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Posted: **Mon May 16, 2016 7:47 am**

Hello there,
while I tried to make a memory manager I wanted to use the mbs_mmap_* tags of the multiboot structure.
But there is something problematic in it.

While using the folowing code I got a mysterious output.

start.S

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.section multiboot
#define MB_MAGIC 0x1badb002
#define MB_FLAGS 0x0
#define MB_CHECKSUM -(MB_MAGIC + MB_FLAGS)

// Der Multiboot-Header
.align 4
.int    MB_MAGIC
.int    MB_FLAGS
.int    MB_CHECKSUM

.section .text

.extern main

.global _start
_start:
    mov $kernel_stack, %esp

    call main

_stop:
    cli
    hlt

    jmp _stop

.section .bss
.space 8192
kernel_stack:

main.c

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#include "descriptor_tables.h"

static void init(void);

int main(void)
{
    init();
    while(1);
    return 0;
}

static void init(void)
{
    asm volatile("push %ebx");
    init_gdt();
    init_idt();
    asm volatile("pop %ebx");
  clear();
    asm volatile("int $0x30");
}

Annotation: The interrupt 0x30 calls "init_memory" with the ebx as parameter.

memory_manager.c

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#include "memory_manager.h"
#include "console.h"

struct map_entry
{
    uint32_t size;
    uint64_t base_addr;
    uint64_t length;
    uint32_t type;
}__attribute__((packed));

void init_memory(void *pointer)
{
    uint32_t flags;
    uint32_t length;
    struct map_entry *map;

    uint8_t i;

    flags = *(uint32_t *)pointer;
    if(flags | 0x40)
    {
        length = *(uint32_t *)(pointer + 0x2c);
        map = *(struct map_entry **)(pointer + 0x30);
        kprintf("%d\n\r", map->length);
        do
        {
            kprintf("%d\n\r", map->size);
            kprintf("0x%x-0x%x-%d\n\r", map->base_addr, map->length, map->type);
            map++;
            i++;
        }
        while(i<7);
    }
    while(1);
}

Output of "init_memory"

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654336
20
0x0-0x0-654336
20
0x9fc00-0x0-1024
20
0xf0000-0x0-65536
20
0x100000-0x0-133160960
20
0x7ffe000-0x0-8192
20
0xfffc0000-0x0-262144
0
0x0-0x0-0

So there is no "free" ram and every ram block has a size of zero?

Posted: **Mon May 16, 2016 9:42 am**

You are doing it wrong

Use the multiboot.h's structure, not a hard-codded and wrong offset.

e.g. should be 0x40 not 0x30 for "map" and 0x3C not 0x2C for "length" - there are 15 uint32_ts before mmap_length (if I counted correctly)
So your offsets into multiboot_info_t is off by 0x10 bytes.

Also do not use inline ASM to push and pop a register!
In fact you should have pushed ebx then called main(multiboot_info_t*), it would be in a place the compiler could handle and wouldn't have to rely on "hacks".

Best regards,

B!

P.S. I don't mean for this to come off offensively

Posted: **Mon May 16, 2016 9:57 am**

Another probable cause: are you sure your kprintf() handles 64-bit integers correctly?

If it interprets %x as 32-bit, then your second %x will print upper 32-bits from map->base_addr, not lowest from map->length.

Correct format specifier should be %llx, but if your kprintf() is not able to handle that, you may temporary try to down-cast the arguments: (uint32_t)map->base_addr

Posted: **Mon May 16, 2016 11:03 am**

Velko wrote: Another probable cause: are you sure your kprintf() handles 64-bit integers correctly?

If it interprets %x as 32-bit, then your second %x will print upper 32-bits from map->base_addr, not lowest from map->length.

Thanks. That solved my problem. But in the memory map are several ram blocks missing.
Here is the memory map.

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start_addr  length     resulting_end
0x0         0x9fc00    0x9fc00
0x9fc00     0x400      0xa0000
0xf0000     0x10000    0x100000
0x100000    0x7efe000  0x7ffe000
0x7ffe000   0x2000     0x8000000
0xfffc0000  0x40000    0x0

The missing blocks are

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start_addr  length     resulting_end
0xa0000     0x50000    0xf0000
0x8000000   0xf7fc0000 0xfffc0000

Has anyone an idea why they are missing?

Posted: **Mon May 16, 2016 11:55 am**

1) Memory need not be contiguous.
2) There is almost always a hole at 0xa0000, it is the memory region reserved for the old graphics modes.

- Monk

Posted: **Thu May 19, 2016 2:04 am**

There is "Memory" in RAM and ROM, but not in peripherals. Memory maps can't cover the entire space because the firmware doesn't know how to label all what devices do.

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