Assembly Basic Syntax
An assembly program can be divided into three sections:
The data section
The bss section
The text section
The data Section
The data section is used for declaring initialized data or constants. This data does not change at runtime. You can declare various constant values, file names or buffer size etc. in this section.
The syntax for declaring data section is:
The bss Section
The bss section is used for declaring variables. The syntax for declaring bss section is:
The text section
The text section is used for keeping the actual code. This section must begin with the declarationglobal main, which tells the kernel where the program execution begins.
The syntax for declaring text section is:
Comments
Assembly language comment begins with a semicolon (;). It may contain any printable character including blank. It can appear on a line by itself, like:
or, on the same line along with an instruction, like:
Assembly Language Statements
Assembly language programs consist of three types of statements:
Executable instructions or instructions
Assembler directives or pseudo-ops
Macros
The executable instructions or simply instructions tell the processor what to do. Each instruction consists of an operation code (opcode). Each executable instruction generates one machine language instruction.
The assembler directives or pseudo-ops tell the assembler about the various aspects of the assembly process.
These are non-executable and do not generate machine language instructions.
Macros are basically a text substitution mechanism.
Syntax of Assembly Language Statements
Assembly language statements are entered one statement per line. Each statement follows the following format:
The fields in the square brackets are optional. A basic instruction has two parts, the first one is the name of the instruction (or the mnemonic) which is to be executed, and the second are the operands or the parameters of the command.
Following are some examples of typical assembly language statements:
INC COUNT ; Increment the memory variable COUNT
The Hello World Program in Assembly
The following assembly language code displays the string 'Hel o World' on the screen:
When the above code is compiled and executed, it produces fol owing result:
Compiling and Linking an Assembly Program in NASM
Make sure you have set the path of nasm and ld binaries in your PATH environment variable. Now take the following steps for compiling and linking the above program:
Type the above code using a text editor and save it as hello.asm.
Make sure that you are in the same directory as where you saved hello.asm.
To assemble the program, type nasm -f elf hello.asm
If there is any error, you will be prompted about that at this stage. Otherwise an object file of your program named hello.o will be created.
To link the object file and create an executable file named hello, type ld -m elf_i386 -s -o hello hello.o
Execute the program by typing ./hello
If you have done everything correctly, it will display Hello, world! on the screen.