os_kernel/doc/gate descriptor.md

门描述符(32 位)

63 48	47	46 45	44	43 40	39 32
Offset	P	DPL	0	Gate Type	Reserved

31 16	15 0
Segment Selector	offset

• Offset: A 32-bit value, split in two parts. It represents the address of the entry point of the Interrupt Service Routine.
• Selector: A Segment Selector with multiple fields which must point to a valid code segment in your GDT.
• Gate Type: A 4-bit value which defines the type of gate this Interrupt Descriptor represents. There are five valid type values:
  • 0b0101 or 0x5: Task Gate, note that in this case, the Offset value is unused and should be set to zero.
  • 0b0110 or 0x6: 16-bit Interrupt Gate
  • 0b0111 or 0x7: 16-bit Trap Gate
  • 0b1110 or 0xE: 32-bit Interrupt Gate
  • 0b1111 or 0xF: 32-bit Trap Gate
• DPL: A 2-bit value which defines the CPU Privilege Levels which are allowed to access this interrupt via the INT instruction. Hardware interrupts ignore this mechanism.
• P: Present bit. Must be set (1) for the descriptor to be valid.

For more information, see Section 6.11: IDT Descriptors and Figure 6-2: IDT Gate Descriptors of the Intel Software Developer Manual, Volume 3-A.

Example Code C Struct:

struct InterruptDescriptor32 {
   uint16_t offset_1;        // offset bits 0..15
   uint16_t selector;        // a code segment selector in GDT or LDT
   uint8_t  zero;            // unused, set to 0
   uint8_t  type_attributes; // gate type, dpl, and p fields
   uint16_t offset_2;        // offset bits 16..31
};

Example type_attributes values that people are likely to use (assuming DPL is 0):

• 32-bit Interrupt Gate: 0x8E (p=1, dpl=0b00, type=0b1110 => type_attributes=0b1000_1110=0x8E)
• 32-bit Trap Gate: 0x8F (p=1, dpl=0b00, type=0b1111 => type_attributes=1000_1111b=0x8F)
• Task Gate: 0x85 (p=1, dpl=0b00, type=0b0101 => type_attributes=0b1000_0101=0x85)