Download Instruction level architecture

ISA - Instruction Set
Architecture
Programmer’s Point of View
Copyright © 2005 - Curt Hill
View point
• At this level we disregard the
following important levels
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Digital logic level
Micro-program level
Cache memory
Anything hidden in the CPU
Except as they impact the instructions
• Only things that are of interest to the
programmer
Copyright © 2005 - Curt Hill
We are concerned with
• Memory models
• Registers
– Number and type of registers
– Some registers are hidden and so we
can disregard
– The IR is invisible to us
– The PC may be accessible or not so we
may have to consider it
• The instructions, which is where we
will spend the most time
Copyright © 2005 - Curt Hill
Who uses this level?
• Machine language programmers
– There are hardly any of these
• Assembly language programmers
– Even these are scarce but still viable
– There is a one to one correspondence
between machine and assembly language in
most cases
• Compilers
– The compiler writer generates code very close
to machine language code
– Therefore the ISA must be well understood
Copyright © 2005 - Curt Hill
Memory model issues
• Unit of addressability
• Alignment
• Memory partitioning
Copyright © 2005 - Curt Hill
Addressability
• What is the smallest unit that can be
addressed?
• Bit
– B1700 used a bit addressable scheme
• Byte
– Most machines are byte addressable
• Word
– A few are word addressable
– When the word is short that is no
problem
Copyright © 2005 - Curt Hill
Examples
• 8080 had a 8 bit word
– Byte addressable or word addressable?
• 8086-80286 had a 16 bit word
– Byte addressable
• 80386-Pentium had a 32 bit word
– Byte addressable
• PDP-8 had a 12 bit word
• IBM 360 had a 32 bit word
• CDC Cyber had a 60 bit word
– Word addressable
Copyright © 2005 - Curt Hill
Alignment
• Even in byte addressable machine
there may be times when there is a
requirement to align operands on
boundaries
• This is the issue of alignment
• Are there ramifications of having
any operand on any address?
• Sometimes it is an error and
sometimes it is a performance issue
Copyright © 2005 - Curt Hill
Alignment Examples
• IBM 360 required words to be on
addresses divisible by 4
– The successor IBM 370 dropped the
requirement but the program ran faster if the
alignment was maintained
• PDP 11
– 16 bit word but byte addressable
– had even and odd addresses
• Pentiums have a 32 bit word but fetch
things 64 bits at a time
– They run faster if you waste a few bytes and
align operands on addresses divisible by 8
– What you especially want to avoid is a single
reference that spans two double words
Copyright © 2005 - Curt Hill
Partitioning memory
• Some machines have a single address
space which can contain data,
instructions or whatever
• Others partition the memory such that
code goes in one part and data in another
• This allows for a doubling of the address
space since the context of the reference
selects the memory
• The Intel machines from 8086 up look like
they partition memory but actually do not
Copyright © 2005 - Curt Hill
Serialization
• If two commands occur in sequence
will they be executed sequentially?
St 10,X -- store reg 10 to x
L 5,X – load x into reg 5
• This would assume that the load gets
the contents saved by the store
• Does that always happen?
Copyright © 2005 - Curt Hill
Violation of serialization
• In pipelined systems and some kinds of
microprogramming this may not be the
case
• If you serialize memory references you
slow performance
• Or you may just specify that there may be
a delay that is imposed on Stores
• This makes the compiler writers job
substantially more difficult, not to mention
the assembler
Copyright © 2005 - Curt Hill
Other topics
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Register types
Instruction Format
Addressing Modes
Assembly language programming
Copyright © 2005 - Curt Hill