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Posted: **Sun Dec 21, 2008 4:59 pm**

Hi. I'm trying to write ACPI code for my OS.
I wrote a simple function to find the RSDP table using the Anchor string "RDS PTR ". My function is (i'm in higher half)

Code: Select all

void acpi_init() {
	char *sign;

	for (sign = 0xC00E0000; sign <= 0xC00FFFFF; sign++) {
		if (!strncmp(sign, "RSD PTR ", 8)) {
			kprintf("Found at %x\n", (dword) sign);
		}
	}
}

My code returns two tables at 0xC00E1ECF and at 0xC00FB970.
Which one should I use. Is this a bochs bug or a my code bug?

Thanks

Posted: **Sun Dec 21, 2008 6:53 pm**

Hi,

AlfaOmega08 wrote:My code returns two tables at 0xC00E1ECF and at 0xC00FB970.

ACPI 3.0 wrote:5.2.5.1 Finding the RSDP on IA-PC Systems
OSPM finds the Root System Description Pointer (RSDP) structure by searching physical memory ranges on 16-byte boundaries for a valid Root System Description Pointer structure signature and checksum match as follows:
The first 1 KB of the Extended BIOS Data Area (EBDA). For EISA or MCA systems, the EBDA can be found in the two-byte location 40:0Eh on the BIOS data area.

The BIOS read-only memory space between 0E0000h and 0FFFFFh.

I'm guessing the table at 0xC00E1ECF isn't on a 16-byte boundary (and the checksum probably doesn't match either)...

Cheers,

Brendan

Posted: **Mon Dec 22, 2008 4:21 pm**

Probably a bug in your code, but it is possible case that you have 2 copies of the main table one for 32-bit and one for 64-bit.

Posted: **Mon Dec 22, 2008 4:27 pm**

I doubt it, it is far more likely that his code to find the entry point doesn't search on a 16-byte boundary (pos += 16, instead of pos++).

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