complex instruction set computer(CISC):
A complex instruction set computer is a computer in which single instructions can execute several low-level operations or are capable of multi-step operations or addressing modes within single instructions.
*Some of the instructions are exceuted in the lower level operations, they are load from memory, an arithmetic operation, and a memory store.
*The term was retroactively coined in contrast to reduced instruction set computer (RISC) and has therefore become something of an umbrella term for everything that is not RISC, from large and complex mainframe computers to simplistic microcontrollers where memory load and store operations are not separated from arithmetic instructions.
*A modern RISC processor can therefore be much more complex than a modern microcontroller using a CISC-labeled instruction set.
*The complexity of its electronic circuits, but also in the number of instructions or the complexity of their encoding patterns.
*The only typical differentiating characteristic is that most RISC designs use uniform instruction length for almost all instructions, and employ strictly separate load/store-instructions.
* For examples:
the instruction set architectures that have been retroactively labeled CISC are System/360 through z/Architecture, the VAX architectures.
*The famous microprocessors and microcontrollers that have also been labeled CISC in many academic publications include the Motorola 6800, 6809 and 68000-families; the Intel 8080, iAPX432 and x86-family; the Zilog Z80, Z8 and Z8000-families.
SOME OF THE CISC USING PROCESSORS ARE ;
*IBM 370/168:It was introduced in the year 1970. CISC design is a 32 bit processor and four 64-bit floating point registers.
*VAX 11/780: CISC design is a 32-bit processor and it supports many numbers of addressing modes and machine instructions which is from Digital Equipment Corporation.
*Intel 80486:It was launched in the year 1989 and it is a CISC processor, which has instructions varying lengths from 1 to 11 and it will have 235 instructions.
CHARACTERISTICS OF THE CISC DESIGN PHILOSOPHY;
*Instruction-decoding logic will be complex.
*One instruction is required to support multiple addressing modes.
*Less chip space is enough for general purpose registers for the instructions that are operated directly on memory.
*Various CISC designs are set up two special registers for the stack pointer, handling interrupts, etc.
*MUL is referred to as a “complex instruction” and requires the programmer for storing functions.
