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Detecting Memory (x86): Difference between revisions
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Taking this into account, our example code would look like the following. Note that if you prefer a version that does not require the multiboot.h header downloaded from the link above, there is a version listed in the code examples section of this article.
<source lang="c">
#include "multiboot.h"
void _main(multiboot_info_t* mbd, uint32_t magic)
{
/* Make sure the magic number matches for memory mapping*/
if(magic != MULTIBOOT_BOOTLOADER_MAGIC) {
panic("invalid magic number!");
}
/* Check bit 6 to see if we have a valid memory map */
if(!(mbd->flags >> 6 & 0x1)) {
panic("invalid memory map given by GRUB bootloader");
}
/* Loop through the memory map and display the values */
int i;
for(i = 0; i < mbd->mmap_length;
i += sizeof(multiboot_memory_map_t))
{
multiboot_memory_map_t* mmmt =
(multiboot_memory_map_t*) (mbd->mmap_addr + i);
printf("Start Addr: %x | Length: %x | Size: %x | Type: %d\n",
mmmt->addr, mmmt->len, mmmt->size, mmmt->type);
if(mmmt->type == MULTIBOOT_MEMORY_AVAILABLE) {
/*
* Do something with this memory block!
* BE WARNED that some of memory shown as availiable is actually
* actively being used by the kernel! You'll need to take that
* into account before writing to memory!
*/
}
}
}
</source>
'''WARNING:''' If you downloaded the multiboot header from gnu.org (linked above) you probably got a version which defines the base address and length fields as one 64-bit unsigned integer each, rather than two 32-bit unsigned integers each. [https://forum.osdev.org/viewtopic.php?t=30318 This may cause gcc to pack the structure incorrectly] which can lead to nonsensical values when you try to read it.
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