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Assembler directives in ARM programming

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• Dictactic disclaimer
  • CS255 is not a course where you primarily learn assembler programming

    The purpose of the CS255 course is:

    In CS255, I use assembler programming to reveal the inner workings of high level programming constructs I.e.: I let you look "under the hood" and show you what if-statements, while-statements, method calls, etc. are actually doing.

    Therefore:

    I will only discuss a subset of the assembler directives You will learn enough about assembler directives to understand what they do to support high level programming features Also, by limiting to a useful subset, I don't waste time teaching the other directives that are not often used.

    BTW, the complete list of GNU assembler directives are in this online document: click here

    You can see it's a very long list and I don't have to time to cover it all !!!

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• The "text" and "data" segments of a computer program
  • A computer program consists in general of 2 parts:

    A part that contains computer instructions In assembler programming, the program instruction part is called: the text segment A part that contains data (constants and variables) In assembler programming, the program instruction part is called: the data segment

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• .text
  • I'll start the discussion with the easier assembler directive first.

  • Syntax:

    .text // Tell the assmbler to put the lines that follows .text in the text segment // I.e.: the lines that follows .text will considered as assembler instructions

    Example: see the hello.s program in the webpage: click here

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• .data

  • Syntax:

    .data // Tell the assmbler to put the lines that follows .data in the data segment // I.e.: treat these lines as variable definitions

    Example: see the string data defined in after .data in the hello.s program in the webpage: click here

    Note:

    I will discuss how to define different kinds of variables in the next webpage

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• Identifiers in assembler programming
  • Identifiers in assembler programming are names used represent a value

  • The syntax of an identifier in assembler programming is similar to that in Java:

    Identifier must start with a letter or an underscore Then it can be followed by a letter, digit or underscore Identifiers are case-sensitive !!!

    Examples of identifiers:

    radius pi X x _X

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• Labels
  • Label:

    An identifier that is used to identify a location in memory is called a label

    As you recall, a location in memory is uniquely identified by its memory address

    Therefore, a label is actually a symbolic name for some memory address

    I.e.: a label is an identifier that is used to represent some memory address

    We will learn about labels in the next webpage.

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• Notations used
  • In the explanation of the directives, I used the following notations:

    [ stuff ] - the stuff inside the square brackets [ ] are optional stuff ... - means: the stuff may be repeated

    You will see in the syntax examples in each directive hwo I use these notations

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• .global
  • Syntax:

    .global identifier [, identifer ... ] Examples: .global main .global main, radius, pi

    The .global directive makes an identifier accessible to the "loader" program ld that combine multiple program files into one executable program

    You don't need to know the details how the ld (loader) program work, you only need to know the effect of the .global directive:

    An identifier that has been declared by .global is visible (= accessible) by EGTAPI I.e.: EGTAPI can access the identifier

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  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255001/demo/1-directives/global.s

    How to run the program:

    Get a copy of this program into your cs255 directory first To compile:   as255 global To run:          use EGTAPI

    Type x into the Variable Name area.

    Select SI (signed integer)

    Then click Add.

    The value in the variable x will be be displayed in EGTAPI (you should see 4444)

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• .end
  • Syntax:

    .end

    Tells the assembler to stop.

    All text that follows the .end directive will be ignored by the assembler