Understanding Assembler

Assignment

• Using the information below determine which basic element of the assembly language is used in each line of the code.
• Students with an interest can check the notes on the code revisited page

Basic elements of assembly language are:

• Labels
• Instructions
• Operands
• Directives
• Comments

Labels

• Identifies a line in a program that the program can "goto"
• are text based, must start with a letter of the alphabet or a "_", length may be up to 32 characters.
• A label is placed in the first column of the programming code.

Instructions

• are defined for a specific microcontroller.
• A list of all instructions for the PIC 16F628A are listed in the microcontrollers data sheet.
• Note: the 14 bit op codes are the actual machine language that is associated with the assembler instruction. When you see the hex codes in ic-prog, these are the machine language. Note that the codes are 3FFF or in binary B'11111111111111' for all the unused address program codes. Using the chart below you could decipher many of the machine codes back into assembler.

Operands

• are the registers, variables or constants used by the instructions in the program.

Directives

• are like instructions but they are not specific to the microcontroller.
• A complete list of assembler directives ( and link to describing their function and examples) can be found in MPLAB under the Help menu > MPASM Assembler Help > Reference, Directive Summary > By Alphabetical Listing or By Type
• Here is a list of many of the directives we will likely use. (They are mostly the control and conditional type)

Directive	Description	Syntax	Type
CONSTANT	Declare Symbol Constant	constant <label> [= <expr>,...,<label> [= <expr>] ]	CONTROL
#DEFINE	Define a Text Substitution Label	#define <name> [[(<arg>,...,<arg>)]<value>]	CONTROL
END	End Program Block	end	CONTROL
EQU	Define an Assembly Constant	<label> equ <expr>	CONTROL
#INCLUDE	Include Additional Source File	include <<include_file>> include "<include_file>"	CONTROL
ORG	Set Program Origin	<label> org <expr>	CONTROL
PROCESSOR	Set Processor Type	processor <processsor_type>	CONTROL
RADIX	Specify Default Radix	radix <default_radix>	CONTROL
SET	Define an Assembler Variable	<label> set <expr>	CONTROL
#UNDEFINE	Delete a Substitution Label	#undefine <label>	CONTROL
VARIABLE	Declare Symbol Variable	variable <label> [= <expr>,..., <label> [= <expr>] ]	CONTROL
ELSE	Begin Alternative Assembly Block to IF	else	CONDITIONAL
ENDIF	End Conditional Assembly Block	endif	CONDITIONAL
ENDW	End a While Loop	endw	CONDITIONAL
IF	Begin Conditionally Assembled Code Block	if <expr>	CONDITIONAL
IFDEF	Execute If Symbol is Defined	ifdef <label>	CONDITIONAL
IFNDEF	Execute If Symbol is Not Defined	ifndef <label>	CONDITIONAL
WHILE	Perform Loop While Condition is True	while <expr>	CONDITIONAL
_ _CONFIG	Set configuration fuses	_ _config <expr> OR _ _config <addr>, <expr>	DATA

Comments

• are ignored by the assembler but are put into the code to make the reading of the code by programmers more clear and legible.
• are written after a ";" .(anything after the ";" is ignored by the assembler)
• can be a whole line or placed after an instruction.

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  More information about assembly programming can be found at http://www.mikroe.com/en/books/picbook/3_chapter.htm