Control Structure

1
Control Structure
2
Control Structures

• Write an algorithm first and then convert it to
  an assembly program.
• Learn how to translate basic control structures
  into machine language.
• Basic control structures
• do loop
• while loop
• for loop
• if-then-else

3
Comparison Instruction

• To compare two values (register vs. register or
  register vs. constant)
• cmp o2, 30
• or
• cmp g2, i1
• The result of this instruction effects the
  following flags
• Z - whether the result was zero.
• N - whether the result was negative.
• V - whether the result was overflow.
• C - whether the result generated a carry.

4
Branch Instructions

• The comparison instruction can be used with
  branch instructions
• ba branch always goto
• bn branch never nop
• bl branch on less (than zero)
• ble branch on less or equal (to zero)
• be branch on equal (to zero)
• bne branch on not equal (to zero)
• bge branch on greater or equal (to zero)
• bg branch on greater (than zero)

5
DoWhile Loop
S1Sn

• Structure
• do
• some statements here
• while(logical expression)
• Translate to
• loop assembly code for S1
• assembly code for S2 Sn
• assembly code for L1
• branch to loop if L1 is true

L1
6
Our Second Program

• Let modify our first program to include a loop
• main()
• int x, y
• x 0
• do
• y ((x - 1) (x - 7)) / (x - 11)
• x x 1
• while (x lt 11)
• printf(d\n, y)

7
Our Second Program

• / Variables
• Store x in l0
• Store y in l1 /
• fmt .asciz d\n
• .align 4
• .global main
• main save sp, -64, sp
• clr l0 ! x 0
• loop sub l0, 1, o0 !(x - 1) to o0
• sub l0, 7, o1 !(x - 7) to o1
• call .mul !(x - 1)(x - 7)
• nop

8
Our Second Program

• sub l0, 11, o1 !(x - 11) to o1
• call .div !(x-1)(x-7)/(x-11)
• nop
• mov o0, l1 ! Store it in y
• add l0, 1, l0 ! x x 1
• cmp l0, 11 ! x lt 11 ?
• bl loop
• nop ! Delay slot

9
Our Second Program

• set fmt, o0
• mov l1, o1
• call printf ! printf(d\n, y)
• nop
• mov 1, g1 ! Exit request
• ta 0

10
While Loop
L1

• The condition is tested first.
• while (logical expression)
• some statements here
• Translate to
• loop assembly code for L1
• branch to exit if L1 is false
• assembly code for S1
• assembly code for S2 Sn
• unconditional branch to loop
• done

S1Sn
11
While Loop Example
L1

• while(a lt 17)
• a a b
• c c 1
• loop cmp l0, 17
• bg done ! Branch if L1 is false
• nop ! Delay slot
• add l0, l1, l0 ! a a b
• add l2, 1, l2 ! c c 1
• ba loop ! Always branch to loop
• nop ! Delay slot
• done

S1 and S2
12
For Loop

• Structure
• for( ex1 ex2 ex3 )
• st
• Translate to
• ex1
• while( ex2 )
• st
• ex3

13
For Loop Example

• for (a1 a lt b a)
• c c a
• Translate to
• a 1
• while( a lt b )
• c c a
• a / a a 1 /
• Note a l0, b l1, c l2

14
For Loop Example

• mov 1, l0 ! a 1
• loop cmp l0, l1 ! Compare a and b.
• bg exit ! Exit for-loop if a gt b.
• nop
• mov l2, o0 ! First param for .mul
• mov l1, o1 ! Second param for .mul
• call .mul ! o0 c b
• nop
• mov o0, l2 ! Store result in c
• add l0, 1, l0 ! a
• ba test
• nop
• exit

15
If-Then

• Structure
• if( ex1 )
• st
• Test "ex1"
• Skip "st" if ex1 is false

16
If-Then Example

• d a
• if((ab) gt c)
• a a b
• c c 1
• a c d
• NOTE
• a l0
• b l1
• c l2
• d l3
• tmp l4 -- keep ab value

17
If-Then Example

• mov l0, l3 ! d a
• add l0, l1, l4 ! tmp (a b)
• cmp l4, l2
• ble next ! Jump if((ab) lt c)
• nop ! Delay slot
• ! Inside if statement.
• add l0, l1, l0 ! a a b
• add l2, 1, l2 ! c c 1
• ! End of inside if statement.
• next
• add l2, l3, l0 ! a c d

18
If-Then-Else

• Structure
• if( ex1 )
• st1
• else
• st2

• Test "ex1"
• If ex1 is false, goto ELSE-BLOCK.
• IF-BLOCK
• Execute st1
• Goto DONE
• ELSE-BLOCK
• Execute st2
• DONE

19
If-Then-Else Example

• d a
• if((ab) gt c)
• a a b
• c c 1
• else
• d c
• a c d

20
If-Then-Else Example

• mov l0, l3 ! d a
• add l0, l1, l4 ! tmp (a b)
• cmp l4, l2
• ble else-block ! Jump if((ab) lt c)
• nop ! Delay slot
• ! if-block Inside if statement.
• add l0, l1, l0 ! a a b
• add l2, 1, l2 ! c c 1
• ba done ! Skip the else-block.
• nop ! Delay slot
• ! End of inside if statement.

21
If-Then-Else Example

• else-block
• ! Inside else-block.
• mov l2, l3 ! d c
• ! End of else-block.
• done
• add l2, l3, l0 ! a c d