Question

A Switch/case statement allows multiway branching based on the value of an integer variable. In the following example, the switch variable s is specified to be assumed to be one of three values ​​of [0, 2], and a different action for each case becomes:

case 0: a = a - 1; break;

case 1: a = a + 2; break;

case 2: b = 3 * b; break;

Shows how such statements can be compiled into MiniMIPS assembly instructions.

Answer 1

Switch statement

The switch statement is one of the statements we had in computer programming. It is a multiway branch statement. Using the switch statement we can choose one of the code blocks to be executed. As in, using a variable, we test the value of the variable, against list of values. Each value of the variable is called a case.

Given problem:
We have a variable ‘s’ with the possible values from the set [0,2]
We also have cases given:
1.   case 0: a=a-1;
break;
2.   case 1: a=a+2;
break;
3.   case 2: b=3*b;
break;

The computer programming code/pseudocode can be as follows:
switch (s)
{
case 0: a=a-1;
break;
case 1: a=a+2;
break;
case 2: b=3*b;
break;
default: a = 0; //assume for example
break; //suppose
}

Coming to the MINIMIPS assembly instructions:

Using the mapping as follows:
a: s0, b: s1, s: s2

Code in MIPS

lw $s0,a # load a from memory--assume
lw $s1,b # load b from memory--assume

bne s2, 0, L1 # case s!=0
sub s0, s0, 1 # case s=0 so a=a-1
b Exit # end of case so Exit

L1:
bne s2, 0, L2 # case s != 1
addi s0, s0, 2 # case s=1 so a=a+2
b Exit # end of case so Exit

L2:
bne s2, 0, Exit # case s != 2
muli s1, s1, 3 # case s=2 so b=b*3
Exit: