ECS154A-L17

📗 -> 02/14/25: ECS154A-L17

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Lecture Video

🎤 Vocab

❗ Unit and Larger Context

How to build a machine? Microarchitecture

✒️ -> Scratch Notes

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• Minimal example of CPU, capable of executing instructions

• X and Z are buffers, to hold the data and prevent signals from mixing (bus can’t hold A and B at same time, need temp storage in X) (same with Z)

• FDE Cycle:

  1. Fetch - Gets the instruction
  2. Decode - Decodes the fetched information, translates it
  3. Execute - Does what the instruction told it to

• Going through this cycle is effectively a big Moore model, going through the states of each step.

Example Steps for Instructions

Cycle	Instructions
0	PC -> Bus ()	Bus -> MAR ( )	Send the READ signal	BUS -> X ()
1	INC B ()
2	Z -> Bus	Bus -> PC
3	MDR -> Bus	Bus -> IR
4	ACC -> Bus	Bus -> X
5	R -> Bus	ADD
6	Z -> Bus	Bus -> ACC

• Instructions for ADD R
  • ADD R: ACC = ACC + R

Cycle	Instructions
n	R -> Bus ()	Bus -> MAR ( )	Send the READ signal
n+1	ACC -> Bus	Bus -> X
n+2	MDR -> Bus	ADD
n+3	Z -> Bus	B -> ACC
n+4
n+5

• Instructions for ADD mem(R)
  • ACC (R): CC = ACC + mem(R)

Comparison:

ADD R takes 3 execute cycles
ADD (R) takes 4 execute cycles

We can see that the limiting factor here is sharing the bus. Multiple buses could mean faster execution!

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• Visualized as a Moore Model

Continuation

You can speed it up by adding another bus! Or another! Or another!

With 3 buses, you can do the entire instruction of add in ONE cycle!

CISC - Complex Instruction Set Computer
RISC - Reduced Instruction Set Computer

Which is faster?

• Define faster!
• CISC has more instructions than RISC, but RISC manages them better.

🧪 -> Refresh the Info

Did you generally find the overall content understandable or compelling or relevant or not, and why, or which aspects of the reading were most novel or challenging for you and which aspects were most familiar or straightforward?)

Did a specific aspect of the reading raise questions for you or relate to other ideas and findings you’ve encountered, or are there other related issues you wish had been covered?)

🔗 -> Links

Resources

• Put useful links here

Connections

• Link all related words