Programming Loops in Computer Systems: Types, Mechanisms, and Implementation, Study notes of Microcomputers

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ECE 3120

Computer Systems

Programming Loops

Manjeera Jeedigunta

http://blogs.cae.tntech.edu/msjeedigun

Email: [email protected]

Tel: 931-372-6181, Prescott Hall 120

Prev: „

Write programs to do arithmetic

Today: „

Loops

Do Statement S forever

Infinite loop

Possible to add “If C then exit”

Figure 2.4 An infinite loop

S

For i=n1 to n2 Do S

i = loop counter

S = Statement

1) Initialize the loop counter

2) Compare the loop counter with the limit

3) Perform the operations in S if loop counter within the limit

4) Increment the loop counter ‘i’ and go to step 2

While C Do S

Logical expression C is evaluated „

Only while C if true, S will be executed

C

S

true

false

Figure

T

he

W

hile

... Do

looping

construct

1) Initialize the logical expression C

2) Evaluate the logical expression C

3) If C is true perform the functions specified by S,

go back to 2, if not exit

4) Exit

Repeat S until C

Figure 2.7 The Repeat ... Until looping construct

initialize C

S C

true

false

1) Initialize loop counter

2) Execute statement S

3) Evaluate C, if true go to step 2, else exit

Unary (unconditional) branch:

always execute

Simple branches:

branch is taken when a specific bit of CCR is in a specific

status

Unsigned branches:

branches are taken when a comparison or test of unsigned

numbers results in a specific combination of CCR bits

Signed branches:

branches are taken when a comparison or test of signed

quantities results in a specific combination of CCR bits

Three categories

of Branches

Short Branches:

in the range of -128 ~ +127 bytes

Long Branches:

in the range of 64KB

bit-conditional branches

Branch Instructions

Four types of branch instructions:

T a b l e 2 .2 S u m m a r y o f s h o r t b r a n c h i n s t r u c t i o n s

M n e m o n i c

F u n c t i o n

U n a r y B r a n c h e s

B R A B R N

B r a n c h a l w a y sB r a n c h n e ve r

E q u a t i o n o r O p e r a t i o n

S i m p l e B r a n c h e s

M n e m o n i c

F u n c t i o n

B C C

B C S B E Q

B M I B N E

B P L B V C

B V S

B r a n c h i f c a r r y c l e a rB r a n c h i f c a r r y s e tB r a n c h i f e q u a lB r a n c h i f m i n u sB r a n c h i f n o t e q u a lB r a n c h i f p l u sB r a n c h i f o ve r f l o w c l e a rB r a n c h i f o ve r f l o w s e t

1 = 11 = 0

U n s i g n e d B r a n c h e s

M n e m o n i c

F u n c t i o n

B H I

B H SB L O

B L S

B r a n c h i f h i g h e rB r a n c h i f h i g h e r o r s a m eB r a n c h i f l o w e rB r a n c h i f l o w e r o r s a m e

C = 0C = 1Z = 1

N

= 1

Z = 0

N

= 0

V = 0V = 1

E q u a t i o n o r O p e r a t i o n E q u a t i o n o r O p e r a t i o n

C +

Z =

0

C = 0C = 1

C +

Z =

1

M n e m o n i c

F u n c t i o n

E q u a t i o n o r O p e r a t i o n

S i g n e d B r a n c h e s

B G EB G T

B L EB L T

B r a n c h i f g r e a t e r t h a n o r e q u a lB r a n c h i f g r e a t e r t h a nB r a n c h i f l e s s t h a n o r e q u a lB r a n c h i f l e s s t h a n

N

⊕

V =

0

Z + ( N

⊕

V ) =

0

Z + ( N

⊕

V ) =

1

N

⊕

V =

1

Compare and Test Instructions -

Condition flags need to be set up before conditional branch instruction should

be executed.

The 68HCS12 provides a group of instructions for testing the condition flags.

Table 2.4 Summary of compare and test instructions

Mnemonic

Function

Compare instructions

CBA

CMPACMPB

CPD

CPS

CPX

CPY

Compare A to BCompare A to memoryCompare B to memoryCompare D to memoryCompare SP to memoryCompare X to memoryCompare Y to memory

Operation

(A) - (B) (A) - (M)(B) - (M)

(D) - (M:M+1)

(SP) - (M:M+1)

(X) - (M:M+1)(Y) - (M:M+1)

Test instructions

Mnemonic

Function

TST

TSTATSTB

Test memory for zero or minusTest A for zero or minusTest B for zero or minus

Operation(M) - $

(A) - $00(B) - $

Decrementing & Incrementing Instructions

DEC, DECA,DECB,DES,DEX,DEY

INC,INCA,INCB,INS,INX,INY

ldaa i

adda #

staa i

Loop Primitive Instructions-

68HCS12 provides a group of instructions that either decrement or increment aloop count to determine if the looping should be continued.

The range of the branch is from $80 (-128) to $7F (+127).

Table 2.5 Summary of loop primitive instructions

Mnemonic

Function

DBEQ cntr, relDBNE cntr, rel

IBEQ cntr, relIBNE cntr, rel

TBEQ cntr, relTBNE cntr, rel

Decrement counter and branch if = 0(counter = A, B, D, X, Y, or SP)Decrement counter and branch if

≠

0

(counter = A, B, D, X, Y, or SP)Increment counter and branch if = 0(counter = A, B, D, X, Y, or SP)Increment counter and branch if

≠

0

(counter = A, B, D, X, Y, or SP)Test counter and branch if = 0(counter = A, B, D, X, Y, or SP)Test counter and branch if

≠

0

(counter = A, B, D, X, Y, or SP)

Equation or Operation

counter

←

(counter) - 1

If (counter) = 0, then branchelse continue to next instructioncounter

←

(counter) - 1

If (counter)

≠

0, then branch

else continue to next instructioncounter

←

(counter) + 1

If (counter) = 0, then branchelse continue to next instructioncounter

←

(counter) + 1

If (counter)

≠

0, then branch

else continue to next instructionIf (counter) = 0, then branchelse continue to next instructionIf (counter)

≠

0, then branch

else continue to next instruction

Note. 1.

cntr

is the loop counter and can be accumulator A, B, or D and register X, Y, or SP.

rel

is the relative branch offset and is usually a label

Example 2.15’

Write a program to find the maximum element from an array of

N 8-bit elements using the

repeat S until C

looping construct.

N

equ

org

max_val

ds.b

org

ldaa

array

; set array[0] as the temporary max max

staa

max_val

ldx

#array+N-1 ; start from the end of the array

ldab

#N-

; set loop count to N - 1

loop

ldaa

max_val

cmpa

0,x

bge

chk_end

ldaa

0,x

staa

max_val

chk_end

dexdbne

b,loop

; finish all the comparison yet?

forever

bra

forever

array

db

db

end

Instructions for Variable Initialization 1.

[] CLR

opr

[]

where

opr

is specified using the

extended

or

index

addressing modes. The

specified memory location (1 bye) is cleared.

[] CLRA []Accumulator A is cleared to 0

[] CLRB

[]

Accumulator B is cleared to 0

Next…

Shift & Rotation

Read Chapter 2.