arch.x86.multiboot¶

Multiboot specification structures and boot-time information access.

Overview¶

This unit defines the data structures described by the Multiboot specification that the bootloader (typically GRUB) passes to the kernel at startup. It exposes the multiboot information pointer (multibootinfo) and magic number (multibootmagic) as global variables that are set by the early boot assembly stub before Pascal code is entered. Other subsystems, particularly the physical memory manager and framebuffer initialisation, read from these structures to discover available RAM, modules, and display parameters.

Constants¶

KERNEL_STACKSIZE¶

$4000 (16 384 bytes) — Size of the kernel stack allocated in the boot assembly.

MULTIBOOT_BOOTLOADER_MAGIC¶

$2BADB002 — Magic value placed in EAX by a Multiboot-compliant bootloader. The boot stub compares the value in EAX against this constant and stores it in multibootmagic.

Types¶

elf_section_header_table_t / Pelf_section_header_table_t¶

elf_section_header_table_t = packed record
    num   : uint32;
    size  : uint32;
    addr  : uint32;
    shndx : uint32;
end;

Embedded inside multiboot_info_t. Describes the ELF section header table if the bootloader provided it (Multiboot flags bit 5).

Field	Description
num	Number of section headers
size	Size in bytes of each section header
addr	Physical address of the section header table
shndx	Index of the string-table section header

multiboot_info_t / Pmultiboot_info_t¶

The primary Multiboot information structure. Selected fields:

Field	Description
flags	Bitmask indicating which fields are valid
mem_lower	Amount of lower memory in kilobytes
mem_upper	Amount of upper memory in kilobytes
boot_device	Boot device identifier
cmdline	Physical address of the kernel command line string
mods_count	Number of loaded modules
mods_addr	Physical address of the first module_t entry
elf_sec	ELF section header information
mmap_length	Byte length of the memory map buffer
mmap_addr	Physical address of the memory map buffer
drives_legnth	Byte length of the drives table (note: field name has a typo in the source)
drives_addr	Physical address of the drives table
config_table	Physical address of ROM configuration table
boot_loader_name	Physical address of the bootloader name string
apm_table	Physical address of the APM table
vbe_control_info	Physical address of VBE control information
vbe_mode_info	Physical address of VBE mode information
vbe_mode	Current VBE video mode number
vbe_interface_seg	VBE protected-mode interface segment
vbe_interface_off	VBE protected-mode interface offset
vbe_interface_len	VBE protected-mode interface length
framebuffer_addr	64-bit physical address of the framebuffer
framebuffer_pitch	Bytes per framebuffer row
framebuffer_width	Framebuffer width in pixels
framebuffer_height	Framebuffer height in pixels
framebuffer_bpp	Bits per pixel

module_t / Pmodule_t¶

module_t = packed record
    mod_start : uint32;
    mod_end   : uint32;
    name      : uint32;
    reserved  : uint32;
end;

Describes a single Multiboot module (e.g., an initial ramdisk).

Field	Description
mod_start	Physical start address of the module
mod_end	Physical end address of the module
name	Physical address of the module name string
reserved	Must be zero

memory_map_t / Pmemory_map_t¶

memory_map_t = packed record
    size      : uint32;
    base_addr : uint64;
    length    : uint64;
    mtype     : uint32;
end;

A single entry in the Multiboot memory map. Entries are variable-length; advance by size + sizeof(size) bytes to reach the next entry.

Field	Description
size	Size of this entry, not including this field itself
base_addr	64-bit base physical address of the region
length	64-bit length of the region in bytes
mtype	Region type: 1 = available RAM, other values = reserved/ACPI/etc.

Variables¶

multibootinfo¶

var multibootinfo : Pmultiboot_info_t = nil;

Pointer to the Multiboot information structure passed by the bootloader. Set by the assembly boot stub before kmain is called. Must not be dereferenced until after virtual memory is initialised (the address requires the kernel virtual base offset applied).

multibootmagic¶

var multibootmagic : uint32;

The value found in EAX at kernel entry, saved by the boot stub. Should equal MULTIBOOT_BOOTLOADER_MAGIC to confirm a valid Multiboot load.

Notes¶

All addresses inside multiboot_info_t are physical addresses provided by the bootloader. After paging is enabled with a higher-half kernel layout, they must have KERNEL_VIRTUAL_BASE added before dereferencing.
The physical memory manager (arch.x86.memory.physical) walks the memory_map_t array pointed to by mmap_addr and mmap_length to determine which 4 MiB blocks are available for allocation.