Instruction Format (With reference to
address)
Instruction
contains some specific fields: opcode, source/destination operand, source
operand address, destination operand address, next instruction address.
Computers
may have instructions of several different length containing varying number of
addresses. According to address
reference, there are mainly four types of instruction:
1. Three
address instruction
2. Two
address instruction
3. One
address instruction
4. Zero
address instruction
Three Address Instruction
Syntax of
three address instruction is,
Operation Destination, Source 1, Source 2
For Example:
ADD
A, B, C
Where A, B, C are the operands. These variable names are assigned to distinct locations in the memory. In above example, operands B and C are called source operands and operand A is called destination operand and ADD is the operation to be performed on the operands. Bits required the three memory addresses of the three operands. If n-bit are required to specify one memory address, 3n bits are required to specify three memory addresses.
Two Address Instruction
Syntax of
two address instruction is,
Operation Destination, Source
For
Example: ADD A, B
Where A, B are the operands. These variable names are assigned to distinct locations in the memory. In above example, operands B are called source operands and operand A is serves as both source & destination operand. ADD is the operation to be performed on the operands. Bits required the two memory addresses of the two operands. If n-bit are required to specify one memory address, 2n bits are required to specify two memory addresses.
One Address Instruction
Syntax of
one address instruction is,
Operation Source
For
Example: ADD A
This
instruction adds the contents of variable A into the processor register called accumulator
and stores the sum back into the accumulator destroying the previous content of
the accumulator. In this instruction the second operand is assumed implicitly
in a unique location accumulator.
Example
of one address instruction:
LOAD A
(Copy content of memory location A into accumulator)
STORE B (Copies the contents of accumulator into memory location B)
Zero Address Instruction
In this instruction, the locations of all operands are defined implicitly. Such instructions are found in machines that stores operands in a structure called a pushdown stack. Though all the functionalities (like push, pop, TOS...) of stack is included in the instruction execution.
The
instruction with only one address will require a smaller number of bits to
represent it, and instruction with three addresses will require a greater
number of bits to represent it. Three addresses instruction requires more
memory accesses (more time required to execute) and one address requires less
memory accesses (less time required to execute).
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