Example: Average Integers

The following Pascal statement averages three integers:

avg := (i1 + i2 + i3) / 3 ;

The same computation in SAL requires 3 instructions:

        add   avg, i1, i2 
        add   avg, avg, i3 
        div   avg, avg, 3

To run the program on the SPIM simulator, certain declarations must be added.

• The variables used in the program must be declared in the .data section of the program;
• A .text directive must precede the executable instructions;
• The first instruction in the program must begin with the label __start: (this label begins with two underscores);
• The program must end with a done instruction, which tells the CPU to terminate execution of the program.

A terminal session from the SPIM simulator can be logged using the Unix script command. The output from the simulator for the above program is shown below.

Note the use of the simulator load, step, and print commands to control program execution. Each line of output from the simulator step command consists of:

1. The address of the instruction in hexadecimal.
2. The instruction in hexadecimal.
3. The TAL instruction.
4. A comment field beginning with # containing the SAL instruction.

For more information on running the simulator, please refer to the SPIM simulator reference manual.