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The "Hello World" program in ARM assembler language

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• The famous "Hello World" program
  • In CS170, you may have seen the famous "Hello World" program in Java:

    public class Hello { public static void main(String[] args) { System.out.println("Hello World"); } }

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  • Here is the Hello World program written in ARM assembler code:

    // // hello.s: hello world in ARM assembler // // How to compile: // // as -o hello.o hello.s // gcc -o hello hello.o // .global main // ************************ Starting variable definition ***************** .data // *********************************************************************** HelloStr: // Label marking this location in memory .asciz "Hello World\n" // ASCII codes for the string // ************************ Starting computer instruction **************** .text // *********************************************************************** main: push {lr} // Save the return address on the stack push {fp} // Save the frame pointer on the stack // Explained later in CS255 /* ---------------------------------------- Pass the string to printf function ---------------------------------------- */ movw r0, #:lower16:HelloStr movt r0, #:upper16:HelloStr /* ---------------------------------------- Call the printf function ---------------------------------------- */ bl printf // Call printf function with input string pop {fp} // Pop the frame pointer pop {pc} // Pop the return address into PC // Explained later in CS255 .end Any text that follws ".end" is ignored (treated as comment)

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  • In order to compile and run this ARM assembler program, you need a computer with an ARM processor

    We have a machine called cs255host1 in the CS department that is running on an ARM processor to support this course

  • Example Program: (Demo above code)                                                
    • Prog file: /home/cs255/cs255/hello/hello.s on cs255host1

    How to run the program:

    ssh -X cs255@cs255host1 (passwd: abc123) cd /home/cs255/cs255/hello as -o hello.o hello.s gcc -o hello hello.o hello

    Output:

    cs255@cs255host1(314)> hello Hello World

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• Anatomy of an assembler program
  • Recall how a computer program is organized (stored) inside the computer memory:

    (See: click here )

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  • When a program begins execution, the heap and stack area are empty; and you only find these program parts in the memory:

    Notice that the assembler program contain the key words:

    .text - this keyword announces that program instructions will follow .data - this keyword announces that constant and variables will follow

    The instructions that follows the keyword .text will be stored in the program section (green area)

    The data and variable definitions that follows the keyword .data will be stored in the class variable section (yellow area)

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• The program instructions in the "Hello World" program
  • There is only one statement in the Java "Hello World" program:

    System.out.println("Hello World");

    This statement is a method (function) call statement where:

    The function is println The parameter of the function is the string "Hello World"

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  • The program instruction section in the assembler program is:

    // ************************ Starting computer instruction **************** .text // *********************************************************************** main: push {lr} // Save the return address on the stack push {fp} // Save the frame pointer on the stack // Explained later in CS255 /* ---------------------------------------- Pass the string to printf function ---------------------------------------- */ movw r0, #:lower16:HelloStr movt r0, #:upper16:HelloStr /* ---------------------------------------- Call the printf function ---------------------------------------- */ bl printf // Call printf function with input string pop {fp} // Pop the frame pointer pop {lr} // Pop the return address // Explained later in CS255

    I will skip the push and pop instructions - this is advanced material in assembler programming that will be explained later in CS255.

    The main: identifier (called: label in assembler programming) corresponds to the location of the start of the program.

    Every assembler program starts executing at the main: location.

    The program instruction in the Hello World program consists of the following 3 (assembler) instructions (given in red in the diagram below):

    /* ---------------------------------------- Pass the string to printf function ---------------------------------------- */ movw r0, #:lower16:HelloStr movt r0, #:upper16:HelloStr /* ---------------------------------------- Call the printf function ---------------------------------------- */ bl printf // Call printf function with input string

    The first 2 instruction passes the parameter HelloStr to the function (= method)

    The last instruction calls the function (method) printf (which is the same as println in java).

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• Important: assembler programming will need to use registers in the processor
  • Fact:

    When you write an assembler program, you must use the registers in the processor For example: you can see this assembler instruction in the "Hello World" program: movw r0, #:lower16:HelloStr The notation r0 is the name of a specific register of the ARM processor

    We call assembler programming: low level programming because an assembler programmer must know every detail of the computer processor in order to write the program.

    In contrast: Java (and C) is a high level programming language where the programmer does not need to know the details of the computer processor !!!

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• The constants and variables in the "Hello World" program
  • When you look at the Java "Hello World" program:

    public class Hello { public static void main(String[] args) { System.out.println("Hello World"); } }

    you may think that there are no constants defined in the program.

    That's wrong, because:

    "Hello World" is a String constant !!!

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  • Correspondingly, the .data section of the assembler program contains the defining of a string constant expressed as follows:

    // ************************ Starting variable definition ***************** .data // *********************************************************************** HelloStr: // Label marking this location in memory .asciz "Hello World\n" // ASCII codes for the string

    The string constant is defined using the syntax:

    .asciz "Hello World\n"

    The identifier HelloStr: (called "label" in assembler programming - later) is used to mark the memory location where this string constant is stored.

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• Organization of the Hello World Program
  • According to this organization:

    The program instructions and String constant are stored inside the computer memory as follows:

    Without a suitable tool, I cannot show you the internal organization of the program

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• EGTAPI: Emory Graphical Tool for Assembler Programming Instruction
  • We have develop a special teaching tool for CS255 assembler instruction called: EGTAPI

    This tool is design to:

    Reveal all the secrets of programming features in high level languages (i.e.: show you how are these features implemented in assembler) Run assmbler programs without using a real computer (We run the assembler program using an emulator. This feature will allow the CS lab machines to run ARM assembler programs -- so we don't need another computer lab to support CS255 projects)

    I will use EGTAPI to show you the program organization in the next webpage