4.1 Switch arithmetic operations

;Write a program that, given two operands opa and opb of type word in memory, of value respectively of 2043 and 5, executes an operation between integers, chosen by user and that saves the result into the word variable 'res'
;Depending on the character typed by user, the program must execute:
;1 --> res = a+b
;2 --> res = a-b
;3 --> res = a*b
;4 --> res = a/b (integer division)

;IMPLEMENTATION
;Need to implement a switch construct:

;      switch (expression)
;      {
;          case val1: sequence1;
;          break;
;          case val2: sequence2;
;          break;
;          ...
;          default: sequence_def;
;      }


;It's possible use operations of compare and conditional jumps to a blocks of code

.MODEL small
.STACK
.DATA
opa DW 2043
opb DW 5
res DW ?
.CODE
.START
MOV AH,1
INT 21h
SUB AL,'0'
CMP AL,1
JE sum
CMP AL,2
JE subtr
CMP AL,3
JE product
CMP AL,4
JE division
JNE ex

sum: MOV BX,opa
       ADD BX,opb
       MOV res, BX
       JMP ex

subtr: MOV BX,opa
       SUB BX,opb
       MOV res, BX
       JMP ex

product: MOV AX,opa
         MOV CX,opb
         MUL CX
         MOV res,AX
         JMP ex

division: MOV AX,opa
          MOV DX,0  ;If I don't insert this, it gives me problem at overflow. Because the value of DX is changed when the input window appears
          MOV BX,opb
          DIV BX
          MOV res,AX
          JMP ex

ex:
.EXIT
END