Computer Architecture: Operations and Operands

OPERATIONS AND OPERANDS

Every computer must be able to perform arithmetic. The MIPS assembly language notation add a, b, c instructs a computer to add the two variables b and c and to put their sum in a. This notation is rigid in that each MIPS arithmetic instruction performs only one operation and must always have exactly three variables. For example, suppose we want to place the sum of four variables b, c, d, and e into variable a.

The following sequence of instructions adds the four variables:

add a, b, c # The sum of b and c is placed in a

add a, a, d # The sum of b, c, and d is now in a

add a, a, e # The sum of b, c, d, and e is now in a

Fig 1.5 MIPS Instructions

Thus, it takes three instructions to sum the four variables. The words to the right of the sharp symbol (#) on each line above are comments for the human reader, so the computer ignores them.

Fig. 1.6 MIPS Operands