arch.x86.idt¶

Interrupt Descriptor Table (IDT) data structures, gate registration, and loading.

Overview¶

This unit defines the 256-entry IDT used by the x86 interrupt system. It provides constants for the four privilege-level gate flag bytes, a procedure to install individual interrupt gates, and an init routine that clears the table and loads it into the CPU via LIDT. Each call to set_gate immediately reloads the IDT so that the new handler is active as soon as the procedure returns.

Dependencies¶

core.util
arch.x86.util
io.syslog

Constants¶

ISR_RING_0¶

$8E — Gate flags for a ring-0 interrupt gate (present, DPL 0, 32-bit interrupt gate type).

ISR_RING_1¶

$AE — Gate flags for a ring-1 interrupt gate (present, DPL 1, 32-bit interrupt gate type).

ISR_RING_2¶

$CE — Gate flags for a ring-2 interrupt gate (present, DPL 2, 32-bit interrupt gate type).

ISR_RING_3¶

$EE — Gate flags for a ring-3 interrupt gate (present, DPL 3, 32-bit interrupt gate type).

Types¶

TIDT_Entry / PIDT_Entry¶

TIDT_Entry = packed record
    base_low  : uint16;
    selector  : uint16;
    always_0  : uint8;
    flags     : uint8;
    base_high : uint16;
end;

An 8-byte x86 interrupt gate descriptor in the hardware layout.

Field	Description
base_low	Lower 16 bits of the handler entry-point address
selector	Code segment selector for the handler
always_0	Reserved byte, must be zero
flags	Gate type and privilege flags (use `ISR_RING_*` constants)
base_high	Upper 16 bits of the handler entry-point address

TIDT_Pointer / PIDT_Pointer¶

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

The 6-byte operand passed to LIDT. limit is always (sizeof(TIDT_Entry) * 256) - 1; base is the linear address of IDT_Entries.

Variables¶

IDT_Entries¶

var IDT_Entries : Array [0..255] of TIDT_Entry;

Static array of 256 interrupt gate descriptors.

IDT_Pointer¶

var IDT_Pointer : TIDT_Pointer;

The LIDT operand, initialised during init.

Functions and Procedures¶

init¶

procedure init();

Sets up IDT_Pointer, zeroes the entire IDT_Entries array using core.util.memset, and loads the IDT via LIDT. Must be called before any interrupt gates are installed.

set_gate¶

procedure set_gate(Number : uint8; Base : uint32; Selector : uint16; Flags : uint8);

Installs an interrupt gate at index Number. Splits Base into base_low and base_high, sets the code segment Selector (typically $08 for the ring-0 code segment), writes the Flags byte, and zeroes always_0. Calls LIDT immediately after writing so the gate is live.

Parameters: - Number — IDT vector number (0–255). - Base — Linear address of the ISR entry point. - Selector — GDT code segment selector used when the handler is invoked. - Flags — Gate type and DPL flags; use one of the ISR_RING_* constants.

Notes¶

The IDT is reloaded after every set_gate call. This is safe but slightly inefficient when installing many gates in sequence.
Vectors 0–31 are reserved for CPU exceptions; vectors 32–255 are available for hardware IRQs and software interrupts.