CISC

Question 1

Why was Microcode used for CISC?

Answer 1

Adding more and more instructions increased hardware complexity to decode / control functions with direct decode from opcode.

Question 2

What did CISC do?

Answer 2

Added more functionality to single instructions (and a lot more of them)

Easier to program in assembler

Less instructions, less memory usage (more efficient)

Question 3

How did CISC implement instructions?

Answer 3

Decode for opcode converted into microinstructions by group stored in ROM (faster than ram)

One microinstruction per clock cycle (many cycles for one regular instruction)

Question 4

Advantages of CISC microcode

Answer 4

Simplify hardware design

All instructions decoded in a common way

Easy to implement/update ROM instruction groups instead of hardware decode circuits

Question 5

What is a microinstruction?

Answer 5

Textual representation of the control signals required to perform a phase of an instruction

Question 6

List 3 types of instruction length formats & adv & disadv

Answer 6

Variable length:
More complex decode hardware (CPU doesn’t know how long instruction is on fetch), limits parallelism

Fixed:
More memory instruction space used (unused fields)

Hybrid:
set of multiple fixed length/format instructions

Question 7

List some features of CISC machines

Answer 7

Large number of functionally complex instructions
Executed over a number of cycles

Multiple addressing modes for memory/operand access

Small number of general purpose registers (lots of special purpose to help with complex calculations)

Complex decoding logic:
Variable length instructions
Microcode/microinstructions

Not much pipelining (limited overlap)