# IDT generation and loading

The IDT management is defined inside `shelter/lib/include/irq/idt.h` and implemented inside `shelter/lib/src/irq/idt.c`. The current IDT generator generate the IDT according to boot process needs, and is therefore not definitive.

## IDT generation

The IDT entry is structured with this structure:
``` C
#pragma pack(1)
typedef struct {
  sh_uint16 offset_low;
  sh_uint16 selector;
  sh_uint8 ist;
  sh_uint8 type_attr;
  sh_uint16 offset_mid;
  sh_uint32 offset_high;
  sh_uint32 reserved;
} sh_idt_IDT_ENTRY;
#pragma pack()
```

The full IDT is stored in this structure:
``` C
#pragma pack(1)
typedef struct {
  sh_idt_IDT_ENTRY entries[256];
} sh_idt_IDT;
#pragma pack()
```

IDT entries are generated using the following functions:
- `sh_idt_fill_type_attr_byte(sh_bool present,sh_uint8 dpl,sh_uint8 type)`: generate the `type_attr` byte
- `sh_idt_fill_idt_entry(sh_uint64 offset,sh_uint16 selector,sh_uint8 ist,sh_uint8 type_attr)`: generate an IDT entry, expect the `type_attr` argument as `sh_idt_fill_type_attr_byte()` would generate it

The IDT is generated by `sh_idt_fill_idt(sh_idt_IDT *idt)`. 

Using the following structure:
``` C
#pragma pack(1)
typedef struct {
  sh_uint16 limit;
  sh_uint64 base;
} sh_idt_IDTR;
#pragma pack()
```

`sh_idt_load_idtr(sh_idt_IDT *idt)` load the IDT.

The following vectors are filled for the moment: 0 to 21, 28 to 30, 32 to 47, 254 and 255. Please see the [IRQ docs](irq.md) to see how the handlers work.