programming(1).pdf

EMCH 367

Fundamentals of Microcontrollers

367pck S01.doc

PROGRAMMING THE MICROCONTROLLER

ASSEMBLY LANGUAGE

Assembly language is of higher level than machine language and hence easier to use.

An assembly language code consists of

a) Program statement lines

b) Comment lines

A program statement is a line that contains 4 fields in the following format:

[<LABEL>]

[<OPCODE MNEMONIC>]

[<OPERANDS>]

[;<comments>]

[<LABEL>]

[<DIRECTIVE MNEMONIC>]

[<OPERANDS>]

[;<comments>]

where [ ] indicates an optional field that may not be always required. The fields are separated by a tab

or space. (Tab is recommended, since it ensures an orderly appearance to your code. For the same

reason, when a field is not used, the tab or blank should still to be used, such that the fields of the same

type stay aligned in same columns.) When writing <LABEL>, <OPCODE MNEMONIC> or <DIRECTIVE

MNEMONIC>, and <OPERANDS>, use upper case characters. When writing <comments>, use lower

case.

The <OPCODE MNEMONICS> correspond to the microcontroller opcodes. These mnemonics are

found in the Motorola MC68HC11 programming reference guide and related literature.

The <DIRECTIVE MNEMONICS> are native to the Assembly language. A list of directives is given in

Table 1. The directives that you will use often are shown in bold .

Table 1

Assembler directives

Name of Assembler directive

what it does

Alias for

END

end program

define bytes

FCB

define words

FDB

define storage

RMB

EQU

equate

FCB

form constant byte

FCC

form constant characters

FDB

form double bytes

ORG

set origin

RMB

reserve memory bytes

#INCLUDE

include source file

$INCLUDE

include source file

#INCLUDE

The <OPERAND> contains a value, an expression, an address, or a label that the opcodes or the

directives need. The operand could be up to 4 bytes long, separated by commas. Some opcodes or

directives do not require operands (inherent mode).

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EMCH 367

Fundamentals of Microcontrollers

367pck S01.doc

The constants used in the <OPERAND> can be hex, decimal, binary, or octal numbers. Table 2 gives

the assembler symbols used to this purpose.

Table 2

Assembler symbols for constants

Symbol

Meaning

Example

$<number>

hex number

$A1

decimal number

%<number>

binary number

%11001010

@<number>

octal number

@73

‘<string>’, ‘<string>

ASCII string

‘A’ or ‘A (the latter does not work with #INCLUDE)

The expressions used in the <OPERAND> can use any of the operators listed in Table 3

Table 3 Assembler symbols for expressions

Symbol

Meaning

Example

unary minus

-4

binary AND

%11111111&%10000000

binary OR

%11111111!%10000000

multiplication

3 * $2A

division

$7E/3

addition

1+2

subtraction

3-1

( )

parentheses used for grouping

3 * (1+2)

Important conventions used in the <OPERAND> are given in Table 4:

Table 4 Other important conventions

Symbol

Meaning

Example

immediate mode (IMM)

#$A3

;

start of comment line and of comment inside

a program statement

LDAA #$FF ; Load accA

alternate sign for start of comment line only

* This is a comment

, X

index X mode (IND,X)

LDAA TFLG1,X

, Y

index X mode (IND,Y)

LDAA TFLG2,Y

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EMCH 367

Fundamentals of Microcontrollers

367pck S01.doc

The <LABEL> is a very powerful concept that can greatly simplify the programmer’s task. The

<LABEL> consists of a string of alphanumeric characters that make up a name somehow meaningful to

the programmer. The placement of the <LABEL> can be in one of the following positions:

a) In the first column and terminates with a tab or blank character

b) In any column and terminates with a colon (:)

There are 3 different usages of the <LABEL>:

1) To assign the name inserted in the <LABEL> to a location in a program. The <LABEL>

will be assigned the address of that location

2) To assign the value of an expression or constant to the name inserted in the <LABEL>

using the EQU (equate) or SET directives.

3) To define the name of a subroutine (macro). Essentially, this is the same as 1), since an

address (the subroutine starting address) is assigned to the label.

When labels are assigned to certain addresses, one can tell the program to go to that address by

referring to the label (case 1 above). Alternatively, one can use the contents of a certain address by

referring to its label, just like when using variables (case 2 above).

A comment is prefixed by semicolon (;).When the assembler detects an semicolon, it knows that the

rest of the line is a comment and does not expect any executable instructions from it. A comment can

be a separate line (comment line) or can be inserted in a program statement. A comment line can be

also prefixed by an asterisk ( * ). The comments, either in the comment field or as a separate comment

line, are of great benefit to the programmer in debugging, maintaining, or upgrading a program. A

comment should be brief and specific, and not just reiterate its operation. A comment that does not

convey any new information needs not be inserted. When writing a comment, use lower case

characters.

A program written in Assembly language is called source file . Its extension is .ASM. When the source

file is assembled, two files are generated:

a) Object file that can be run in the microcontroller. The Motorola object file is in ASCII-HEX

format. Its generic name is “S19 file’. Its extension is .S19

b) List file, extension .LST, that contains the original code in Assembly language and the

corresponding hex codes resulting from the Assembly process. The list file is used by the

programmer to verify and debug his/her coding of the program.

The .ASM files can be opened, viewed, edited and saved in the THRSIM11 application. Alternatively, all

three file types (.ASM, .LST, .S19) can be also processed in a text editor, e.g., the Notepad application.

Examples of .ASM and .LST files follow.

Addressing Modes

Inherent Mode is implied and requires no programming action.

Immediate Mode means that the number contained in the operand will be immediately used.

Direct and Extended Modes use the number contained in the operand to signify an address where the

required information should be retrieved from or deposited to. The Extended mode is automatically

used for addresses greater than FF.

Index Mode is used by adding the operand to the value already existing in the Index X or Y, as

selected. In this case, the operand acts as an offset.

Relative Mode uses the operand as an offset relative to the present Program Counter value.

Dr. Victor Giurgiutiu

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EMCH 367

Fundamentals of Microcontrollers

367pck S01.doc

MICROCONTROLLER COMMANDS

(Section 6 and Section A of M68HC11 Reference Manual)

The 6811 microcontroller has 145 different commands. These commands can be grouped into several

categories. The categories and the commands in those categories are listed below:

1) Arithmetic operations:

a) Addition:

ABA, ABX, ABY, ADCA, ADCB, ADDA, ADDB, ADDD, INC, INCA, INCB, INS, INX, INY

b) Subtraction:

SBA, SBCA, SBCB, SUBA, SUBB, SUBD, DEC, DECA, DECB, DES, DEX, DEY

c) Multiplication: MUL

d) Division: FDIV, IDIV

2) Logical operations: (note: logical operations are carried out on a bit by bit basis)

a) Standard logical operations: ANDA, ANDB, EORA, EORB, ORAA, ORAB, COM (Boolean

inverse), COMA, COMB

b) Operations that shift the location of the bits in the register:

ASL, ASLA, ASLB, ASLD, ASR, ASRA, ASRB, LSL, LSLA, LSLB, LSLD, LSR, LSRA, LSRB,

LSRD, ROL, ROLA, ROLB, ROR, RORA, RORB

c) Operations that compare two numbers:

BITA, BITB, CBA, CMPA, CMPB, CPD, CPX, CPY

3) Branching commands: BCC, BCS, BEQ, BGE, BGT, BHI, BHS, BLE, BLO, BLS, BLT, BMI, BNE,

BPL, BRA, BRCLR, BRN, BRSET, BSR, BVC, BVS, JMP, JSR, RTS, RTI, WAI

4) Memory/Register Functions

a) Move data into / out of memory: LDAA, LDAB, LDD, LDS, LDX, LDY, STAA, STAB, STD, STS,

STX, STY

b) Change the values in memory/registers: BCLR, BSET, CLC, CLI, CLR, CLRA, CLRB, CLV,

COM, COMA, COMB, NEG, NEGA, NEGB, SEC, SEI, SEV

c) Transfer data from one register to another: TAB, TAP, TBA, TPA, TSX, TSY, TXS, TYS,

XGDX, XGDY

Stack Pointer Functions: PSHA, PSHB, PSHX, PSHY, PULA, PULB, PULX, PULY

Misc.: NOP, SWI

Note: Boolean inversion commands: COM, COMA, COMB

Dr. Victor Giurgiutiu

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EMCH 367

Fundamentals of Microcontrollers

367pck S01.doc

SAMPLE PROGRAM IN ASSEMBLY LANGUAGE WITH MCU COMMANDS

P ROBLEM S TATEMENT

This simple program is an example of addition. It performs the operation:

VAR0 + VAR1 ã SUM

In addition, the program checks if an overflow happened during the addition process, and sets the flag

OVERFL accordingly.

P ROGRAM D ESCRIPTION

•

The variables are defined in lower memory starting with $0000, in the order VAR0, VAR1, SUM,

OVERFL.

•

LDAB with zero is used to reset the initial value of the overflow flag (optimistic!).

•

LDAA is used to load VAR0 into AccA

ADDA is used to add accA with VAR1. Result of addition stays in accA

•

BVC is used to branch over the next instruction, i.e. to LABEL1, if no overflow occurred

•

If an overflow occurred during the addition process, this instruction is reached and COMB is

used to invert accB from $00 to $FF.

•

Label1: STAA is used to store the result of addition from accA into SUM

•

STAB is used to store accB ($00 or $FF, depending on the logic just discussed) into the

overflow flag OVERFL

F LOWCHART

FLOWCHART

Initialize variables:

VAR0

$0000

VAR1

$0001

SUM

$0002

OVERFL ã

$0003

Load $00 into accB as the initial (optimistic)

guess for the overflow status

Load first variable into accA

Add second variable to accA

(result stay in accA)

Brach if overflow bit

is clear

Since overflow bit was not clear, Invert accB

LABEL1

Store result of addition from accA into SUM

Store current value of overflow flag from

accB into OVERFL

SWI

Dr. Victor Giurgiutiu

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