Prerequisites for CISC processors
Technology in the 1960s and early 1970s was dominated by high hardware cost, in particular by high cost for memory.
Only a small main memory and slow memory access!
Instruction fetch was done from main memory and could be overlapped with decode and execution of previous instructions.
Observation: the number of cycles per instruction was determined by the number of cycles taken to fetch the instruction.
CISC (complex instruction set computer) approach:
- it is acceptable to increase the average number of cycles taken to decode and execute an instruction.
- reduce the number of instructions and
- encode these instructions densely.
Multiple-cycle instructions reduce the overall number of instructions, and thus reduce the overall execution time because they reduce the instruction-fetch time.