Q. Explain fundamental instruction set characteristics?

Let's look into a number of fundamental instruction set characteristics:

• The operands can be addressed in memory, registers or I/O device address.
• Instruction having less number of operand addresses in an instruction may need lesser bits in the instruction though it also restricts range of functionality which can be performed by instructions. This indicates that a CPU instruction set having less number of addresses has longer programs that means longer instruction execution time. On the other side having more addresses may lead to more complex processing circuits anddecoding.
• Most of instructions don't need more than three operand addresses. Instructions having less addresses than three use registers implicitly for operand locations since using registers for operand references can yield in smaller instructions as only few bits are required for register addresses as against memory addresses.
• The kind of internal storage of operands in CPU is most fundamental differentiation.

The three most general types of ISAs are: 

1.  Evaluation Stack: Operands are implicitly on top of stack. 

2.  Accumulator: One operand is implicitly the accumulator. 

3. General Purpose Register (GPR): All operands are explicit either memory or registers locations.