Shortcomings of The Simple CPUs

1
Shortcomings of The Simple CPUs

• Chapter 6.4 - 6.6
• Yok Fai Ho

2
Shortcomings of The Simple CPUs

• In this section of the chapter, it go into more
  detail on how to design a faster and more
  efficient CPU.

3
Main Points

• More Internal Registers And Cache.
• Multiple Buses Within The CPU.
• Pipelined Instruction Processing.
• Larger Instruction Sets.
• Subroutines And Interrupts.
• Internal Architecture of The Intel 8085
  Microprocessor.

4
More Internal registers And Cache

• Enable to build a faster CPU, one of the best
  ways to improve the performance of a
  microprocessor is to incorporate more storage
  within the CPU.
• Adding registers and cache makes it possible to
  replace some external memory accesses with much
  faster internal accesses.

CPU
Registers
RAM
Cache
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More Internal registers And Cache

• In most of our home PC, there are 256K-512K of
  cache in the CPUs. Some early model only have
  16K of cache.
• Because the register and cache are more expensive
  than RAM. So many computer turn to RAM as their
  main memory storage. But at the same time the
  speed decrease.

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More Internal registers And Cache

• Sun Microsystems.
• It has 1 MB of cache.
• Some even go up to 4 MB of cache

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Multiple Buses Within the CPU

• Buses are efficient media for routing data
  between components within a CPU.
• However, a bus may only contain one value at any
  given time. Because of that, most CPUs contain
  multiple buses for multiple data transfers.
• That reduces the time needed to fetch, decode,
  and execute instructions, thus improving system
  performance.

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Diagram of Simple CPU using multiple buses
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Pipelined instruction processing

• In pipelining, instructions are processed like
  goods on an assembly line.
• While one instructions is being decoded, the next
  instruction is fetched, and while the first
  instruction is being executed, the second is
  decoded and a third instruction is fetched.
• This allows programs to be executed more quickly,
  even though each individual instruction requires
  the same amount of time.

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Pipelined instruction processing
washer
dryer
closet
first
washer
closet
dryer
second
closet
dryer
washer
third
Step 1
Step 2
Step 3
Step 4
Step 5
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Larger Instruction Sets

• Having a larger number of instructions in a
  processors instruction set generally allows a
  program to perform a function using fewer
  instructions.

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Larger Instruction Sets
Consider a CPU have a small instruction sets can
only have AND.
(A B) (A B)
OR A and B
Complement A Complement B AND A and B Complement
the result
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Subroutines and Interrupts

• Almost all CPUs have hardware to handle
  subroutines, typically a stack pointer, and
  instructions to call and return from the
  subroutine.
• Most CPUs also have interrupt inputs to allow
  external hardware(like mouse, keyboard ) to
  interrupt the current operations of the CPU.
• That is useful, if you know the program is a
  infinite loop and want to exit the program.

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Internal organization of the 8085 microprocessor
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Summary

• To design a CPU, we first develop its instruction
  set architecture,including its instruction set
  and its internal registers.
• We then create a finite state machine model of
  the micro-operations needed to fetch, decode, and
  execute every instruction in its instruction set.
• Then we develop an RTL specification for this
  state machine.

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References

• Carpinelli, D. John. Computer System Organization
  and Architecture. Addison-Wesley, 2001.
• Http//www.awl.com/carpinelli