Leadership

HISTORY OF CISC

CISC is a CPU that came about to maximize the performance of early computers. At that time computers executed instructions sequentially. The first instructions had to complete the execution cycle before the next instruction could begin. Designers combined sequences of instructions into single instructions. This reduced the amount of time spent retrieving instructions from memory, although these instructions did require multiple clock cycles to execute.

Characteristics of CISC

CISC used microcode to simplify the computers architecture. In a micro programmed system, the ROM contains a group of microcode instructions that correspond w/ each machine language instruction. When a machine language instruction arrives at the processor, it executes the corresponding series of microcode instructions. The microcode acts as a transition layer between the instructions and the electronics of the computer. Another characteristic is their variable length instruction format. This format is used to limit the amount of waste space, although they do require special decoding circuits that count bytes within words and frame the instructions according to their byte length. One more characteristic of CISC is the small number of general purpose registers it typically about 8 registers.

Advantages of CISC

1. Less expensive due to the use of microcode. 2. Upwardly compatible because a new computer would contain a superset of the instructions of the earlier computers. 3. Fewer instructions could be used to implement a given task, allowing for more efficient use of memory. 4. Simplified compiler, because the micro program instruction sets could be written to match the constructs of high level language.

Disadvantages of CISC

1. Instruction sets and chip hardware become more complex with each generation of