Program Design Two - Microcomputer Structures - Lecture Slides, Slides of Microcomputers

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Program Design (2)

Lecture Overview

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

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.

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.

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

Code example

• For using the stack to pass by reference.

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

Figures for this example

• Fig 3.

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.

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, #-
    • Allocate 64 bytes (16 longwords) of on this stack frame
  • Start subroutine

Subroutine example

• Calculate R=(P 2 + Q 2 )/(P 2 – Q 2 )

Link Example

• The stack

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

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

• All algorithms can be programmed using

structured programming techniques

without using a GO TO!!!