GDT

This document describes the Global Descriptor Table (GDT) implementation in the Kyronix kernel. It is the child of x86-64 Architecture.

The GDT provides segment descriptors for kernel and user code/data, plus per-CPU Task State Segment (TSS) descriptors. In long mode, segments are mostly flat (base=0, limit=4 GiB), but the TSS is essential for interrupt stack switching and I/O permission bitmap.

GDT Structure

typedef struct {
    gdt_entry_t entries[5];         // null, kcode, kdata, udata, ucode
    tss_desc_t  tss[MAX_CPUS];     // one TSS descriptor per CPU
} gdt_t;  // aligned(16)

Segment Values

Entry	Selector	Encoded Value	Meaning
Kernel code	`0x08`	`0x00AF9A000000FFFF`	64-bit, ring 0, executable, readable
Kernel data	`0x10`	`0x00CF92000000FFFF`	64-bit, ring 0, writable
User data	`0x18`	`0x00CFF2000000FFFF`	64-bit, ring 3, writable
User code	`0x20`	`0x00AFFA000000FFFF`	64-bit, ring 3, executable, readable

TSS Structure

The TSS (104 bytes on x86-64) provides:

rsp0: Ring 0 stack pointer, used on privilege level transitions (interrupts from ring 3)
ist[7]: Interrupt Stack Table entries for critical handlers (Double Fault, NMI)
iopb_offset: Offset to I/O permission bitmap

typedef struct {
    uint32_t reserved0;
    uint64_t rsp0, rsp1, rsp2;
    uint64_t reserved1;
    uint64_t ist[7];
    uint64_t reserved2;
    uint16_t reserved3;
    uint16_t iopb_offset;  // = sizeof(tss_t)
} tss_t;  // 104 bytes, packed

Key Functions

Function	Description
`gdt_init()`	Initialize GDT, BSP TSS, load GDT and TSS on BSP
`gdt_ap_load(cpu_id)`	Load GDT and TSS on an Application Processor
`gdt_set_kernel_stack(rsp0)`	Update TSS rsp0 for the current CPU
`tss_init(tss)`	Initialize TSS fields (IST stacks, IOPB offset)

IST Stacks

IST Index	Stack	Size	Assigned To
1	`g_ist_df`	16 KiB	Double Fault (#8)
2	`g_ist_nmi`	16 KiB	NMI (vector 2)

NOTE: IST stacks provide dedicated, non-overflowable stacks for the most critical exceptions, preventing kernel stack corruption during Double Faults.

Last reviewed: 2026-07-22

Kyronix Documentation

GDT

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