Direct and Indirect Addressing

Addressing modes are an aspect of the instruction set architecture in most central processing unit (CPU) designs. The various addressing modes that are defined in a given instruction set architecture define how machine language instructions in that architecture identify the operand (or operands) of each instruction. An addressing mode specifies how to calculate the effective memory address of an operand by using information held in registers and/or constants contained within a machine instruction or elsewhere.

what is the difference between direct and indirect instruction ?

 If the second part of an instruction code specifes an operand, then it is said that the instruction has an immediate operand. If the second part specifies the address of an operand, then it is said that the instruction has a direct address.There is also a third possibility called indirect address, where the bits in the second part of the instruction specify an address of a memory word in which the address of the operand is obtained. In the instruction code, one bit is used to distinguish between a direct and an  indirect address.

  Indirect address

An address that serves as a reference point instead of the address to the direct location. For example, if a programmer saved something to an in memory, the data is saved to any free spot in the memory, instead of a specific address.