CISC

Emergence of CISC

CISC,which stands for Complex Instruction Set Computer is a philosophy for designing chips that are easy to program and makes efficient use of memory.Each instruction in CISC instruction set might perform a series of operations inside the processor.This reduces the number of instructions required to implement a given program,and allows the programmer to learn a small but flexible set of instructions

Advantages of CISC

As the time of their initial development,CISC machines used the available technologies to optimize computer performance.

• Microprogramming is as easy as assembly language to implement,and much less expensive than hardwiring a control unit
• The ease of microcoding new instructions allowed the designers to make CISC machines upwardly compatible-a new computer could run the same programs as earlier computers because the new computer would contain a superset of instructions of the earlier programming
• As each instruction became more capable,fewer instructions could be used to implement a given task.This made more efficient use of the relatively slow main menory.
• Because microprogram instruction sets can be written to match the constructs of high level programming languages,the compiler does not have to be as complicated.

Disadvantages of CISC

• Earlier generations of a processor family were generally contained as a subset in every new versions.So instruction set and chip hardware become more complex with each generation of computers
• So that as many instructions as possible could be stored in memory with the least possible wasted space,individual instructions could be of almost any length.This means that different instructions will take different amounts of clock time to execute,slowing down the overall performance of the machine
• Many specialized instructions aren't used frequently enough to justify their existence-approximately only 20% of the available instructions are used in a typical program
• CISC instructions typically set the condition codes as a side effect of the instruction.Not only does setting the condition codes take time,but programmers have to remember to examine the condition code bits before a subsequent instructions changes them