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

6.1 Indexed Addressing Modes Used to Implement Pointers

• 9S12 has 15 addressing modes.

• 9 are related to pointers.

• Assembly—pointers are implemented using indexed addressing.

• The address is placed into RegisterX or RegisterY, then the data is accessed using indexed addressing mode.

Figure 6.2, page 193

• Pointers—PT, 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 elements—the pointer points to the first element then the indexed addressing mode is used to access the rest of the structure.

Ch. 6 Pointers and Data Structures

From the text by Valvano: Introduction to Embedded Systems:

Interfacing to the Freescale 9S12

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.

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 operand—1 byte)– $6A5C staa -4,Y ; [Y-4] = Reg A– Assume RegA is $56, RegY is $0823– $0823 -4 = $081F– See Figure 6.3

More Examples of Indexed Addressing Mode

• Example of 9-bit indexed mode requires 2 bytes to encode the operand:– $6AE840 staa $40,Y ; [Y+$40] = 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 + $0823=$0A23)

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.

Other Addressing Modes

• 6.1.3 Accumulator-Offset Indexed Addressing mode—Accumulator 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).

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

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 size—it is accessed knowing the name of the structure, the size of the elements and the number of elements.

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.

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)

Example 6.2

• Design an exponential function y=10x, 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.)

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.

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.

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 0,25,50,75,100,75,50,25

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

Example 6.3 (cont.)

• main lds #$4000

• movb #$FF, DDRT

• mloop ldx #Data1 ;first string

• bsr DAC

• ldx #Data2 ;second string

• bra mloop

Example 6.4

• Write software to output a ASCII string to the serial port.

• Call by reference parameter passing is used—a pointer to the string will be passed.

• See Program 6.6, page 204.

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)

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.

Structures

• A structure has elements with different types and/or precisions.

• Program 6.14 is an assembly language example of a structure.

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)

Trees

• A graph is a general linked structure without limitations.