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Program Structure and Design. ELEC 330 Digital Systems Engineering Dr. Ron Hayne. Software Facts. Software Cost Expensive to develop Volume reduces unit cost Software (and documentation) Quality Often poor Inadequate testing (debugging) Programming Difficult Tedious. - PowerPoint PPT Presentation
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Program Structure and Design
ELEC 330
Digital Systems Engineering
Dr. Ron Hayne
330_04 2
Software Facts
Software Cost Expensive to develop Volume reduces unit cost
Software (and documentation) Quality Often poor Inadequate testing (debugging)
Programming Difficult Tedious
330_04 3
Program Design
Programmers' Goals Write the shortest
program Write the fastest
program Write an easily
understood program Write and easily
modified program Meet the schedule
Market Conditions Microprocessor speed
increases Chip cost reductions Memory size increases Secondary storage
evolution
330_04 4
Goals vs. Reality
Write the shortest program Tricks usually lead to problems
Write the fastest program Profile and optimize parts that make a difference
Write an easily understood program Simple straightforward problem solutions
Write an easily modified program Modifications happen frequently
Meet the schedule Good software delivered late is often not profitable
330_04 5
Practical Programming
Don't use a single resource for multiple purposes Use no intimate knowledge of hardware configuration Keep the instructions and data separated Use tables to collect program parameters and data Only put constant data within instructions Avoid tricks
Clever use of instructions not easy to alter
Don't write self-modifying programs Use the instructions in the instruction set well
330_04 6
Action Symbol
Begin / End
Process
Decision
Connector
Flowchart Symbols
330_04 7
Example Flowchart
330_04 8
Structured Programming
Fundamental Program Structures SEQUENCE IF-THEN-ELSE DO-WHILE
Extended Program Structures DO-UNTIL IN-CASE-OF
330_04 9
SEQUENCE Structure
330_04 10
IF-THEN-ELSE Structure
330_04 11
IF-THEN-ELSE ExamplePRESS RMB 1
WARNING RMB 1
LIMIT FCB 100
* PRESSURE OVER LIMIT?
START LDAA PRESS
CMPA LIMIT
BHI OVER YES
* SET WARNING TO FALSE
CLR WARNING
BRA LAST
* SET WARNING TO TRUE
OVER LDAA #$01
STAA WARNING
LAST
Set warning to false
Pressureover limit?
Set warning to true
330_04 12
DO-WHILE Structure
330_04 13
DO-WHILE Example
* INITIALIZE NONZERO COUNT
START CLR COUNT
* INITIALIZE TABLE POINTER
LDX #TABLE
* TABLE ENTRY ZERO?
AGAIN TST 0,X
BEQ LAST YES
* INCREMENT NONZERO COUNT
INC COUNT
* ADVANCE POINTER
INX
BRA AGAIN
LAST
Table entryzero?
Incrementnonzero count
Initializenonzero count
Initializetable pointer
Advancepointer
No
Yes
330_04 14
DO-UNTIL Structure
330_04 15
IN-CASE-OF Structure
330_04 16
Top Down Design
Module 2
Start
End
Module 1
Task 2
Module 1
End 1
Task 1
Task 3
Module 2
End 2
330_04 17
Top/Down Design
Develop flow charts in order, top first Start coding after all levels are developed
Run program modules as soon as practical Use structured programming in each module Comment as you code, in accordance with flowcharts
Include program structure info in your comments
Keep flowchart symbols at functional level Keep flowcharts independent of hardware No more than ten symbols per flowchart Make flowcharts conform to structured programming
330_04 18
Top/Down Assembly Language
**************************
** Program Name
** Description
**************************
** Data Section
**************************
ORG $10
DATA1 RMB 2 Comment
*
**************************
** Program Section
**************************
ORG $E010
Module 2
Start
End
Module 1
330_04 19
Top/Down Assembly Language
*-------------------------
* Module 1
*-------------------------
* Task 1
* Subtask 1a
START LDD #0000
STD DATA1
* Subtask 1b
LDAA DATA2
* Task 2
*-------------------------
Task 2
Module 1
End 1
Task 1
330_04 20
Top/Down Design Teams
Teams of people write most programs Flowcharts and written materials document design
Interaction between program modules very important
Team members work on different modules Structured programming necessary to ensure
modules will fit together
330_04 21
Top/Down Implementation
Start writing and running the program soon after starting the design Start writing the program even before designing the
lower levels Dummy modules called stubs
Program Testing Running and demonstrating the program containing stubs
checks the design, overall function and initial coding Stubs replaced by actual modules as they are developed
330_04 22
Summary
Program Design Flowcharts Structured Programming Top/Down Design
