ACOE251Sofware Constructs1 Software Constructs What they are …..

ACOE251Sofware Constructs2 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 Statement StatementN END Assembly: CMP AL,Condition JNE Cont Statement StatementN Cont: Flowchart

ACOE251Sofware Constructs3 IF THEN :- Example 1 Write a procedure that increments the content of AH only if the content of AL is equal to 50H.

ACOE251Sofware Constructs4 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.

ACOE251Sofware Constructs5 The “IF THEN…ELSE” 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

ACOE251Sofware Constructs6 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

ACOE251Sofware Constructs7 Conversion of Case Structure Consider: CASE AX 1 or 2: Add 1 to BX 3 or 4: Add 1 to CX End_CASE

ACOE251Sofware Constructs8 Conversion of Case Structure (cont’d) FIRST THOUGHTBETTER CODE ;CASE AX; CASE AX ;1 or 2; 1 or 2CMP AX,1JE ADD_BXCMP AX,2 JE ADD_BXJE ADD_BX ;3 or 4; 3 or 4CMP AX,3 JE ADD_CXJE ADD_CXCMP AX,4 JE ADD_CXJNE END_CASE_AX JMP END_CASE_AX ADD_CX: ADD_BX:INCCX INC BXJMP END_CASE_AX ADD_BX: ADD_CX:INCBX INCCXEND_CASE_AX: END_CASE_AX:

ACOE251Sofware Constructs9 Branches with Compound Conditions AND conditions (TRUE IF both conditions are true) How do you implement: IF AX=1 AND BX=1 THEN ADD 1 TO CX END_IF In AL we could code: ;IF AX=1 and BX=1 CMP AX,1 JNE END_IF CMP BX,1 JNE END_IF ;Then add 1 to CX INC CX END_IF:

ACOE251Sofware Constructs10 Branches with Comp. Conditions (cont’d) OR conditions (True if either condition is true, false IF both conditions are false) How do you implement: IF AX=1 or BX=1 Then Add 1 to CX END_IF In AL we could code: ; IF AX=1 or BX=1 CMP AX,1 JE ADD_CX CMP BX,1 JNE END_IF ADD_CX: INC CX END_IF: