Code

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Code
2
Overview

• Architecture
• Levels of software
• The software development process

3
Takeaways

• The hardware architecture defines how the
  software architecture is built.
• The software architecture defines how code is
  written.
• Open systems can be more easily modified

4
Overview

• Architecture
• Connect processor to peripherals
• CD/DVD, Hard disk, floppy disk off motherboard
• Network, keyboard, mouse, monitor on
  motherboard
• Connect processor to RAM
• Design instruction set (and circuitry) for CPU
• Levels of software
• The software development process

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CPU is the key component

• Key ideas
• Von Neumann machine
• Fetch execute cycle
• Can only operate on data held locally
• May have many or few machine instructions
• CISC (Complex Instruction Set Computers)
• RISC (Reduced Instruction Set Computers)

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The CPU
Memory Registers
Control Unit(State Machine)
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Machine Snapshot

• Everything is in binary (hex)
• Organized into words of some size (16/32/64 bits)

Registers
R0 0000 R1 0CA8 R2 A9DB R3 0705 R4 1011 R5
90A0 R6 0807 R7 00A0
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Fetch/Execute Cycle

• At each tick of the clock
• Read the Instruction pointer
• Go to that address in RAM
• Fetch the contents of that location to the
  instruction register
• Execute the instruction stored in the instruction
  register
• This instruction may require using the A/LU to
  operate on information in registers
• This instruction may involve information from
  registers, RAM, disk, CD,
• This instruction may change the address in the
  instruction pointer. It it doesnt, the
  instruction pointer is increased by 1 when the
  execution is done.

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Sidebar Question

• My computer is 1.2GHz, that means 1.2 billion
  clock ticks per second.
• When I run IE and ask it to open a page, it takes
• 3 seconds if the page is already open elsewhere
• 7 seconds otherwise
• Questions
• Why the difference in timings?
• What happens to the 3.6-8.4 billion clock ticks?

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Sample Instructions

• Add RegisterA to RegisterB and put the result in
  RegisterC
• Load the value X into RegisterA
• Store the contents of RegisterB into location X
  in RAM
• Load the contents of memory location X into
  RegisterA
• Move the instruction pointer to location X
• If the value in RegisterA is positive then move
  the instruction pointer to location X

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Adding Numbers

• Simple Program to calculate1 2 3 4 5 6
  21

10 Load R0 ? 0001 (always 1) 11 Load R2 ?
0000 (running total) 12 Load R1 ? 0001
(current number) 13 Add R2 ? R2 R1
(R21) 14 Add R1 ? R1 R0 (R12) 15 Add
R2 ? R2 R1 (R23) 16 Add R1 ? R1 R0
(R13) 17 Add R2 ? R2 R1 (R26) 18 Add
R1 ? R1 R0 (R14) 19 Add R2 ? R2 R1
(R210) 1A Add R1 ? R1 R0 (R15) 1B Add
R2 ? R2 R1 (R215) 1C Add R1 ? R1 R0
(R16) 1D Add R2 ? R2 R1 (R221) 1E halt
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Adding Numbers

• Use a Loop to calculate1 2 3 4 5 N

10 Load R1 ? 0006 (N) 11 Load R2 ?
0000 (running total) 12 Load R0 ? 0001
(always 1) 13 Add R2 ? R2 R1
(add in N) 14 Sub R1 ? R1 - R0 (N N-1)
15 Jump to 13 if (R10) (If N isnt 0
yet, go back) 16 halt (N is
now 0, and R2 12N)
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Von Neumann machine

• Program and data indistinguishable
• The same machine can do different things at
  different times
• So, we write programs

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Overview

• Architecture
• Levels of software
• Operating systems
• Compilers
• Libraries
• Interfaces
• Applications
• The software development process

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When user turns on computer

• System boots
• First the hardware, then the Windows operating
  system
• Windows O/S makes desktop appears
• Windows O/S creates device interaction
• CPU is listening to mouse, keyboard, etc.
• CPU can communicate with RAM, disk,
• O/S enters event-driven fetch/execute cycle
• Fetch event from user (mouse click, keyboard
  stroke, ..)
• Executes command

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Under the hood when machine boots

• Power goes on, each device is turned on
• BIOS lets you talk to hardware
• Change order devices are turned on
• Set system clock
• Identify new interaction devices
• Then, the boot program for Windows starts.
• You tell it an order to look at disks
• It looks at disks in this order to find operating
  system
• Operating system boots
• Simple program called the operating system kernel
  is loaded (at location 10). This is the program
  that runs the computer.

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Fetch/execute cycle

• Fetch is Event driven
• Button clicks in windows
• Command line entries in UNIX
• Execution is done by the operating system
• .exe files in Windows
• How do we create a .exe file?

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How do we create a .exe file?

• Unix
• Write a program in a higher level language
• C, C, Java,
• Compile the program into machine level commands
• Translate the compiled version into binary
  commands
• Link to system libraries (eg for input/output)
• Build the .exe
• Execute the .exe

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Overview

• Architecture
• Levels of software
• Operating systems
• Compilers
• Libraries
• Interfaces
• Applications
• The software development process

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A very simple program
main( ) int a,b,c a 1
b 2 c a b
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Translation
.file "a.c" .version
"01.01" gcc2_compiled. .text .align
4 .globl main .type main,_at_function main
pushl ebp movl esp,
ebp subl 12, esp movl
1, -4(ebp) movl 2, -8(ebp)
movl -8(ebp), eax addl -4(ebp),
eax movl eax, -12(ebp)
leave ret .Lfe1 .size
main,.Lfe1-main .ident "GCC (GNU) 2.96
20000731 (Red Hat Linux 7.3 2.96-112)"
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.file "a.c" gcc2_compiled. .section
".text" .align 4 .global main
.type main,function .proc
04 main !PROLOGUE 0 save
sp, -128, sp !PROLOGUE 1 st
i0, fp68 st i1, fp72
mov 1, o0 st o0,
fp-20 mov 2, o0 st
o0, fp-24 ld fp-20, o0
ld fp-24, o1 add o0, o1,
o0 st o0, fp-28 .LL2
ret restore .LLfe1 .size
main,.LLfe1-main .ident "GCC (GNU)
2.95.2 19991024 (release)"
.file "a.c" .version
"01.01" gcc2_compiled. .text .align
4 .globl main .type main,_at_function main
pushl ebp movl esp,
ebp subl 12, esp movl
1, -4(ebp) movl 2, -8(ebp)
movl -8(ebp), eax addl -4(ebp),
eax movl eax, -12(ebp)
leave ret .Lfe1 .size
main,.Lfe1-main .ident "GCC (GNU) 2.96
20000731 (Red Hat Linux 7.3 2.96-112)"
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After translation

• A binary file is created

0000000 7f45 4c46 0101 0100 0000 0000 0000
0000 0000020 0200 0300 0100 0000 e082 0408 3400
0000 0000040 1029 0000 0000 0000 3400 2000 0600
2800 0000060 1d00 1a00 0600 0000 3400 0000 3480
0408 0000100 3480 0408 c000 0000 c000 0000 0500
0000 0000120 0400 0000 0300 0000 f400 0000 f480
0408 0000140 f480 0408 1300 0000 1300 0000 0400
0000 0000160 0100 0000 0100 0000 0000 0000 0080
0408 0000200 0080 0408 7804 0000 7804 0000 0500
0000 0000220 0010 0000 0100 0000 7804 0000 7894
0408 0000240 7894 0408 1001 0000 2801 0000 0600
0000 0000260 0010 0000 0200 0000 8c04 0000 8c94
0408 0000300 8c94 0408 d000 0000 d000 0000 0600
0000 for 15,000 bytes
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At execution

• The binary file is moved from disk to RAM
• The instruction pointer is set to the first line
  of the file in RAM
• Execution happens
• Fetch/execute cycle until program exits

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Issues

• Different machines have different basic
  instructions
• The hardware guys get to specify the instruction
  set
• CISC vs. RISC vs.
• Different machines interpret slightly different
  versions of the language
• Programs can have different code for different
  architectures
• ifdef SPARC
• SPARC code goes here
• else
• LINUX code goes here
• endif

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Other tidbit

• The compiler might be optimizing
• Generates more efficient code at greater cost
• pushl ebp
• movl esp, ebp
• popl ebp
• ret
• Code may look very different

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Translation issues

• Different compilers do different things
• Source code is easiest to read
• Assembler code is fairly unreadable
• Binary code is totally unreadable

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Overview

• Architecture
• Levels of software
• Operating systems
• Compilers
• Libraries
• Interfaces
• Applications
• The software development process

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Libraries

• Shared code for common functions
• Math function library
• Input/Output library
• String processing library
• Drawing library
• Often linked at execution time.
• .dll dynamically linked library
• A library needs to be loaded into RAM but 1 copy
  is enough
• When you install a program, it brings its dlls
  and see if they are needed

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Overview

• Architecture
• Levels of software
• Operating systems
• Compilers
• Libraries
• Interfaces
• Applications
• The software development process

33
Interfaces

• Tools are designed with APIs (applications
  programmers interface)
• The API is published, but the code isnt
• Who controls the API, controls a lot

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Visual Basic Exercise

• Create 2 buttons
• Add
• Quit
• Give each functionality
• Update buttons to take input

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Public Class Form1 Inherits
System.Windows.Forms.Form Region " Windows Form
Designer generated code " Public Sub New()
MyBase.New() 'This call is
required by the Windows Form Designer.
InitializeComponent() 'Add any
initialization after the InitializeComponent()
call End Sub 'Form overrides dispose to
clean up the component list. Protected
Overloads Overrides Sub Dispose(ByVal disposing
As Boolean) If disposing Then
If Not (components Is Nothing) Then
components.Dispose() End If
End If MyBase.Dispose(disposing) End
Sub 'Required by the Windows Form Designer
Private components As System.ComponentModel.ICon
tainer
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'NOTE The following procedure is required by the
Windows Form Designer 'It can be modified
using the Windows Form Designer. 'Do not
modify it using the code editor. Friend
WithEvents Label1 As System.Windows.Forms.Label
Friend WithEvents Label2 As System.Windows.Forms
.Label Friend WithEvents Label3 As
System.Windows.Forms.Label Friend WithEvents
Button1 As System.Windows.Forms.Button Friend
WithEvents TextBox1 As System.Windows.Forms.TextBo
x Friend WithEvents TextBox2 As
System.Windows.Forms.TextBox Friend
WithEvents TextBox3 As System.Windows.Forms.TextBo
x
Private Sub InitializeComponent()
Me.Label1 New System.Windows.Forms.Label()
Me.Label2 New System.Windows.Forms.Label()
Me.Label3 New System.Windows.Forms.Label()
Me.Button1 New System.Windows.Forms.But
ton() Me.TextBox1 New
System.Windows.Forms.TextBox()
Me.TextBox2 New System.Windows.Forms.TextBox()
Me.TextBox3 New System.Windows.Forms.Text
Box() Me.SuspendLayout() '
'Label1 ' Me.Label1.Location
New System.Drawing.Point(184, 32)
Me.Label1.Name "Label1" Me.Label1.Size
New System.Drawing.Size(176, 88)
Me.Label1.TabIndex 0 Me.Label1.Text
"vALUE OF a" ' 'Label2 '
Me.Label2.Location New
System.Drawing.Point(176, 168)
Me.Label2.Name "Label2" Me.Label2.Size
New System.Drawing.Size(184, 104)
Me.Label2.TabIndex 1 Me.Label2.Text
"vLAUE OF b" '
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'Label3 ' Me.Label3.Location
New System.Drawing.Point(184, 416)
Me.Label3.Name "Label3" Me.Label3.Size
New System.Drawing.Size(184, 112)
Me.Label3.TabIndex 2 Me.Label3.Text
"vALUE OF c" ' 'Button1
' Me.Button1.Location New
System.Drawing.Point(176, 304)
Me.Button1.Name "Button1"
Me.Button1.Size New System.Drawing.Size(168,
64) Me.Button1.TabIndex 3
Me.Button1.Text "aDD THEM" '
'TextBox1 ' Me.TextBox1.Location
New System.Drawing.Point(416, 416)
Me.TextBox1.Name "TextBox1"
Me.TextBox1.Size New System.Drawing.Size(184,
20) Me.TextBox1.TabIndex 4
Me.TextBox1.Text "" '
'TextBox2 ' Me.TextBox2.Location
New System.Drawing.Point(424, 192)
Me.TextBox2.Name "TextBox2"
Me.TextBox2.Size New System.Drawing.Size(176,
20) Me.TextBox2.TabIndex 5
Me.TextBox2.Text "" '
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'TextBox3 ' Me.TextBox3.Location
New System.Drawing.Point(424, 40)
Me.TextBox3.Name "TextBox3"
Me.TextBox3.Size New System.Drawing.Size(184,
20) Me.TextBox3.TabIndex 6
Me.TextBox3.Text "" ' 'Form1
' Me.AutoScaleBaseSize New
System.Drawing.Size(5, 13) Me.ClientSize
New System.Drawing.Size(672, 622)
Me.Controls.AddRange(New System.Windows.Forms.Cont
rol() Me.TextBox3, Me.TextBox2, Me.TextBox1,
Me.Button1, Me.Label3, Me.Label2, Me.Label1)
Me.Name "Form1" Me.Text "Form1"
Me.ResumeLayout(False) End Sub End
Region
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Dim A, B, C As Integer Private Sub
Button1_Click(ByVal sender As System.Object,
ByVal e As System.EventArgs) Handles
Button1.Click C A B
TextBox1.Text Str(c) End Sub Private
Sub TextBox3_TextChanged(ByVal sender As
System.Object, ByVal e As System.EventArgs)
Handles TextBox3.TextChanged A
Val(TextBox3.Text) End Sub Private Sub
TextBox2_TextChanged(ByVal sender As
System.Object, ByVal e As System.EventArgs)
Handles TextBox2.TextChanged b
Val(TextBox2.Text) End Sub End Class
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Overview

• Architecture
• Levels of software
• Operating systems
• Compilers
• Libraries
• Interfaces
• Applications
• The software development process

42
Applications

• Few applications start from scratch
• Use classes methods
• Heres how to store the data
• Heres how to manipulate the data
• Use tools built by others
• Use APIs to share information as objects among
  applications

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Overview

• Architecture
• Levels of software
• The software development process

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Software development

• In the small
• Write specifications
• Write code to realize the specifications
• In the large
• Write overall specifications
• Break the task into pieces (modules)
• Carefully design interfaces between modules
• Have programmers work on individual modules

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Software development tales

• The mythical man month
• There is no silver bullet
• It isnt a bug, its a feature.
• Dont break the build
• If specifications are wrong
• Machines behave differently. Same implementation
  on different machines can be different
• IE vs Netscape
• Side effects can be serious
• If it aint broke, dont fix it
• Remember backwards compatibility
• Think ahead to forward compatibility