Q. Diffrence between RISC and CISC architecture?
CISCs provide better support for high-level languages since they include high-level language constructs such as CASE, CALL etc.
Yes CISC architecture attempts to narrow gap between High level and Assembly Level Language; however, this support comes at a cost. Actually the support can be measured as inverse of costs of employing typical HLL constructs on a particular machine. If architect offers a feature which looks same as the HLL construct however runs slowly, or has many options, the compiler writer may omit feature or even HLL programmer may avoid the construct as it is cumbersome andslow. So the comment above doesn't hold.
It is more difficult to write a compiler for a RISC than a CISC.
The studies have revealed that it isn't so because of the following reasons:
If an instruction can be executed in more ways than one then more cases should be considered. For it compiler writer required to balance speed of compilers to get good code. In CISCs compilers require to analyse potential usage of all available instruction that is time consuming. So it is suggested that there is at least one good way to do something. In RISC there are few choices; for illustration if an operand is in memory it should first be loaded in a register. So RISC needs simple case analysis that means a simple compiler though more machine instructions would be produced in every case.
RISC is tailored for C language and won't work well with other high level languages.However studies of other high level languages revealed that most frequently executed operations in other languages are also same as simple HLL constructs found in C for that RISC has been optimized. Except a HLL changes paradigm of programming we will get comparable result.
The good performance is because of the overlapped register windows; reduced instruction set has nothing to do with it.
Surely a major part of the speed is because of overlapped register windows of RISC which offer support for function calls. Though please note this register windows is likelybecause of reduction in control unit size from 50 to 6 per cent. Additionally the control is simple in RISC than CISC so further helping simple instructions to execute faster.