Two Address Instructions

Most common in commercial computers. Each address field can specify either a processes register on a memory word.

MOV      R1, A     R1 ® M [A]
ADD      R1, B    R1 ® R1 + M [B]
MOV      R2, C   R2 ® M [C]    X = (A + B) * ( C + D)
ADD      R2, D    R2 ® R2 + M [D]
MUL      R1, R2    R1 ® R1 * R2
MOV      X1 R1    M [X] ® R1

Three Address Instructions

Computer with three addresses instruction format can use each address field to specify either processor register are memory operand.

ADD R1, A, B A1 ® M [A] + M [B]
ADD R2, C, D R2 ® M [C] + M [B] X = (A + B) * (C + A)

The advantage of the three address formats is that it results in short program when evaluating arithmetic expression. The disadvantage is that the binary-coded instructions require too many bits to specify three addresses.