Home/ Questions/Q 3344886
In Process

The Archive Base Latest Questions

Editorial Team
  • 0
Asked: 2026-05-18T01:07:17+00:00

Can anybody explain me this piece of assembly code? LINEAR_DATA_SEL equ $-gdt dw 0FFFFh

  • 0

Can anybody explain me this piece of assembly code?

LINEAR_DATA_SEL equ $-gdt
    dw 0FFFFh
    dw 0
    db 0
    db 92h      ; present, ring 0, data, expand-up, writable
    db 0CFh     ; page-granular (4 gig limit), 32-bit
    db 0

Here I have already googled about the command equ, dw and db but I can’t understand what this code actually do(especially the first line). what is this $-gdt and the parameters of dw and db? Kindly explain in detail if possible. Thanks in advance.

Share

Leave an answer

You must login to add an answer.

Need An Account,

1 Answer

  1. Editorial Team

    It’s actually an 8-byte entry in the global descriptor table. It creates a descriptor addressing the entire 4G address space as a selector.

    The equ $-gdt sets up a value in the assembler equal to the difference between this location ($) and the gdt label. In other words, it’s the offset of this entry within the GDT itself.

    The structure of a GDT entry is as follows:

         alt text

    where the individual parts are explained below.

         alt text

    For your specific values:

    (a) dw FFFFh
(b) dw 0
(c) db 0
(d) db 92h      ; present, ring 0, data, expand-up, writable
(e) db CFh      ; page-granular (4 gig limit), 32-bit
(f) db 0

    1. The base address is calculated from the f, c and b fields, from most significant to least – because these are all zero, the base is at zero.

    2. The selector limit is calculated from the rightmost 4 bits of e and all of a, to give 0xfffff in this case. This has 1 added to it to give 0x100000. See point 3 below for what this means.

    3. The top 4 bits of e (the flags) set the granularity (4K rather than 1 byte) and the operand size (32-bit). With a granularity of 4K (12 bits) and page count of 0x100000 (20 bits), that gives you your full 32-bit (4G) address space.

    4. The d field is the access byte and sets the following properties based on 0x92:

      • Pr present (in-memory) bit to true.
      • Privl privelege level to 0 (need to be ring 0 to get access).
      • Ex executable bit 0 (data selector).
      • DC, direction bit is 0, segment grows up.
      • RW of 1, memory is writable.
      • Ac accessed bit set to 0.
    • 0