1. CS 101 – Sept. 25 Continue Chapter 5
Review of pipelined instruction execution
Memory system
Limited memory on chip
Main memory (RAM)
Secondary memory (e.g. disk)

2. How many cycles to execute 7 instructions?F D X 1 2 3 4 5 6 7 8 9
How many cycles to execute 7 instructions?
Real answer is “too many” 
This is as far as we got.

3. Cycle F D X 1 2 3 4 5 6 7 8 9
Better approach:
Technique is called “Pipelining”
On each cycle, try to do several jobs at once.
Like an assembly line.

4. Pipelining the CPU
CPU consists of parts: fetcher, decoder, executor, etc.
Without pipelining,
total cycles = (# inst) * (CPI)
With pipelining,
total cycles = ?
(Hint: do first instruction; then handle the rest)
# inst + # stages - 1

5. Example
How many cycles are needed to execute 50 instructions, given that the CPU uses 4 stages? [Fetch, decode, execute, writeback]
4 cycles for the first instruction
1 cycle for each of the rest
4 + (50 – 1) = 53 cycles
CPI = 53 cycles / 50 instructions = 1.06

6. Stalls
What if instruction 2 needs more time? During cycle 5, instruction 3 has to wait.
Cycle F D X W 1 2 3 4 5 6

7. Cycle F D X W 1 2 3 4 5 6 7 8 9 10

8. Deli pipeline! Five stages in deli to make sandwich:
Place order
Get right bread and slice
Put meat on bread
Add condiments
Cashier and pickup.
What are some possible stalls?

9. Summary Time = total cycles / clock rate Is the machine pipelined?
If no, cycles = (# instructions) * (CPI)
If yes, cycles = # stages + # inst – 1 + stalls
Simultaneous stalls
With pipelining, CPI is almost 1.
Can we do better than this?

10. Memory system After the CPU, next most important feature
CPU contains registers – only enough for immediate calculations
Main memory – running programs
Secondary memory – long-term storage
“open” vs. “save”

11. Main memory Runs on electricity: “volatile” but fast
Principle of random access
Examples:
RAM (vast majority of main memory is here)
Cache (small version of RAM, much faster)
BIOS (“Basic I/O System”)

12. Secondary memory Non-electric, “non-volatile” technologies
Magnetic
Optical
How to find information? It may be:
Random access (e.g. flash memory; stick)
Direct access (disk)
Sequential access (tape)

13. Example comparison Type Size Access time Cost per MB CPU regs
256 bytes
1 ns
Forget it.
Cache
16 KB
2 ns
$ 20
RAM
128 MB
20 ns
$ 1
disk
100 GB
100,000 ns $