CISC vs RISC
• Consider i486 instruction set format shown in
fig 1.
• It can be noted that each time an instruction is
fetched the various sub-sections of the format
need to be decoded for effective execution of
that instruction
• The fields are similar to those of i86 but
complicated since other fields are added like ss,
index, base, sreg2, sreg3, mn
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RISC
Compared to the long, complex, and many
addressing modes of CISC formats, RISC
processors have very simple and short
instructions.
RISC instructions are fixed in length (usually 32
bit) and have simple formats and stored in
memory aligned in a double word boundary
The formats are simply decoded and hence
shortening the overall instruction cycle.
Any operation simple or complex can be
synthesized using these simple instructions.