rulimine/PROTOCOL.md

# The Limine Boot Protocol

The Limine boot protocol is a modern, minimal, fast, and extensible boot
protocol, with a focus on backwards and forwards compatibility,
created from the experienced gained by working on the
[stivale boot protocols](https://github.com/stivale).

This file serves as the official centralised collection of features that
the Limine boot protocol is composed of. Other bootloaders may support extra
unofficial features, but it is strongly recommended to avoid fragmentation
and submit new features by opening a pull request to this repository.

## General Notes

All pointers are 64-bit wide. All pointers point to the object with the
higher half direct map offset already added to them, unless otherwise noted.

## Features

The protocol is centered around the concept of request/response - collectively
named "features" - where the kernel requests some action or information from
the bootloader, and the bootloader responds accordingly, if it is capable of
doing so.

In C terms, a feature is composed of 2 structure: the request, and the response.

A request has 3 mandatory members at the beginning of the structure:
```c
struct limine_example_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_example_response *response;
    ... optional members follow ...
};
```
* `id` - The ID of the request. This is an 8-byte aligned magic number that the
bootloader will scan for inside the executable file to find requests. Requests
may be located anywhere inside the executable as long as they are 8-byte
aligned. There may only be 1 of the same request. The bootloader will refuse
to boot an executable with multiple of the same request IDs.
* `revision` - The revision of the request that the kernel provides. This is
bumped whenever new members or functionality are added to the request structure.
Bootloaders process requests in a backwards compatible manner, *always*. This
means that if the bootloader does not support the revision of the request,
it will process the request as if were the highest revision that the bootloader
supports.
* `response` - This field is filled in by the bootloader at load time, with a
pointer to the response structure, if the request was successfully processed.
If the request is unsupported or was not successfully processed, this field
is *left untouched*, meaning that if it was set to `NULL`, it will stay that
way.

A response has only 1 mandatory member at the beginning of the structure:
```c
struct limine_example_response {
    uint64_t revision;
    ... optional members follow ...
};
```
* `revision` - Like for requests, bootloaders will instead mark responses with a
revision number. This revision is not coupled between requests and responses,
as they are bumped individually when new members are added or functionality is
changed. Bootloaders will set the revision to the one they provide, and this is
*always backwards compatible*, meaning higher revisions support all that lower
revisions do.

This is all there is to features. For a list of official Limine features, read
the "Feature List" section below.

## Executable memory layout

The protocol mandates kernels to load themselves at or above
`0xffffffff80000000`. Lower half kernels are *not supported*.

At handoff, the kernel will be properly loaded and mapped with appropriate
MMU permissions at the requested virtual memory address (provided it is at
or above `0xffffffff80000000`).

No specific physical memory placement is guaranteed. In order to determine
where the kernel is loaded in physical memory, see the Kernel Address feature
below.

Alongside the loaded kernel, the bootloader will set up memory mappings as such:
```
 Base Physical Address -                    Size                    ->  Virtual address
  0x0000000000001000   - 4 GiB plus any additional memory map entry -> 0x0000000000001000
  0x0000000000000000   - 4 GiB plus any additional memory map entry -> HHDM start
```
Where HHDM start is returned by the Higher Half Direct Map feature (see below).
These mappings are supervisor, read, write, execute (-rwx).

The bootloader page tables are in bootloader-reclaimable memory (see Memory Map
feature below), and their specific layout is undefined as long as they provide
the above memory mappings.

If the kernel is a position independent executable, the bootloader is free to
relocate it as it sees fit, potentially performing KASLR (as specified by the
config).

## Entry machine state

### x86_64

`rip` will be the entry point as defined as part of the executable file format,
unless the an Entry Point feature is requested (see below), in which case,
the value of `rip` is going to be taken from there.

At entry all segment registers are loaded as 64 bit code/data segments, limits
and bases are ignored since this is 64-bit mode.

The GDT register is loaded to point to a GDT, in bootloader-reserved memory,
with at least the following entries, starting at offset 0:

  - Null descriptor
  - 16-bit code descriptor. Base = `0`, limit = `0xffff`. Readable.
  - 16-bit data descriptor. Base = `0`, limit = `0xffff`. Writable.
  - 32-bit code descriptor. Base = `0`, limit = `0xffffffff`. Readable.
  - 32-bit data descriptor. Base = `0`, limit = `0xffffffff`. Writable.
  - 64-bit code descriptor. Base and limit irrelevant. Readable.
  - 64-bit data descriptor. Base and limit irrelevant. Writable.

The IDT is in an undefined state. Kernel must load its own.

IF flag, VM flag, and direction flag are cleared on entry. Other flags undefined.

PG is enabled (`cr0`), PE is enabled (`cr0`), PAE is enabled (`cr4`),
LME is enabled (`EFER`).
If 5-level paging is requested and available, then 5-level paging is enabled
(LA57 bit in `cr4`).
The NX bit will be enabled (NX bit in `EFER`).

The A20 gate is opened.

Legacy PIC and IO APIC IRQs are all masked.

If booted by EFI/UEFI, boot services are exited.

`rsp` is set to point to a stack, in bootloader-reserved memory, which is
at least 16KiB (16384 bytes) in size. An invalid return address of 0 is pushed
to the stack before jumping to the kernel.

All other general purpose registers are set to 0.

## Feature List

Request IDs are composed of 4 64-bit unsigned integers, but the first 2 are
common to every request:
```c
#define LIMINE_COMMON_MAGIC 0xc7b1dd30df4c8b88, 0x0a82e883a194f07b
```

### Bootloader Info Feature

ID:
```c
#define LIMINE_BOOTLOADER_INFO_REQUEST { LIMINE_COMMON_MAGIC, 0xf55038d8e2a1202f, 0x279426fcf5f59740 }
```

Request:
```c
struct limine_bootloader_info_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_bootloader_info_response *response;
};
```

Response:
```c
struct limine_bootloader_info_response {
    uint64_t revision;
    char *name;
    char *version;
};
```

`name` and `version` are 0-terminated ASCII strings containing the name and
version of the loading bootloader.

### HHDM (Higher Half Direct Map) Feature

ID:
```c
#define LIMINE_HHDM_REQUEST { LIMINE_COMMON_MAGIC, 0x48dcf1cb8ad2b852, 0x63984e959a98244b }
```

Request:
```c
struct limine_hhdm_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_hhdm_response *response;
};
```

Response:
```c
struct limine_hhdm_response {
    uint64_t revision;
    uint64_t offset;
};
```

* `offset` - the virtual address offset of the beginning of the higher half
direct map.

### Framebuffer Feature

ID:
```c
#define LIMINE_FRAMEBUFFER_REQUEST { LIMINE_COMMON_MAGIC, 0xcbfe81d7dd2d1977, 0x063150319ebc9b71 }
```

Request:
```c
struct limine_framebuffer_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_framebuffer_response *response;
};
```

Response:
```c
struct limine_framebuffer_response {
    uint64_t revision;
    uint64_t framebuffer_count;
    struct limine_framebuffer **framebuffers;
};
```

* `framebuffer_count` - How many framebuffers are present.
* `framebuffers` - Pointer to an array of `framebuffer_count` pointers to
`struct limine_framebuffer` structures.

```c
// Constants for `memory_model`
#define LIMINE_FRAMEBUFFER_RGB 1

struct limine_framebuffer {
    void *address;
    uint16_t width;
    uint16_t height;
    uint16_t pitch;
    uint16_t bpp;
    uint8_t memory_model;
    uint8_t red_mask_size;
    uint8_t red_mask_shift;
    uint8_t green_mask_size;
    uint8_t green_mask_shift;
    uint8_t blue_mask_size;
    uint8_t blue_mask_shift;
    uint8_t unused;
    uint64_t edid_size;
    void *edid;
};
```

### 5-Level Paging Feature

ID:
```c
#define LIMINE_5_LEVEL_PAGING_REQUEST { LIMINE_COMMON_MAGIC, 0x94469551da9b3192, 0xebe5e86db7382888 }
```

Request:
```c
struct limine_5_level_paging_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_5_level_paging_response *response;
};
```

Response:
```c
struct limine_5_level_paging_response {
    uint64_t revision;
};
```

Notes: The presence of this request will prompt the bootloader to turn on
x86_64 5-level paging. It will not be turned on if this request is not present.
If the response pointer is unchanged, 5-level paging is engaged.

### SMP (multiprocessor) Feature

ID:
```c
#define LIMINE_SMP_REQUEST { LIMINE_COMMON_MAGIC, 0x95a67b819a1b857e, 0xa0b61b723b6a73e0 }
```

Request:
```c
struct limine_smp_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_smp_response *response;
    uint64_t flags;
};
```

* `flags` - Bit 0: Enable X2APIC, if possible.

Response:
```c
struct limine_smp_response {
    uint64_t revision;
    uint32_t flags;
    uint32_t bsp_lapic_id;
    uint64_t cpu_count;
    struct limine_smp_info **cpus;
};
```

* `flags` - Bit 0: X2APIC has been enabled.
* `bsp_lapic_id` - The Local APIC ID of the bootstrap processor.
* `cpu_count` - How many CPUs are present. It includes the bootstrap processor.
* `cpus` - Pointer to an array of `cpu_count` pointers to
`struct limine_smp_info` structures.

Notes: The presence of this request will prompt the bootloader to bootstrap
the secondary processors. This will not be done if this request is not present.

```c
typedef void (*limine_goto_address)(void);

struct limine_smp_info {
    uint32_t processor_id;
    uint32_t lapic_id;
    uint64_t reserved;
    limine_goto_address goto_address;
    uint64_t extra_argument;
};
```

* `processor_id` - ACPI Processor UID as specified by the MADT
* `lapic_id` - Local APIC ID of the processor as specified by the MADT
* `goto_address` - An atomic write to this field causes the parked CPU to
jump to the written address, on a 16KiB stack. A pointer to the
`struct limine_smp_info` structure of the CPU is passed in `RDI`. Other than
that, the CPU state will be the same as described for the bootstrap
processor. This field is unused for the structure describing the bootstrap
processor.
* `extra_argument` - A free for use field.

### Entry Point Feature

ID:
```c
#define LIMINE_ENTRY_POINT_REQUEST { LIMINE_COMMON_MAGIC, 0x13d86c035a1cd3e1, 0x2b0caa89d8f3026a }
```

Request:
```c
typedef void (*limine_entry_point)(void);

struct limine_entry_point_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_entry_point_response *response;
    limine_entry_point entry;
};
```

* `entry` - The requested entry point.

Response:
```c
struct limine_entry_point_response {
    uint64_t revision;
};
```

### Module Feature

ID:
```c
#define LIMINE_MODULE_REQUEST { LIMINE_COMMON_MAGIC, 0x3e7e279702be32af, 0xca1c4f3bd1280cee }
```

Request:
```c
struct limine_module_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_module_response *response;
};
```

Response:
```c
struct limine_module_response {
    uint64_t revision;
    uint64_t module_count;
    struct limine_module **modules;
};
```

* `module_count` - How many modules are present.
* `modules` - Pointer to an array of `module_count` pointers to
`struct limine_module` structures.

```c
struct limine_module {
    void *base;
    uint64_t length;
    char *path;
    char *cmdline;
    struct limine_file_location *file_location;
};
```

* `base` - The address of the module.
* `length` - The size of the module.
* `path` - The bootloader-specific path of the module.
* `cmdline` - A command line associated with the module.
* `file_location` - A pointer to the file location structure for the module.

#### File Location Structure

```c
struct limine_uuid {
    uint32_t a;
    uint16_t b;
    uint16_t c;
    uint8_t d[8];
};

struct limine_file_location {
    uint64_t revision;
    uint64_t partition_index;
    uint32_t tftp_ip;
    uint32_t tftp_port;
    uint32_t mbr_disk_id;
    struct limine_uuid gpt_disk_uuid;
    struct limine_uuid gpt_part_uuid;
    struct limine_uuid part_uuid;
};
```

* `revision` - Revision of the `struct limine_file_location` structure.
* `partition_index` - 1-based partition index of the volume from which the
module was loaded. If 0, it means invalid or unpartitioned.
* `tftp_ip` - If non-0, this is the IP of the TFTP server the file was loaded from.
* `tftp_port` - Likewise, but port.
* `mbr_disk_id` - If non-0, this is the ID of the disk the module was loaded
from as reported in its MBR.
* `gpt_disk_uuid` - If non-0, this is the UUID of the disk the module was
loaded from as reported in its GPT.
* `gpt_part_uuid` - If non-0, this is the UUID of the partition the module
was loaded from as reported in the GPT.
* `part_uuid` - If non-0, this is the UUID of the filesystem of the partition
the module was loaded from.

### RSDP Feature

ID:
```c
#define LIMINE_RSDP_REQUEST { LIMINE_COMMON_MAGIC, 0xc5e77b6b397e7b43, 0x27637845accdcf3c }
```

Request:
```c
struct limine_rsdp_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_rsdp_response *response;
};
```

Response:
```c
struct limine_rsdp_response {
    uint64_t revision;
    void *address;
};
```

* `address` - Address of the RSDP table.

### SMBIOS Feature

ID:
```c
#define LIMINE_SMBIOS_REQUEST { LIMINE_COMMON_MAGIC, 0x9e9046f11e095391, 0xaa4a520fefbde5ee }
```

Request:
```c
struct limine_smbios_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_smbios_response *response;
};
```

Response:
```c
struct limine_smbios_response {
    uint64_t revision;
    void *entry_32;
    void *entry_64;
};
```

* `entry_32` - Address of the 32-bit SMBIOS entry point. NULL if not present.
* `entry_64` - Address of the 64-bit SMBIOS entry point. NULL if not present.

### EFI System Table Feature

ID:
```c
#define LIMINE_EFI_SYSTEM_TABLE_REQUEST { LIMINE_COMMON_MAGIC, 0x5ceba5163eaaf6d6, 0x0a6981610cf65fcc }
```

Request:
```c
struct limine_efi_system_table_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_efi_system_table_response *response;
};
```

Response:
```c
struct limine_efi_system_table_response {
    uint64_t revision;
    void *address;
};
```

* `address` - Address of EFI system table.

### Boot Time Feature

ID:
```c
#define LIMINE_BOOT_TIME_REQUEST { LIMINE_COMMON_MAGIC, 0x502746e184c088aa, 0xfbc5ec83e6327893 }
```

Request:
```c
struct limine_boot_time_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_boot_time_response *response;
};
```

Response:
```c
struct limine_boot_time_response {
    uint64_t revision;
    int64_t boot_time;
};
```

* `boot_time` - The UNIX time on boot, in seconds, taken from the system RTC.

### Kernel Address Feature

ID:
```c
#define LIMINE_KERNEL_ADDRESS_REQUEST { LIMINE_COMMON_MAGIC, 0x71ba76863cc55f63, 0xb2644a48c516a487 }
```

Request:
```c
struct limine_kernel_address_request {
    uint64_t id[4];
    uint64_t revision;
    struct limine_kernel_address_response *response;
};
```

Response:
```c
struct limine_kernel_address_response {
    uint64_t revision;
    uint64_t physical_base;
    uint64_t virtual_base;
};
```

* `physical_base` - The physical base address of the kernel.
* `virtual_base` - The virtual base address of the kernel.