The Instruction Set Architecture

1
The Instruction Set Architecture
2
Hardware Software boundary
Java Program
Ada Program
C Program
Compiler
Compiler
Compiler
Instruction Set Architecture
Microcode
Hardware
3
Data types

• The ISA defines the data types native to a
  particular machine. Some examples of numeric
  data types that may or may not be present on the
  machine
• Signed integer
• Unsigned integer
• Floating point
• Binary Coded Decimal (BCD) integers
• Packed numeric representations
• Java does not support unsigned, BCD, or packed
  integer representations.

4
Instruction Format

• The ISA defines the instruction format(s) for the
  machine
• Fixed or variable length instructions
• 0, 1, 2, or 3 operand instructions
• Addressing modes
• The Java Virtual Machine
• 9 instruction formats
• Variable length
• Little in the way of addressing modes immediate,
  indexed

5
Examples

• See handout of IBM System 360 ISA
• 5 instruction formats
• A long list of instructions
• Condition codes
• This is a CISC (Complex Instruction Set Computer)
  Architecture

6
Addressing modes

• Why more than one type?
• Short is good, but not sufficient for all
  purposes
• Immediate address the data is in the same
  memory word as the instruction
• Register the data is in one of the registers,
  memory locations on the processor chip itself
• Absolute specify the exact memory location in
  use
• Indexed Data is specified by a base and
  displacement
• Indirect address specified contains an address,
  rather than data

7
ISA and Microarchitecture

• So, you have this wonderful idea for improving
  performance of your processor
• Will all the existing software still run?
• Will the same operating system work?
• The microarchitecture separates the actual
  hardware from the ISA.
• Do the hardware improvements.
• Modify the microcode to match the existing ISA to
  the new hardware. All existing applications
  continue to run. New applications can take
  advantage of new features.

8
Language Levels
Higher level images of the computer
Spreadsheets, Word processors, databases
closer to the problem
C, Java, etc. designed for representing a
problem solution independent of the machine
ISA instruction set, addressing modes of a
specific line of computers
Microarchitecture bit settings that activate
specific data flows
Hardware understands voltages
9
Access to the hardware

• Programs that can manipulate the hardware can be
  dangerous.
• Hardware is protected by providing several
  classes of access rights.
• Operating system has complete access
• User programs are limited.
• Users can access the hardware by invoking
  operating system functions.

10
Why many high level languages?

• Notice the position of the high level languages
  in our chart
• They exist to make it easier to express the
  problem
• Different application areas have different needs
  in terms of expressing problems.
• Ex. APL single instruction to multiply two
  matrices

11
Compiler

• A compiler is a program
• Its input is the text of a program in a
  particular language, say c
• The compiler translates the c code into the
  instruction set understood by the computer
• The translation is not a line by line
  transliteration. Optimization.

12
Instructions and Data

• Two distinct kinds of things
• The machine understands a finite subset of
  possible bit patterns as instructions.
• Data can take any bit pattern.
• Usually stored separately. Data accessed when an
  instruction calls for it.

13
Black boxes

• To reduce complexity, we sometimes show only the
  input and output of an operation without the
  details of how it gets done. We call it a black
  box, because the content of the box is hidden.

a b
a, b
Turing machine that adds
We can combine the boxes and use them any way we
want.
14
Turing Machines

• Mathematical model of a machine
• Turing was interested in understanding what could
  be computed and what could not be computed.
• The general purpose computers of today are
  implementations of the Turing model
• Thats why all computers can do the same things,
  given enough time and memory because they are
  all Turing machines.