Download Fundamentals of Design

Computer Architecture
Part II-B: CPU Instruction Set
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Levels of Representation
temp = v[k];
High Level Language
Program
v[k] = v[k+1];
v[k+1] = temp;
Compiler
mov BL, 02h
mov AX,0A56h
mul BL
Assembly Language
Program
Assembler
Machine Language
Program
0000
1010
1100
0101
1001
1111
0110
1000
1100
0101
1010
0000
0110
1000
1111
1001
1010
0000
0101
1100
1111
1001
1000
0110
0101
1100
0000
1010
1000
0110
1001
1111
Machine Interpretation
Control Signal
Specification
ALUOP[0:3] <= InstReg[9:11] & MASK
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Instruction Set Design
software
instruction set
hardware
Which is easier to change?
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Instruction Set

The set of machine level instructions
for a particular type of computer or
hardware
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Instruction Cycle

The sequence of operations involved
in processing an instruction
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Instruction Cycle
Instruction
Obtain instruction from program storage
Fetch
Determine required actions
and instruction size
Instruction
Locate and obtain operand data
Operand
Decode
Fetch
Compute result, value or status
Deposit results in storage for later use
Determine successor instruction
Execute
Result
Store
Next
Instruction
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CPU-Memory Handshake


The CPU operates, then memory
There is a need for synchronization
between CPU and memory


Can be done through handshaking
Memory can inform the CPU that it has
completed its read/write request
through a Memory Function Complete
(MFC) signal
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Types of Control


Hardwired
Microprogrammed
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Hardwired Control

Pure hardware mode of control


The control unit tells the rest of the CPU
what is to be done every clock cycle
during the execution of instructions
Involves complex circuit design
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Hardwired Control Considerations

Fast


Permanent


Everything is done through hardware
Difficult and costly to do modifications
Difficult to implement in a large
instruction set
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Microprogrammed Control

Each instruction is broken down into a
series of steps, i.e. microinstructions
(or microcode)


The control unit becomes a miniature
computer
The complete set of steps for each
instruction is stored in a ROM called
the control store, and is executed
sequentially
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Microprogrammed Control

Decoding
involves obtaining
the starting
address in the
control store of
the first control
word (or
microinstruction)
for the instruction
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Microprogrammed Control
Considerations

Flexible



Only the control store needs
modifications to implement/modify
instruction execution
Slower
May require more hardware
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Microprogrammed Control Units


Introduced by IBM in 1964
Microprogrammed control units have
seen widespread use for several
reasons:



Easy to implement complex instructions
Cost of microprogramming is far less
than the cost of implementing a
conventional hardwired control unit
Easy to add instructions and/or modify
existing ones
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CISC and RISC

There are two schools of thought on
instruction set design:


Complex instruction set computers
(CISC)
Reduced instruction set computers
(RISC)
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Complex Instruction Set
Computers (CISC)

Upward compatibility




New processor designs had to provide
more power over older models while
maintaining compatibility
More complex and powerful instructions!
Generally requires the use of
microcode
Easily benefits from improved
technology
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CISC

Richer or more complex instruction
sets simplify compiler design


Reduced software development and
learning costs
More powerful instructions means
fewer instructions are needed to
implement software routines

Reduced system memory and bandwidth
requirements
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CISC Issues



The more complex the instruction set,
the larger the required control store
Compilers tend to make use of few of
the available instructions
80/20 rule: Analysis of the instruction
mix generated by CISC machines
shows that 80% of the instructions
executed used only 20% of the
instructions set
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CISC Issues



CISC designers may have introduced a lot
of instructions simply because it was easy to
do OR because its competitors did so
Manufacturing cost is increased because of
the need to test all operations and
combinations of operations
Design cost is increased for future systems
because of the need to maintain upward or
forward compatibility
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Reduced Instruction Set
Computers (RISC)



80/20 rule: If the most often used
instructions (the 20%) was speeded up then
the performance benefits would be greater
Studies showed that these instructions tend
to perform simpler operations and would
use only the simpler addressing modes
Complex operations can still be done by
using several simpler operations
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Why RISC?

Basic hardware is simpler to
implement



Easier to adopt hardwired control.
Easier to compile for a RISC than a
CISC architecture
Design effort and development cost is
less for RISC than for a CISC
processor
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RISC CPU Characteristics



Fewer instructions
More CPU registers
More instructions needed to execute
routines

May mean more memory space and
increased bandwidth requirements
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