CODEWITHSUNDEEP
assemblyx86nasm
Matan
Matan

Reputation: 179

Alternative to @data in NASM32

I'm learning assembly right now and I realized that the symbol @data isn't defined in NASM. Here is my code:

section .data
    var1 db 0x3
    var2 db 0x4

section .text
global main
main:
    mov eax, @data
    mov ds, eax
    
    mov eax, var1
    mov ebx, var2
    
    mov eax, 0

I search the web for alternative symbols and I didn't found anything. So is there any alternative to @data at all? Thanks for your help.

Upvotes: 1

Views: 254

Answers (1)

vitsoft
vitsoft

Reputation: 5805

Symbol @data represents the paragraph address of the first byte of data segment in some assemblers. See also the example of 16bit .EXE file at NASM Chapter 9. Loading segment registers before accessing data is essential in real-mode 16bit programs for DOS or Windows 3. When such program starts, its DS and ES point to Program Segment Prefix structure, not to the data segment of your program. When was the segment declared, assembler also created a relocatable symbol with corresponding name, such as @data or data, which can be used in your program to initialize segment register. Other assemblers may use a different syntax, for instance MOV AX,PARA# [data].

When a flat 32|64bit protected-mode program starts, its segment registers are already preloaded with valid indexes to Segment Descriptor Table and you don't have to care about segment registers at all.

Upvotes: 5

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