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High Memory Overhead
The programmers who made the CISC processor were going for one goal;
to conserve memory as best they could. Back when CISC processors
were first made, memory was very expensive. This made them compact
everything instruction to as small as they could get it so to save memory.
Some common mechinisms, used by CISC processsor designers, to reduce memory
usage are:
Variable Lenth Instructions
Variable length instructions were used in CISC processors to limit the
amount of memory each instruction took up. It also made it so that
any instruction length would fit in one cycle so it wouldn't have to make
two cycles to get one piece of data. Saving memory is not a bad idea,
but one down side to this design guide line is it is difficult for the
I-unit to preform out of order execution, which slows the computer down
greatly. Another down side is it is very difficult to find the opcodes
in the upcomming instructions. Both RISC and EPIC processors use
fixed length instructions because it is easier to see whats comming up.
Extensive Instructions Set
Frequently CISC supports SS (Storage to Storage) instructions. A
SS instruction is the act of allowing data to be moved from one place in
memory to another in one instruction. The good thing about SS instructions
is CISC processors only have to use one instruction to preform what would
take a RISC processor two instructions to do; one to load the data from
memory into a register and another to store it in memory again at the new
destination..
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