Else implemented with branch:
Branch to skip if body for else case
If body ends with jump to skip else body
High Level
Assembly
if(i == j) { k = 1;
} else {
k = 2; } …
Branch to else if r1 != r r3 = jump to endif else:
r3 = 2
endif:
.data
i: .word 0x2
j: .word 0x2
k: .word 0x999
.text
main:
lw $8, i #load value at i in memory into $8
lw $9, j #load value at i in memory into $8
bne $8, $9 else #if NOT same, skip ahead to else
nop #branch delay!!
ori $10, $0, 1 #store 1 into $10
sw $10, k #store the 1 into k
j endif #skip else part
else:
endif:
ori $30, $0, 0 #random instruction = rest of program...

13. If / Else If/Else implemented with branch:
Branch to skip if body for else case
If body ends with jump to skip else body
.data
i: .word 0x2
j: .word 0x2
k: .word 0x999
.text
main:
lw $8, i #load value at i in memory into $8
lw $9, j #load value at i in memory into $8
bne $8, $9 else #if NOT same, skip ahead to else
nop #branch delay!!
ori $10, $0, 1 #store 1 into $10
sw $10, k #store the 1 into k
j endif #skip else part
else:
endif:
ori $30, $0, 0 #random instruction = rest of program...

14. Loop = jump backwards # # $8 is loop control variable (i) init:
ori $8, $0, # count = 0
test: sltiu $9, $8, # count < 10
beq $9, $0, endLp # end loop if count >= 10
sll $0, $0, # delay
# do stuff
addiu $8, $8, # count++
j test
nop # delay
endLp: sll $0,$0, # end loop target

15. Loop = jump backwards int i = 0; while(i < 10) { //do stuff i++; }#
# $8 is loop control variable (i)
init:
ori $8, $0, # count = 0
test: sltiu $9, $8, # count < 10
beq $9, $0, endLp # end loop if count >= 10
sll $0, $0, # delay
# do stuff
addiu $8, $8, # count++
j test
nop # delay
endLp: sll $0,$0, # end loop target

16. Counting Loop Test jumps to end when done End branches back to test #
# $8 is loop control variable (i)
init:
ori $8, $0, # count = 0
test: sltiu $9, $8, # count < 10
beq $9, $0, endLp # end loop if count >= 10
sll $0, $0, # delay
# do stuff
addiu $8, $8, # count++
j test
nop # delay
endLp: sll $0,$0, # end loop target

17. Real Code Counting Loop
Naïve loop implementation :
4 instructions / iteration

18. Real Code Counting Loop
Compilers usually move test to end of loop
Start by jumping to test
3 instructions / iteration

19. Sum
Sum 0…10

20. ARM Specific Tricks: Set Bits & Conditional Execution

21. Condition Bits
ARM instructions feature 4 condition bits:

22. Condition Bits
Lots of instructions start with E

23. Condition Bits
Specify condition to execute instruction under

24. Condition Bits
E = Always execute

25. Conditional Execution
Apply conditions to most instructions

26. Condtional Without Branch
Can implement if/else as conditional instructions

27. Fun Fact
Setting Flags
CMP, CMN, TST, TEQ set status register

28. Setting Flags Fun Fact CMP, CMN, TST, TEQ set status register
Data instructions can do so optionally instructionS = Set status

29. Fun Fact
Status Flag If
If with CMP & conditional execution:

30. Fun Fact
Status Flag If
Setting status with subtraction instead:

31. Fun Fact
Status Flag If
Loop using set flags: