Software Constructs

1
Software Constructs

• What they are ..

2
The IF THEN . Construct

• The IF THEN construct is used when a sequence
  of instructions must be executed only if a
  condition is satisfied.

• Pseudo-Code
• IF (Condition True) THEN
• BEGIN
• Statement1
• . . . . . . . . .
• StatementN
• END
• Assembly
• CMP AL,Condition
• JNE Cont
• Statement1
• . . . . . . . . .
• StatementN
• Cont . . . . . . . . . .

Flowchart
3
IF THEN - Example 1

• Write a procedure that increments the content of
  AH only if the content of AL is equal to 50H.

4
IF THEN - Example 2

• Write a procedure that adds BL to AL. If the sum
  is greater than 255 then increment the contents
  of AH.

5
The IF THENELSE Construct

• The IF THEN ELSE construct is used when a
  sequence of instructions must be executed only if
  a condition is satisfied, while another sequence
  must be executed if the condition is not
  satisfied..

• Pseudo-Code
• IF (Condition True) THEN
• BEGIN Statement1
• Statement2 END
• ELSE BEGIN Statement1
• Statement2 END
• Assembly
• CMP AL,Contition
• JNE No
• Yes_Statements
• JMP Cont
• No No_Statements
• Cont . . . . . . . . . .

Flowchart
6
CASE Structure

• CASE structure is a multi-way branch instruction
• CASE expression
• Values_1 Statements to perform if
• expression values_1
• Values_2 Statements to perform if
• expression values_2
• .
• .
• .
• Values_N Statements to perform if
• expression values N
• END_CASE

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Conversion of Case Structure

• Consider
• CASE AX
• 1 or 2 Add 1 to BX
• 3 or 4 Add 1 to CX
• End_CASE

8
Conversion of Case Structure (contd)

• FIRST THOUGHT BETTER CODE
• CASE AX CASE AX
• 1 or 2 1 or 2
• CMP AX,1 CMP AX,1
• JE ADD_BX JE ADD_BX
• CMP AX,2 CMP AX,2
• JE ADD_BX JE ADD_BX
• 3 or 4 3 or 4
• CMP AX,3 CMP AX,3
• JE ADD_CX JE ADD_CX
• CMP AX,4 CMP AX,4
• JE ADD_CX JNE END_CASE_AX
• JMP END_CASE_AX ADD_CX
• ADD_BX INC CX
• INC BX JMP END_CASE_AX ADD_BX
• ADD_CX INC BX
• INC CX END_CASE_AX
• END_CASE_AX

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Branches with Compound Conditions

• AND conditions (TRUE IF both conditions are true)
• How do you implement
• IF AX1 AND BX1
• THEN ADD 1 TO CX
• END_IF
• In AL we could code
• IF AX1 and BX1
• CMP AX,1
• JNE END_IF
• CMP BX,1
• JNE END_IF
• Then add 1 to CX
• INC CX
• END_IF

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Branches with Comp. Conditions (contd)

• OR conditions (True if either condition is true,
  false IF both conditions are false)
• How do you implement
• IF AX1 or BX1
• Then Add 1 to CX
• END_IF
• In AL we could code
• IF AX1 or BX1
• CMP AX,1
• JE ADD_CX
• CMP BX,1
• JNE END_IF
• ADD_CX
• INC CX
• END_IF