Objectives

1
Objectives

• Implement pointers using indexed addressing modes
• Use pointers to access arrays, strings,
  structures, tables, and matrices
• Present finite-state machines as an abstractive
  design methodology
• Describe how the paged memory system allows
  memory sizes above 64KiB
• Present minimally intrusive methods for
  functional debugging

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6.1 Indexed Addressing Modes Used to Implement
Pointers

• 9S12 has 15 addressing modes.
• 9 are related to pointers.
• Assemblypointers are implemented using indexed
  addressing.
• The address is placed into RegisterX or
  RegisterY, then the data is accessed using
  indexed addressing mode.

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Figure 6.2, page 193

• PointersPT, SP, GetPt, PutPt
• Arrows show the memory location to which they
  point.
• Shaded regions represent data.
• Array or string simple structure with multiple,
  equal-sized elementsthe pointer points to the
  first element then the indexed addressing mode is
  used to access the rest of the structure.

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Ch. 6 Pointers and Data Structures

• From the text by Valvano Introduction to
  Embedded Systems Interfacing to the Freescale
  9S12

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Figure 6.2 (cont.)

• The Stack SP points to the top element of the
  stack.
• Linked List contains some data elements that
  are pointers.
• FIFO Queue important for I/O programming.

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6.1.1 Indexed Addressing Mode

• Uses a fixed offset with the 16-bit registers (X,
  Y, SP, and PC).
• The offsets that are possible include
• 5-bits (-16 to 15)
• 9-bits (-256 to 127)
• 16- bits
• Example of 5 bit indexing (5C is index mode
  operand1 byte)
• 6A5C staa -4,Y Y-4 Reg A
• Assume RegA is 56, RegY is 0823
• 0823 -4 081F
• See Figure 6.3

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More Examples of Indexed Addressing Mode

• Example of 9-bit indexed mode requires 2 bytes to
  encode the operand
• 6AE840 staa 40,Y Y40 Reg A
• See Figure 6.4 (note 40 8023 8063)
• Example of 16-bit indexed mode requires 3 bytes
  to encode the operand
• 6AEA2000 staa 200,Y Y 200 RegA
• See Figure 6.5 (note 200 08230A23)

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6.1.2 Auto Pre/Post Decrement/Increment Indexed
Addressing Mode

• The 16-bit pointers are used (X, Y, SP).
• These pointers are modified either before (PRE)
  or after (POST) the access of the memory.
• Used for accessing memory sequentially.
• Example Post-increment addressing.
• Staa 1,Y Store a copy of value in Reg A at
  2345, then Reg Y 2346.
• More examples on the bottom of page 195.

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Other Addressing Modes

• 6.1.3 Accumulator-Offset Indexed Addressing
  modeAccumulator A,B, or D are used with
  registers X, Y, SP, and PC.
• Example staa B,Y
• Efficient for accessing arrays.
• 6.1.4 Indexed-Indirect Addressing Mode
• A fixed offset of 16 bits is used (X, Y, SP or
  PC).
• A second address is fetched the load or store is
  operated on the second address.
• Useful when data structures include pointers (pg.
  196).

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And More Addressing Modes

• 6.1.5 Accumulator D Offset Indexed-Indirect
  Addressing Mode.
• Also useful when data structures includes
  pointers.
• Offset is in D and is added to X, Y, SP, or PC.
• 6.1.6 Post-Byte machine Coded for Indexed
  Addressing
• 6.1.7 Load Effective Address Instructions
• 6.1.8 Call-By-Reference Parameter Passing

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6.2 Arrays

• Random Access
• Read and write data in any order.
• Allowed for all indexed data structures.
• An indexed data structure has elements of the
  same sizeit is accessed knowing the name of the
  structure, the size of the elements and the
  number of elements.

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6.2 (cont.)

• Sequential access
• The elements are read and written in order.
• Examples strings, linked lists, stacks, queues,
  and trees.
• Arrays
• Made of elements of equal precision and allows
  random access.
• Length is the number of elements.
• Origin is the index of the first element.

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Example 6.1

• Write a stepper module to control the read/write
  head of an audio tape recorder.
• Three public functions
• Initialization
• Rotate one step clockwise
• Rotate one step counterclockwise
• Stepper motor has four digital control lines
• To make it spin, output 5, 6, 10, and 9 over and
  over.
• To make it spin in the opposite direction, output
  the values in reverse order.
• Figure 6.9 illustrates the byte array.
• Program 6.1 shows the code (fcb means form
  constant byte)

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Example 6.2

• Design an exponential function y10x, with a
  16-bit output.
• X is an input from 0 to 4.
• Figure 6.10 illustrates a word array
• Program 6.2 (page 201) illustrates the use of the
  array (fdb means form double byte and is used to
  define word constants.)

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Termination Codes

• Termination codes can be used to handle variable
  length.
• Table 6.3 illustrates typical termination codes.
• Program 6.3 illustrates the use of the null
  termination code.
• Program 6.4 illustrates a pointer method to
  access the array.

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6.3 Strings

• A data structure with equal-sized elements that
  are only accessed sequentially.
• The length of the string is stored in the first
  position, when data can take on any value thus
  termination codes are not needed.

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Example 6.3

• Example 6.3 (page 203)
• Write software to output a sequence of values to
  a digital to analog converter.
• The length is stored in the first byte
• Data1 fcb 4 length
• fcb 0,50,100,50 data
• Data2 fcb 8
• fcb 0,25,50,75,100,75,50,2
  5

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Example 6.3 (cont.)

• The DAC is connected to Port T.
• The function (subroutine) DAC outputs the string
  data to the digital to analog converter.
• DAC ldab 0,x length
• loop inx next element
• ldaa 0,x data
• staa PTT out to the dig/analog
  conv
• decb
• bne loop
• rts

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Example 6.3 (cont.)

• main lds 4000
• movb FF, DDRT
• mloop ldx Data1 first string
• bsr DAC
• ldx Data2 second string
• bra mloop

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Example 6.4

• Write software to output a ASCII string to the
  serial port.
• Call by reference parameter passing is useda
  pointer to the string will be passed.
• See Program 6.6, page 204.

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6.4 Matrices

• Two dimensional data structure accessed by rows
  and columns.
• Figure 6.11 illustrates row major and column
  major.
• Program 6.8 illustrates a function to access a
  two by three column-major matrix. (page 205)

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Example 6.5

• Develop a set of driver functions to manipulate a
  32 by 12 graphics display.
• Bit arrays are used to store pixel values.
• 0 represents a blank, 1 turns on a pixel.
• Program 6.11, 6.12 and 6.13 illustrate the
  access, modification, and reading of the bit
  matrix.

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Structures

• A structure has elements with different types
  and/or precisions.
• Program 6.14 is an assembly language example of a
  structure.

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Tables

• A table is a collection of identically sized
  structures.
• Fig. 6.14 shows a table made up of a simple data
  base.
• Program 6.16 (page 211)

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Trees

• A graph is a general linked structure without
  limitations.