Program Design (2)

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Program Design (2)
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Lecture Overview

• Have seen the initial part of
• Top down design
• Modular design
• Now
• Parameter Passing
• Stack and Local Variables
• Structured Programming

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Parameter Passing to Modules

• Modules (subroutines) should have no side effects
• Receive (access) only the data passed to it
• Modify and store/update only parameters in its
  formal output list
• Some subroutines trigger an event such as
  sounding an alarm, running a pump, etc.

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Parameter passing example

• Subroutine PUT_CHAR displays a single character
  on the monitor.
• Main program action
• Push character on stack
• Call subroutine BSR PUT_CHAR
• PUT_CHAR knows the location, and as only 1 byte,
  the data might be passed in a data register
• More formal method places the data on the stack
• It can use 4(A7) or 6(A7) to access the character.

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Parameter passing in Registers

• Works when the quantity of data is small or the
  processor has many, many registers.
• Passing arguments in registers is code that is
  position independent and reentrant.
• Position independent as no absolute address
  location involved in data transfer
• Reentrant as the registers would be saved before
  they are reused.

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Mechanisms for Parameter Passing

• Ways in which parameters are passed.
• Pass-by-value The current value of the variable
  at the time the subroutine call is made, is
  passed to the subroutine
• Pass-by-reference The address of the variable is
  passed to the subroutine, i.e., a pointer. This
  allows modification of the variable back in the
  calling routine, by the subroutine.
• Mechanisms
• Registers - Memory Location - Stack

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Pass by Reference

• Subroutine to search a region of memory
• Text in memory
• Want to check for sequence of characters
• Info needed by subroutine
• Starting and ending address of text
• Start and ending address of the string
• Subroutine receives addresses when called
• Information in call what string to find
• Note Code is not reentrant and cannot be used
  by an interrupt routine

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Code example

• For using the stack to pass by reference.

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Figures for example

• Memory contents and The stack

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Parameter Passing on the Stack

• Can use the stack to transfer the actual data
• Can be used to pass the addresses for
  pass-by-reference

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Figures for this example

• Fig 3.6

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Stack and Local Variables

• Subroutines need access to space for local
  variables
• Possible that they fit in register but may not
• Allocate local storage space on stack.
• Keeps the subroutine reentrant
• Also bind the variables not only to the
  subroutine, but to the specific invocation

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Stack frame

• Allocation space on the stack
• LEA -200(SP),SP
• To restore stack prior to return
• LEA 200(SP),SP
• Refer to Fig 3.6 of text.

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Simplification

• LINK and UNLINK
• A instruction to specifically do this
• MOVE.L D0-D7,/A3-A6, -SP
• Save the specified registers on the stack
• LINK A1, -64
• Allocate 64 bytes (16 longwords) of on this stack
  frame
• Start subroutine

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Figures
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Subroutine example

• Calculate R(P2 Q2)/(P2 Q2)

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Link Example

• The stack

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Structured Programming

• A semiformal method of program development
• 3 benefits
• Inproves programmer productivity
• Program is easier to read
• Program is more reliable

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Three fundamental components

• These are the basic elements of all high level
  functional programming languages.
• Sequence
• Action A, then action B, then action C, etc.
• A looping mechanism
• Finite loop a set number of iterations
• Conditional loop exit when condition occurs
• A decision mechanism
• If (condition) THEN action A ELSE action B

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GO TO????????????????????

• All algorithms can be programmed using structured
  programming techniques without using a GO TO!!!

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Assignment 4

• Problem 2-62