Variables

Programming languages have global variables. Also, a program block nested within other program blocks can use variables of the outer blocks. Let us skip these details and implement local variables, only. The details may be convered in a course on compilers or in a course on programming languages.

The picture shows a stack frame of an active subroutine. As always (for these notes), each item on the stack is four bytes long.

As previously, the caller pushes values from T registers that it will need to restore when control returns. The subroutine (the callee) pushes values from S registers that it might change.

In this example, space is reserved in the stack for implementing four local variables a, b, i and j. In the picture, the space reserved for variable a is labeled "a", but of course what is in that space is the 32-bit pattern that the variable holds. Usually our variables will be integer variables.

A variable is a location in the run-time stack that is used to store data. The values stored in a variable may change as the program executes.

Variable a is the space on the stack. There is no other "thing" that implements the variable. In the program, manipulating a variable is done by using registers to load and store values from the variable's space in the stack. However, the variable is the space in the stack, not a register.