Instruction Formats (Zero, One, Two and Three
Address Instruction)
Computer perform task on the basis of instruction provided. An instruction in
computer comprises of groups called fields. These field contains different
information as for computers everything is in 0 and 1 so each field has different
significance on the basis of which a CPU decide what to perform. The most
common fields are:
Operation field which specifies the operation to be performed like addition.
Address field which contain the location of operand, i.e., register or memory location.
Mode field which specifies how operand is to be founded.
An instruction is of various lengths depending upon the number of addresses it contains.
Generally CPU organization is of three types on the basis of number of address fields:
1. Single Accumulator organization
2. General register organization
3. Stack organization
In first organization operation is done involving a special register called accumulator.
In second on multiple registers are used for the computation purpose.
In third organization the work on stack basis operation due to which it does not contain any
address field.
It is not necessary that only a single organization is applied a blend of various organizations is
mostly what we see generally.
On the basis of number of address, instructions are classified as:
Note that we will use X = (A+B)*(C+D) expression to showcase the procedure.
1. Zero Address Instructions –
A stack based computer do not use address field in instruction. To evaluate an
expression first it is converted to revere Polish Notation i.e. Post fix Notation.
Expression: X = (A+B)*(C+D)
Postfixed : X = AB+CD+*
TOP means top of stack
M[X] is any memory location
PUSH A TOP = A
PUSH B TOP = B
ADD TOP = A+B
PUSH C TOP = C
PUSH D TOP = D
ADD TOP = C+D
MUL TOP = (C+D)*(A+B)
POP X M[X] = TOP
2. One Address Instructions –
These uses an implied ACCUMULATOR register for data manipulation. One
operand is in accumulator and other is in register or memory location. Implied
means that the CPU already knows that one operand is in accumulator so there is
no need to specify it.
This use an implied ACCUMULATOR registers for data manipulation. One
operand is in accumulator and other is in register or memory location. Implied
means that the CPU already knows that one operand is in accumulator so there is
no need to specify it.
Expression: X = (A+B)*(C+D)
AC is accumulator
M[] is any memory location
M[T] is temporary location
LOAD A AC = M[A]
ADD B AC = AC + M[B]
STORE T M[T] = AC
LOAD C AC = M[C]
ADD D AC = AC + M[D]
MUL T AC = AC * M[T]
STORE X M[X] = AC
3. Two Address Instructions –
This is common in commercial computers. Here two addresses can be specified in
the instruction. Unlike earlier in one address instruction the result was stored in
accumulator here result can be stored at different location rather than just
accumulator, but require more number of bit to represent address.
Here destination address can also contain operand.
Expression: X = (A+B)*(C+D)
R1, R2 are registers
M[] is any memory location
MOV R1, A R1 = M[A]
ADD R1, B R1 = R1 + M[B]
MOV R2, C R2 = C
ADD R2, D R2 = R2 + D
MUL R1, R2 R1 = R1 * R2
MOV X, R1 M[X] = R1
4. Three Address Instructions –
This has three address fields to specify a register or a memory location. Program
created are much short in size but number of bits per instruction increase. These
instructions make creation of program much easier but it does not mean that
program will run much faster because now instruction only contain more
information but each micro operation (changing content of register, loading
address in address bus etc.) will be performed in one cycle only.
Expression: X = (A+B)*(C+D)
R1, R2 are registers
M[] is any memory location
Expression: X = (A+B)*(C+D)
R1, R2 are registers
M[] is any memory location
Addressing Modes– The term addressing modes refers to the way in which the
operand of an instruction is specified. The addressing mode specifies a rule for
interpreting or modifying the address field of the instruction before the operand
is actually executed.
