Computer Architecture 1 / Chapter 2: Operations on Numbers / Add Instruction

The add (ADD) instruction reads numbers from two memory cells, adds them, and stores the sum in a specified memory cell. The ADD instruction format is:

add dst=destination src1=first src2=second

• first is the memory cell location of the first operand.
• second is the memory cell location of the second operand.
• destination is the location of the memory cell where the sum will be stored.

When an ADD instruction is executed:

• the value in memory cell #first will be added to the value in memory cell #second and the sum will be stored in destination.
• the PC increases by 1.
• memory cells other than destination do not change.

More precisely, if the following are true:

• instruction #i is add dst=d src1=s1 src2=s2
• the PC at time t = i

Then we can conclude the following:

• value of cell d at time (t + 1) = (value of cell s1 at time t) + (value of cell s2 at time t)
• the PC at time (t + 1) = i + 1

For example, if the following are true:

• instruction #0 is add dst=3 src1=1 src2=2
• the PC at time 0 = 0
• value of cell 1 at time 0 = 2
• value of cell 2 at time 0 = 6

Then we can conclude the following:

• value of cell 3 at time (0 + 1) = (value of cell 1 at time 0) + (value of cell 2 at time 0)
• the PC at time (0 + 1) = 0 + 1

Since the value of cell #1 is 2 and the value of cell #2 is 6, we know that: value of cell 3 at time (0 + 1) = 2 + 6

Then after some Algebra, we conclude:

• value of cell 3 at time 1 = 8
• the PC at time 1 = 1

Try executing the ADD instruction in the simulator:

PC is 0, so the simulator is going to execute instruction #0. src1 is 1 and src2 is 2. Thus, the operation is

value at memory cell #1 + value at memory cell #2

= 2 + 6

= 8

dst is 3, which means the result will be stored in memory cell #3.

Previous Lesson Next Lesson