COCO

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COCO 18Programming Basics

• Programming Pragmatics
• Standard Constructs
• C-like Pseudo-Code
• Program Design
• Programming model

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Build a computer!
Basic Idea The Stored Program Computer Concept
RBUS
An instruction-controlled digital system with a
memory A sequence of instructions are stored in
memory This set of instructions constitute the
program that defines the machines behavior
MBUS
AC
A
B
Memory N bits wide 2M words
Memory
Address
ALU
FSM
MAR
S
ABUS
Opcode
IR
PC
MBUS
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Definitions
Program the sequence of instructions and
associated data values Programmer one who
creates, modifies, or maintains
programs Programming the act of creating
programsInitial design and planning,
documenting, coding into a language, testing, and
debugging Software often used as a synonym for
program
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Cold Hard Facts

• Software is expensive
• program development costs are more than an hour
  of labor per machine instruction
• Quality of much software and documentation is
  poor
• "All programs have at least one bug."
• "Testing proves the presence of bugs, not their
  absence."
• Testing and debugging usually takes more time
  than the original planning and writing.
• Thus, new software is usually completed later
  than scheduled!
• Programming is hard work
• There is no magic to remove the work, only a
  method to work efficiently

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Kelley's 50 Rule
How do you write a program?
THINK
CODE
WRITE
TEST
50
75
87.5
Design the program
Write the program
Document the program
Debug, fix,and retest, redesign as needed
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Realistic Program Goals
What goals do you have when you write a program?

• Write the shortest program
• use the fewest number of bytes of memory
• Write the fastest program
• run as quickly as possible
• Write an easily understood program
• know what to change, fix, or modify
• Write an easily modified program
• cope with changes of environment or hardware
• Meet the schedule
• often at the cost of not meeting the other goals

Less important due to technology advances
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Pragmatic Programming

• Don't use a single item for multiple purposes
• makes debugging difficult
• Use no intimate knowledge of the hardware
  configuration
• 'reserved' bits/bytes might be used in later
  hardware revisions
• Avoid tricks
• avoid the 'clever' use of instructions
• Keep the instructions and data in separate
  sections
• avoids need to jump over data
• Use tables or structures for parameter and
  related data
• group data logically

• Put only constant data within the instructions
• changing data in instructions requires an
  intimate understanding of its function in the
  program
• Don't write self-modifying programs
• can't run in ROM
• difficult to understand, debug, and modify
• Do use the instructions of the instruction set
  well
• use 'special/odd' instructions for certain
  functions

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Flowcharting Symbols
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Standard Constructs
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Constructs ? Pseudo-code
Do Procedure 1 Do Procedure 2
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C ?Pseudo-Assembly Language
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More C ?Assembler
IF-THEN-ELSE IF( A 0) B ELSE B-- ENDIF
Fetch A, Test A, If (A 0) then Fetch
B, Increment B, Go to NEXT else Fetch
B' Decrement B, NEXT Store B
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More Constructs
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IN-CASE-OF Construct
ALL Standard Constructs have a single entry and
exit point
IF Case 1 True Do Procedure 1, Break IF
Case 2 True Do Procedure 2, Break
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Structured Loops
Unstructured premature loop termination
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More Loops
Unstructured premature loop termination
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Top-Down Design
An old and simple idea with a new name

• Divide and conquer
• Level 1
• Identify the major functional parts, or modules
• Level 2
• Next each module is broken into smaller parts. .
  .
• Level N1
• This process is continued until the parts are
  known in enough detail to write and document the
  original program

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Top-Down Implementation
An outside-in approach to integration

• Starts with the root of the top-down tree
• Implements the top level infrastructure first
• Integrates next level modules one at a time
• Requires stubs for unimplemented modules
• and so on for each module
• implement infrastructure then module-by-module
  integration with stubs until the bottom level is
  reached

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Bottom-Up Implementation
An assembly-line approach to integration

• Starts with the leaves of the top-down tree
• Implements the bottom level modules first
• Requires test programs for each modules
• Integrates fully tested modules to form higher
  level module
• Debugs the integration components
• and so on for each module
• integrate and debug with test programs until
  finally the complete, integrated program is
  working

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Programming Model (1)
Tri-Bus Computer
What does a programmer need to know to program
the computer?
Programming Model
1. Memory Model
2. Registers
3. Instruction Set
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Programming Model (2)
Memory Model
Number Notation denotes hexadecimal q 00 ..
11 h 0000 .. 1111
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Programming Model (3)
Registers
What registers do we need?
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Programming Model (4)
Instruction Set
Through the bus
How can we do this?
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Programming Model (5)
Instruction Format
16-bit Instruction
How many bits are needed for the OpCode? 2 How
many bits are needed to access any memory
location? 14
2-bit OpCode
14-bit Operand