CS101 Lecture 10

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Lecture 10
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What will I learn in this lecture?

• What is the difference between machine
• code and C?
• Why use C?
• How do execution of C and Matlab
• programs differ?
• What are three methods of expressing a
• program algorithm?
• Related Chapters FER Chapters 1 2

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Digital Computer Hardware
Input Devices
instr1
0 - 3
Control Unit (CU)
4 - 7
instr2
.
.
.
.
.
.
Arithmetic -Logic Unit (ALU)
Auxiliary Storage

100 -107
101.101
108 -109
-75
110 -121
label
Output Devices
CPU
Main Memory(RAM)
Memory Addresses
System Unit
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Digital Computer Hardware

• Main (Internal) Memory
• All data and instructions are stored in main
  memory as a sequence of 0s and 1s called Bits
  (Binary Digits)
• Byte (smallest addressable element of memory)
  Storage size for one character of information
  (ASCII-8 8 bits). Every (single) address refers
  to a byte in memory.
• 210 bytes is defined as 1 kilo-byte (1KB)220
  bytes is defined as 1 mega-byte (1MB)230 bytes
  is defined as 1 giga-byte (1GB)

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Digital Computer Hardware

• Central Processing Unit (CPU)
• Transfers information into, out of, and between
  memory locations.
• Executes instructions stored in memory.
• Set of instructions for a CPU is known as a
  machine language.
• Each CPU (Pentium III, Power PC, ...) has its own
  specific machine language.

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Machine Cycle
Instruction Cycle
Control Unit
Main Memory
1
Fetch
Decode
2
Execution Cycle
CPU
RAM
3
Execute
Store
4
Arithmetic/Logic Unit
Includes Cache (very fast memory)
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Programming Languages

• Classified as
• Low Level
• Machine Language
• (binary-based code machine dependent)
• Assembly Language
• (mnemonic form of machine language)

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Programming Languages

• High Level
• Closer to natural languages.
• Generally, machine independent
• Usually, several machine instructions are
  combined into one high-level instruction.
• Examples
• FORTRAN COBOL BASIC Java
• Pascal Ada PL/I Lisp
• C GPSS C Matlab

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Language Comparison
To illustrate differences in syntax for language
levels, consider how a computer could be
instructed to subtract two numbers stored in
memory address locations 63 and 2047 and put the
result in location 2047
Variable Names
increment
63
Memory Addresses
value
2047
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Language Comparison
Variable Names
increment
63
Memory Addresses
value
2047
Machine Language 01111100000011111100111111111
1
(6-bit OpCode, 12-bit address fields with values
63 and 2047)
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Language Comparison
Variable Names
increment
63
Memory Addresses
value
2047
Assembly Language S increment,value
C Language value value - increment
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Processing a High-Level Language Program

• Programs written in high-level languages must be
  converted to machine language.
• Two approaches
• (1) Compilation (see p. 28 FER Figure 2.4) Used
  with C, C, Fortran,...(2) Interpretation Used
  with Matlab, Visual Basic,...

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Compilation
Step 1) Use Xemacs to create a source file.We
will name source files with a suffix .c
Step 2) Run the gcc compiler on the source file
to create anexecutable or object file with the
default name a.out . For CS101 we will only
create executable files.
Step 3) Check to see if gcc caught any syntax
errors , if so go back to step 1)
Step 4) Run the program by typinggt a.outat the
Unix prompt
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Problem Solving and C
Computer programming is the art/science of
transforming a real world problem into a finite
sequence of instructions(statements) in a
computer language. The method stated in the
following slide will be used throughout the rest
of the semester. Follow this method in your Labs
and for any Machine Problem.
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Software Development Method
(these vary just slightly from the method
described on p.37 in FER) 1. Requirements
Specification (Problem Definition) 2.
Analysis---Refine, Generalize, Decompose the
problem definition (i.e., identify sub-problems,
I/O, etc.) 3. Design---Develop Algorithm (process
ing steps to solve problem)Use one of the
following Natural-Language Algorithm Flowchart
Algorithm Pseudo-code Algorithm 4.
Implementation --- Write the "Program" (Code) 5.
Verification and Testing --- Test and Debug the
Code
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C Program Example
1. Requirements Specification (Problem
Definition)Given a light-bulb with a
pre-measured power consumption(in watts), compute
the resistance (in ohms) of the bulb. 2.
Analysis---Refine, Generalize, Decompose the
problem definition (i.e., identify sub-problems,
I/O, etc.) Input real number representing
power Outputreal number representing
resistance
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C Program Example
3. Design---Develop Algorithm Natural-Language
Algorithm Prompt user for the power
dissipation Read power Store value in storage
location called power. Compute the resistance
solving the formula power (voltagevoltage)/re
sistance in terms of resistance. resistance
(voltage voltage)/ power Print out the value
stored in location resistance.
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C Program Example

• 3. Design---Develop Algorithm
• Pseudo-code Algorithm
• print enter power in watts
• read power
• resistance (117.0 117.0)/power
• print resistance

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• 3. Design---Develop Algorithm
• Flowchart Algorithm

start
Output enter power in watts
Flow is assumed down unless otherwise specified
with an arrow. Trapezoid used to designate
I/O. Rectangle used to designate one or more
statements in a block. Circle used as
continuation symbol for transfer to another page.
Input power
Compute resistance(117117)/power
Outputresistance
stop
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C Program Example

• 4. Implementation --- Write the "Program" (Code)
  (see the next slide)

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C Code Implementation of the Algorithm
/ C Program to compute the resistance / / of
a light-bulb./ include ltstdio.hgt define VAC
117.0 void main(void) / Declare
variables. / float power, resistance /
request user input power of / / light-bulb
in watts. / printf(Please enter power(watts)
) / read value power / scanf("f",
power) / Compute resistance assuming VAC
117. / resistance (VAC VAC) /power /
Output the calculated resistance. /
printf(Resistance is f (ohms)\n", resistance)
(Note indentation scheme in above code.)
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Open Xemacs and enter your code..
C program compilation
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C program compilation
Click the Save button in Xemacs to save your
code but dont close the Xemacs window. Click
the xterm button to open another window.
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C program compilation
Use the gcc program to compile your C source
code in the file resistance.c. gt
ls resistance.c resistance.c Note backup
files begin and/or end with a or a symbol.
Do not edit or compile these files! gt gcc
resistance.c The gcc program will not alter
the file resistance.c. The output of gcc is
by default contained in the file a.out. The
file a.out is called an executable file.
(continued on next slide)
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C program compilation
Note that the gcc program noted an error on
line 16 in the program and gcc generated a
warning for line 7. Also, note that there is no
a.out file listed. Errors cause gcc to abort and
you will not get an a.out file. Go back to
Xemacs line 16 and fix the syntax error.
(continued on next slide)
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The error on line 16 was actually caused by a
missing semicolon on line 14. In C a semicolon
means end of statement. This differs from
Matlab where a semicolon means suppress output
and is optional. Since there was no semicolon
after line 14, C assumed that lines 14 - 16
represented one C statement. That is incorrect
syntactically, so gcc (the compiler program or
compiler for short) generated an error message
and terminated before producing an a.out file.

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C program compilation
After adding a semicolon to the end of line 14,
Click Save in Xemacs and go back to the xterm
and type again... Note The dot-slash in
./a.out means in this working directory.
If you type a.out at the Unix prompt Unix
will first search your home directory for a file
a.out . Of course this would not be the same
a.out file you created in another directory.
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What have I learned in this lecture?
A CPU only executes machine code which consists
of instructions that are specific to a particular
manufacturer. The machine code is a sequence of
ones and zeros. The C language is more
human-language-like than machine code and can be
converted to machine code by using a compiler
(gcc in CS101). C code is more portable than
machine code. In the design and development of
an algorithm we can use any combination of
Natural Language, Pseudo-code and
Flow-chart . Matlab code is executed by means
of an interpreter that is running in the Matlab
environment. Each time the Matlab code runs
the interpreter converts the code into machine
code. C code is compiled into an executable
file (machine code) once. We then run the
program (default name a.out) by typing the name
of the executable file (a.out)_at the Unix
prompt.