Transcript 330_04
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 330_04 2 Program Design Programmers' Goals Market Conditions Write the shortest program Write the fastest program Write an easily understood program Write and easily modified program Meet the schedule 330_04 Microprocessor speed increases Chip cost reductions Memory size increases Secondary storage evolution 3 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 4 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 5 Flowchart Symbols Action Symbol Begin / End Process Decision Connector 330_04 6 Example Flowchart 330_04 7 Structured Programming Fundamental Program Structures SEQUENCE IF-THEN-ELSE DO-WHILE Extended Program Structures DO-UNTIL IN-CASE-OF 330_04 8 SEQUENCE Structure 330_04 9 IF-THEN-ELSE Structure 330_04 10 IF-THEN-ELSE Example Pressure over limit? Set warning to false Set warning to true PRESS 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 330_04 LAST 11 DO-WHILE Structure 330_04 12 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 Initialize nonzero count Initialize table pointer Table entry Yes zero? No Increment nonzero count Advance pointer 330_04 13 DO-UNTIL Structure 330_04 14 IN-CASE-OF Structure 330_04 15 Top Down Design Start Module 1 Module 1 Module 2 Task 1 Task 2 Module 2 Task 3 End 1 End End 2 330_04 16 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 17 Top/Down Assembly Language Start Module 1 Module 2 End ************************** ** Program Name ** Description ************************** ** Data Section ************************** ORG $10 DATA1 RMB 2 Comment * ************************** ** Program Section ************************** ORG $E010 330_04 18 Top/Down Assembly Language Module 1 Task 1 Task 2 End 1 *------------------------* Module 1 *------------------------* Task 1 * Subtask 1a START LDD #0000 STD DATA1 * Subtask 1b LDAA DATA2 * Task 2 *------------------------- 330_04 19 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 20 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 21 Summary Program Design Flowcharts Structured Programming Top/Down Design 330_04 22