Chapter 6 An Introduction to System Software and Virtual Machines'

1
Chapter 6 An Introduction to System Software and
Virtual Machines.
2

• Von Neumann has the capability of executing
  programs in machine language
• No support tools to make problem solving easier
• This is prone to errors. We call this the naked
  machine since there is only hardware support and
  no software.
• To use the machine we must
• Write programs in binary machine instructions,
  data should be in binary
• Put the program and data in memory
• Put the start address into PC and start the
  Fetch/Decode/Execute cycle.
• But doing this makes changing a program difficult
• Eg. if there is a jump to address 500 instruction
  and an instruction is inserted before address 500
  in the machine. Then the previous jumpto address
  will not be correct anymore. It has to be
  updated as well.
• We need an interface between user and hardware.
• Users view of computer is those operations the
  system software makes available.

3

• System software is a collection of programs that
  manage the resources of a computer and facilitate
  access to those resources.
• Includes the user interface
• Acts as an intermediary between users and the
  hardware.
• There is no black box around software
• Software is a sequence of instructions programs
  that solve a problem.
• The set of services and resources created by the
  software and seen by the user is called a virtual
  machine or virtual environment.
• Figure 6.1

4

• Functions of the System software
• Hide from user details of internal structure of
  Von Neumann
• Present important information in a way that is
  easy to understand
• Allow the user to access hardware resources in a
  simple and efficient way
• Provide a secure and safe environment in which to
  operate
• Eg. to add, using abc is easier than loading
  registers, activating ALU, selecting output of
  circuit and sending result to a.
• Program should be loaded into RAM without the
  programmer specifying where to place it.

5

• There are two main types of user interfaces
• Command line interface
• User types commands and command interpreter
  executes it.
• Graphical User interface(GUI)
• A mouse or pointing device is used to click icons
  which the user interface detects.
• The various types of system software in the
  Operating system are
• Operating System (OS) eg. Win 2000, Win XP,
  Linux, UNIX.
• Language translators
• Assemblers
• Compilers
• Memory Managers
• Loaders
• Linkers
• Information managers
• File systems
• Database systems
• Scheduler
• Utilities
• Text editors

6
Assemblers and assembly language

• Machine language is complicated (binary, numeric
  memory addresses, difficult to change,
  difficult/tedious to create data in binary).
• We use assembly language. And an assembler
  associated with it. The assembler falls under the
  system software. We also call them second
  generation languages/low level programming
  languages.
• Figure 6.3, Programming languages.
• Advantages of using assembly language instead of
  machine language
• Uses symbolic operation codes instead of numeric
• Uses symbolic memory addresses instead of numeric
• Pseudo operations that provide user oriented
  services such as data generation.
• Assembly language - a programming language that
  provides symbolic representation of machine
  instructions and memory locations.
• Assembler - a program that inputs an assembly
  language program produces the machine language
  for the instructions.
• Source program - a program written by a user. It
  is in ASCII text.
• Object program - a machine language program. It
  is binary numbers.
• Assembly language instructions consist of 4
  fields, not all fields are used by every
  instruction.label opcode_mnemonic address(es)
  --comment

Load (0000) Add (0011) Store (0001)
7

• Figure 6.4 The translation, loading and execution
  process.
• Figure 6.5 Typical Assembly language instruction
  set.
• We can use symbolic addresses now so the JUMP 500
  instruction no longer needs to be changed.
• Solution Attach a symbolic label.
• Example
• Begin Load X
• Jump Begin
• This leads to program clarity.

Begin is equivalent to the address of the memory
cell that holds instruction load X
8

• Symbolic labels have two advantages
• Program clarity and meaningful (Ex. Begin, Loop,
  Count, Error, etc.)
• Maintainability
• (refers to an instruction via its label and not
  its address)
• Example
• JUMP LOOP
• LOOP LOAD X

You can add a new instruction here now without
problems That we had before.
9
Data generation

• Final advantage of assembly language
• We saw an algorithm used to represent unsigned
  numbers, singed integers, floating point values
  and characters.
• These conversions need not be done by the
  programmer in assembly.
• To make this request we use pseudo-ops ( special
  type of assembly language op code) (mentioned
  before)
• Pseudo-ops
• Always precedes by .
• Does not generate machine language instructions
  like other op codes.
• Invokes the service of the assembler to generate
  data in the proper binary form.
• Examples
• FIVE .DATA 5
• In this case, DATA builds a signed integer
• Generates binary representation of 5
• Puts it in memory
• Makes label FIVE equivalent to address of that
  cell in memory.
• Draw figure.
• NEGSEVEN .DATA -7 (what would this look
  like?)
• Count .DATA 0
• One .data 1
• LOAD FIVE would be equivalent to LOAD 53 in our
  example that we drew.

10

• Example
• ADD NEGSEVEN
• Adds -7 to current contents of register R.
• When generating data values we should be careful
  not to place them in memory locations where they
  could be misinterpreted as instructions.
• The way a computer can tell if a binary value is
  a number or character is in its context.
• The same thing goes for instructions and data.

11

• We could write the following Incorrect sequence
  of instructions.
• LOAD X
• .DATA 1
• After loading X, the processor will fetch decode
  and execute the instruction 1.
• 1 would look like 0000 000000000001
• The leftmost 4 bits would be the opcode and since
  it is 0 in our machine this is considered to be a
  LOAD. See figure 6.5
• So in effect the data value 1 has accidentally
  been turned into the instruction LOAD 1

12

• Structure of a Typical Assembly Language Program
• .BEGIN --This must be the first line of the
  program
• -- Here are assembly language
  instructions
• -- of the type shown in Figure 6.5
• HALT
• -- Data generation pseudo-ops such as
• -- .DATA are placed here after the HALT
• .END -- This must be the last line of the
  program

13

• Pseudocode
• Set the value of I to 1
• Add 1 to value of I
• Assembly language
• LOAD ONE --put 1 into R
• STORE I --store constant 1 into I
• INCREMENT I --add 1 to memory location I
• I .DATA 0 --Index value initially 0
• ONE .DATA 1 --Constant 1

14

• Pseudocode
• Set value of A to BC-7
• Assembly language instructions
• LOAD B --Put value B into register R
• ADD C --R holds (BC)
• SUBTRACT SEVEN --R holds (BC-7)
• STORE A
• A .DATA 0
• B .DATA 0
• C .DATA 0
• SEVEN .DATA 7

15
Example of testing and comparing

• Pseudocode
• Input value of X
• Input value of Y
• If XgtY then
• Output value of X
• Else
• Output value of Y
• Assembly language
• IN X
• IN Y
• LOAD Y
• COMPARE X -- compare X to Y and set CC
• JUMPLT PRINTY
• OUT X
• JUMP DONE
• PRINTY OUT Y
• DONE -- program continued
• -- the following data go after the HALT

16
Example of Looping

• Pseudocode
• Set I to 0
• While value of I lt 10000 do
• Add 1 to value of I
• End of loop
• Stop
• Assembly language
• LOAD ZERO
• STORE I
• LOOP LOAD MAXVALUE
• COMPARE I
• JUMPEQ DONE
• INCREMENT I
• JUMP LOOP
• DONE HALT
• ZERO .DATA 0

17
Another example

• Read in a sequence of non negative numbers one
  number at a time and compute a running sum. When
  you encounter a negative number, print out the
  sum of the non negative values and stop.
• First write pseudocode
• Set value of sum to 0
• Input the first number N
• While N is not negative, execute lines 45
• Add value of N to Sum
• Input next data value N
• End of loop
• Print out Sum
• Stop

18
Figure 6.8

• .BEGIN
• CLEAR SUM
• IN N
• AGAIN LOAD ZERO
• COMPARE N
• JUMPLT NEG
• LOAD SUM
• ADD N
• STORE SUM
• IN N
• JUMP AGAIN
• NEG OUT SUM
• HALT
• SUM .DATA 0
• ZERO .DATA 0
• N .DATA 0
• .END

19
Translation and Loading

• What must be done to programs to be executed on a
  processor?
• Transparency Figure 6.4
• Assembler Translate symbolic assembly code to
  machine language.
• The four tasks of the Assembler
• Convert symbolic opcodes to binary (Opcode table)
  Figure 6.9.
• Convert symbolic addresses to binary (symbol
  table)
• Perform assembler services requested by pseudo
  ops.
• Put the translated into a file for future use.

20

• What if the op code is not found? i.e. the
  programmer wrote an illegal opcode.
• Think about the algorithm to use to find the op
  code.
• Maybe sequential search (avg n/2 comparisons)
• or binary search. (lg n)
• This is why algorithm analysis is so important.
• Do similar thing for addresses
• eg, X or LOOP to binary address
• More difficult than converting op code.
• Assembler must determine correct numeric value of
  all symbols used in label field
• No built in system like an address conversion
  table.

21

• Symbol Recall this is what appears in the label
  field or data pseudo-op so the symbol has value
  of address of instruction to which it is
  attached.
• Binding process of associating a symbolic name
  with a physical memory address. (First pass)
• Location counter variable used to determine
  address of instruction.

22
First pass of the Assembler

• The Assembler makes two passes over your assembly
  language code.
• First pass
• Goals
• Bind symbolic names to address values
• Keeps track of memory address where instruction
  will be stored when translated (knows begin and
  end and number of cells.
• Binding is a process of associating a symbolic
  name with a physical memory address.
• Location counter is a variable used to determine
  address of instruction.
• Enter bindings in symbol table.
• Determines if there is a symbol in label field of
  instruction. If yes, then enter symbol and
  address into special table called symbol table.
• Figure 6.11 for an algorithm for Pass One.

23
Second Pass of the Assembler

• The goal of the second pass is to produce the
  object file.
• It translates source program into machine
  language.
• There are two table lookups.
• Opcode table ? Binary value
• Mnemonic
• Symbol Table ? Binary value
• Symbolic Address
• GOALS
• Handle data generation pseudo-ops
• Produce the object file needed by loader
• Figure 6.12 Algorithm for Pass 2.
• Object file contains the translated machine
  language object program.

24

• Eg. if our instruction format is 4-bit op code
  followed by 12-bit address then SUBTRACT X
• The assembler will
• Look up SUBTRACT in opcode table and place 4-bit
  value in op code field.(0101)
• Loop up symbol X in symbol table of Figure
  6.10(B)and place the binary address 0000 0000
  1001 in address field.
• So the instruction will be 0101 0000 0000 1001
• Figure 6.13, Example of an object program

25
The Loader

• It reads the object file and stores it in memory
  for execution
• It does this by reading the address and
  instruction for each of your assembly language
  instructions
• When done putting in memory, the loader places
  address of the first instruction into the program
  counter(PC) to initiate execution.
• The hardware then begins the Fetch/Decode/Execute
  cycle.

26
OS

• What is an operating system?
• It is a set of programs that control and make
  available to users the hardware resources.
  Maintenance and support software is also
  included.
• Responsibilities of OS

• Loading and running programs
• Multitasking
• Managing memory providing memory to programs
  and reclaiming memory
• Maintaining the file system
• System security
• Authorize users, passwords etc

• Efficient resource allocation
• High priority to more important programs
• Managing the processor by keeping it busy.
• Switching between users
• Deadlock detection
• Deadlock prevention
• Error detection

27
Responsibilities of OS contd..

• Deadlock detection a set of processes (running
  programs) are deadlocked if each is waiting for a
  resource held by another.
• Safe use of resources Prevent users from
  attempting operations that could cause system to
  enter a state where it cannot do any work.
• Eg.
• Prog A requests tape drive then laser printer and
  then wants to print file
• Prog B requests laser printer and then tape drive
  and then wants to print its file.
• In this case the first request of each program is
  satisfied but then we have deadlock. WHY??
• OS should prevent deadlock from occurring.
• One way to do it is for a program to get all its
  resources in advance and if it cannot get ALL of
  its resources then it must give up all what it
  owns and issue a completely new request all over
  again.
• An analogy would be you calling your friend and
  your friend calling you on the phone at exactly
  the same time!

28
User interfaces

• The OS can use either one or both of these
  interfaces to communicate with the user.
• GUI Graphical User Interface
• Command line interface
• Figure 6.15 for a flowchart of the way the OS
  deals with the user interface.

29
Review

• System Software
• Types of system software
• The virtual machine
• Assemblers and Assembly language
• Examples of Assembly language code
• What is the Operating System (OS)?
• Functions of the OS

30

• THE END OF CHAPTER 6