Sections of an ASM File

As you create more asm files and look at various examples, you’ll find there are 4 sections that will appear in some of the files. While not every file will contain every section, it is good to be aware of the usage of each section.

The .data Section

The data section of an asm file is a read/write section. It is commonly used to initialize variables that will be used in the program.

Example Usage

Some example code follows. Before reading the example, here is some additional helpful information.

• The equ means to create a symbolic constant, that is, memory isn’t allocated for it.

• The $ is a reference to the memory address after the last defined data, that is, the memory address after the location of where the 0xA newline character was placed.

• The $ - msg calculates the current position from the address of the variable named msg. Effectively, this is the length of the message Hello, World!\n.

section .data
    msg db "Hello, world!" , 0xA  ; string with newline
    msgLen equ $ - msg            ; Dynamically calculate length

The .bss Section

The .bss section is used for uninitialized variables. That is, we can use this section to reserve space for values that will be set later.

Example Usage

An example of how the .bss section may be used follows. Note that resb stands for reserve bytes.

section .bss
    someInput resb 20   ; reserve 20 bytes for a var named someInput

The .rodata Section

The .rodata section is used for constants. That is, this section is for data that is not modified. For example, this is commonly used for strings.

Example Usage

In the following example, we define \(\pi\) as a constant. The letters dq in this example stand for define quadword. Despite the name quadword, a quadword is a 64-bit/double-precision value.

section .rodata
    pi dq 3.141592653589793  ; Define a quadword/double-precision constant

The .text Section

The text section is where you place the executable code for your asm file. For an example, see the page titled Hello, World!.