With the increasing use of more and higher level languages manufacturers had offered more powerful instructions to support them. It was claimed that a stronger instruction set will decrease the software crisis and will simplify the compilers. Another significant reason for such a movement was desire to improve performance.
Yet even though instructions which were closer to high level languages were implemented in Complex Instruction Set Computers (CISCs) still it was hard to use these instructions because compilers were required to find those conditions which exactly fit those constructs. Additionally task of optimising generated code to minimise code size, decrease instruction execution count and improving pipelining is much more difficult with these type of complex instruction set.
Another motivation for increasingly complex instruction sets was that complex HLL operation would execute more rapidly as a single machine instruction instead of as a series of more primitive instructions. Thoughdue to the bias of programmers in the direction of use of simpler instructions it may turn out otherwise. CISC makes more complex CU with larger microprogram control store to accommodate a richer instruction set. This enhances the execution time for simpler instructions.
So it is far from clear that trend to complex instruction sets is appropriate. This has led some groups to pursue opposite path.