I am trying to write a physical allocator, and to do so I need the base address for available memory, the high address for available memory, and a memory map. This is all provided to me by the BIOS (multiboot1). I have inspected the value passed to eax as the kernel first boots and it matches the multiboot1 value, so no issue there (I am booting with MB1). Also, I am setting bit 1 of the flags (So I am requesting the memmap). However, it is the second parameter, the MBI structure that is giving me issues. I have looked on the MBI docs and have found an exact description of what the MBI should look like and implemented it in C: my C implementation:

/* macros copy-pasted from multiboot spec */
#define MULTIBOOT_MEMORY_AVAILABLE              1
#define MULTIBOOT_MEMORY_RESERVED               2
#define MULTIBOOT_MEMORY_ACPI_RECLAIMABLE       3
#define MULTIBOOT_MEMORY_NVS                    4
#define MULTIBOOT_MEMORY_BADRAM                 5
 
typedef struct {
    uint32_t address_high; // ignored on 32-bit
    uint32_t address_low;
    uint32_t length_high; // ignored on 32-bit
    uint32_t length_low;
    uint32_t type; // (1 == useable)
    uint32_t zero;
} __attribute__((__packed__)) mmap_entry_t;
 
 
/* like 3/4 of this is not relevant to me but i keep it anyway */
typedef struct {
    uint32_t flags;        // flags
    uint32_t mem_lower;    // lower memory (KB)
    uint32_t mem_upper;    // upper memory (KB)
    uint32_t boot_device;  // boot device ID
    uint32_t cmdline;      // address of kernel command line
    uint32_t mods_count;   // number of modules loaded
    uint32_t mods_addr;    // address of first module structure
 
    uint32_t irrelevant[3];
 
    uint32_t mmap_length;  // memory map length
    uint32_t mmap_addr;    // memory map address
    uint32_t drives_length; // drive info length
    uint32_t drives_addr;   // drive info address
    uint32_t config_table;  // ROM configuration table
    uint32_t boot_loader_name; // bootloader name string address
    uint32_t apm_table;     // APM table address
    uint32_t vbe_control_info; // VBE control information
    uint32_t vbe_mode_info; // VBE mode information
    uint32_t vbe_mode;      // VBE mode
    uint32_t vbe_interface_seg;  // VBE interface segment
    uint32_t vbe_interface_off;  // VBE interface offset
    uint32_t vbe_interface_len;  // VBE interface length
} __attribute__((__packed__)) multiboot_info_t;

The multiboot docs say:

The format of the Multiboot information structure (as defined so far) follows:

        +-------------------+
0       | flags             |    (required)
        +-------------------+
4       | mem_lower         |    (present if flags[0] is set)
8       | mem_upper         |    (present if flags[0] is set)
        +-------------------+
12      | boot_device       |    (present if flags[1] is set)
        +-------------------+
16      | cmdline           |    (present if flags[2] is set)
        +-------------------+
20      | mods_count        |    (present if flags[3] is set)
24      | mods_addr         |    (present if flags[3] is set)
        +-------------------+
28 - 40 | syms              |    (present if flags[4] or
        |                   |                flags[5] is set)
        +-------------------+
44      | mmap_length       |    (present if flags[6] is set)
48      | mmap_addr         |    (present if flags[6] is set)
        +-------------------+
52      | drives_length     |    (present if flags[7] is set)
56      | drives_addr       |    (present if flags[7] is set)
        +-------------------+
60      | config_table      |    (present if flags[8] is set)
        +-------------------+
64      | boot_loader_name  |    (present if flags[9] is set)
        +-------------------+
68      | apm_table         |    (present if flags[10] is set)
        +-------------------+
72      | vbe_control_info  |    (present if flags[11] is set)
76      | vbe_mode_info     |
80      | vbe_mode          |
82      | vbe_interface_seg |
84      | vbe_interface_off |
86      | vbe_interface_len |
        +-------------------+
88      | framebuffer_addr  |    (present if flags[12] is set)
96      | framebuffer_pitch |
100     | framebuffer_width |
104     | framebuffer_height|
108     | framebuffer_bpp   |
109     | framebuffer_type  |
110-115 | color_info        |
        +-------------------+

However, when I boot and check the value in the %ebx register, it points to what seems like garbage:

(gdb) p *(multiboot_info_t*)($ebx)
$1 = {flags = 591, mem_lower = 639, mem_upper = 129920, boot_device = 2147549183, cmdline = 2142208, mods_count = 0, mods_addr = 2142208, irrelevant = {0, 0, 0}, mmap_length = 0, mmap_addr = 144, drives_length = 36864, drives_addr = 0, 
  config_table = 0, boot_loader_name = 0, apm_table = 2142229, vbe_control_info = 0, vbe_mode_info = 0, vbe_mode = 0, vbe_interface_seg = 0, vbe_interface_off = 0, vbe_interface_len = 0}

Im not exactly sure what I expected, but it definitely wasnt the memmap having an address but no length. the mmap_length being 0 is what really throws me off here. However, these values are the same every time. (Every time I boot and check the value, I get these duplicated values). So it makes me think that maybe I am doing something wrong with the way I am handling the info. If so, please let me know. Thanks for reading. If you want to check out additional code from my OS, its all on github: