Asuro/doc/src/arch/x86/arch.x86.gdt.md

arch.x86.gdt

Global Descriptor Table (GDT) data structures, gate management, and loading.

Overview

This unit defines the GDT data structures and provides procedures to set individual segment descriptors, flush the GDT into the CPU via LGDT, and reload it after modification. During init, five standard descriptors are installed: a null segment, a ring-0 code segment, a ring-0 data segment, a ring-3 code segment, and a ring-3 data segment. The GDT can hold up to 1024 entries; the GDT pointer limit is updated automatically as new gates are added.

Dependencies

• io.syslog

Boot Registration

Registered with boot.mgr as arch.x86.gdt at the early barrier.

Types

TGDT_Entry / PGDT_Entry

TGDT_Entry = packed record
    limit_low   : uint16;
    base_low    : uint16;
    base_middle : uint8;
    access      : uint8;
    granularity : uint8;
    base_high   : uint8;
end;

An 8-byte x86 segment descriptor in the standard hardware layout.

Field	Description
limit_low	Lower 16 bits of the segment limit
base_low	Lower 16 bits of the segment base address
base_middle	Bits 16–23 of the segment base address
access	Access byte: present, DPL, type flags
granularity	Upper 4 bits of limit plus granularity/size flags
base_high	Bits 24–31 of the segment base address

TGDT_Pointer

TGDT_Pointer = packed record
    limit : uint16;
    base  : uint32;
end;

The 6-byte structure passed to the LGDT instruction. limit is the byte size of the GDT minus one; base is the linear address of the first GDT entry.

Variables

gdt_entries

var gdt_entries : array[0..1023] of TGDT_Entry;

Static array holding up to 1024 GDT descriptors.

gdt_pointer

var gdt_pointer : TGDT_Pointer;

The LGDT operand. base is set to the address of gdt_entries; limit grows as gates are added.

Functions and Procedures

init

procedure init();

Initialises the GDT with five standard entries and calls flush to load it:

Gate	Offset	Access	Description
0	0x00	0x00	Null descriptor
1	0x08	0x9A	Ring-0 code (execute/read, 4 GB flat)
2	0x10	0x92	Ring-0 data (read/write, 4 GB flat)
3	0x18	0xFA	Ring-3 code (execute/read, 4 GB flat)
4	0x20	0xF2	Ring-3 data (read/write, 4 GB flat)

set_gate

procedure set_gate(Gate_Number : uint32; Base : uint32; Limit : uint32; Access : uint8; Granularity : uint8);

Writes a single GDT descriptor at index Gate_Number. Splits Base and Limit across the appropriate fields of the TGDT_Entry record. Automatically updates gdt_pointer.limit if the new gate extends beyond the current table boundary.

flush

procedure flush;

Loads the GDT via LGDT and reloads all data segment registers (DS, ES, FS, GS, SS) with selector 0x10 (ring-0 data). A far jump to selector 0x08 (ring-0 code) flushes the instruction pipeline and reloads CS.

reload

procedure reload;

Reissues LGDT without touching segment registers. Used after adding descriptors such as a TSS when the existing segment mappings remain valid.

Notes

• The far jump in flush_gdt is hand-assembled using db/dw directives because the Free Pascal inline assembler does not support far-jump syntax directly.
• Gates are numbered from 0; the segment selector for gate N is N * 8 with the lower 3 bits encoding the requested privilege level.
• The table supports up to 1024 descriptors (8 KiB), which is the hardware maximum for a 32-bit protected-mode GDT.