What makes a complex instruction complex, and a simple instruction simple? Is the notion of complex and simple in the realm of instruction set architecture meaningful?

What will be an ideal response?

There is no precise definition of simple and complex. However, a simple instruction generally performs a single basic operation, such as data movement or computer arithmetic (or shifting or logic). Complex instructions usually perform multiple operations or use multicomponent addressing modes. For example, the 68K instruction MOVE -(A0),(A3)+ is complex because it requires the source pointer A0 to be predecremented, memory accessed at that address, the data read to be copied to the location pointed at by A3 and then A3 incremented. This instruction requires a pointer adjustment, two memory accesses and a second pointer adjustment and is, therefore, complex. Generally, RISC processors attempt to perform each operation in a single clock cycle and an instruction like MOVE -(A0),(A3)+ could not be implemented by most RISC processors. However, ARM processors implement instructions like ADD r0,r1,r2, lsl r3 that appear complex (allowing a shift and an add) but are, in fact, simple, because the shift and add take place in the same clock cycle.