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Shortcomings of The Simple CPUs
Chapter 6.4 - 6.6
Yok Fai Ho
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.
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
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
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.
More Internal registers And Cache
• Sun Microsystems.
• It has 1 MB of cache.
• Some even go up to 4
MB of cache
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
Diagram of Simple CPU using multiple buses
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.
Pipelined instruction processing
Step 4
Step 5
Step 1
Step 2
Step 3
Larger Instruction Sets
• Having a larger number of instructions in a
processor’s instruction set generally allows a
program to perform a function using fewer
Larger Instruction Sets
Consider a CPU have a small instruction sets can only have AND.
(A’ * B’)’
Complement A
Complement B
AND A and B
Complement the result
(A + B)
OR A and B
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.
Internal organization of the 8085 microprocessor
• 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.
• Carpinelli, D. John. Computer System Organization and Architecture.
Addison-Wesley, 2001.
• Http://