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Yeah, do they? And how? There's no magic bytes in the BPB, how do you check for the presence of a BPB? It's not possible not even in theory. You could use the strings "FAT16", "FAT32" etc., but even M$ says never do that, and there's no guarantee that an NTFS boot sector will have those at all.Octocontrabass wrote:They can check for and use it.bzt wrote:Nothing else uses it, and BIOSes are not checking for it (they actually can't check for it, that's just not possible).
I call this bullshit.Octocontrabass wrote:I've found one Dell PC that uses the BPB to decide how INT 0x13 AH=0x02 should translate CHS to LBA. It hangs during POST if the BPB is mostly correct but specifies 0 heads per cylinder. (This only happens with unpartitioned drives, of course.)
That's 100% true, therefore rumors about BIOS checking BPB is nothing more than myth.neon wrote:If the intent is to write an MBR you wouldn't need a BPB.
I don't think I ever figured out exactly what the Dell BIOS was looking for when deciding whether a disk contained a BPB or not. I do know that it wasn't very good: if I formatted an unpartitioned flash drive as FAT and overwrote offset 0xD (sectors per track) with 0, it decided no BPB was present, but doing the same with offset 0xF (heads) caused it to hang during POST.bzt wrote:Yeah, do they? And how? There's no magic bytes in the BPB, how do you check for the presence of a BPB? It's not possible not even in theory. You could use the strings "FAT16", "FAT32" etc., but even M$ says never do that, and there's no guarantee that an NTFS boot sector will have those at all.
Where did I say it doesn't?bzt wrote:First, you say that Dell BIOS checks for the BPB, then you say it doesn't. Which one is it then?
That depends on the firmware. Some will look for a valid BPB, and if they don't find one, they assume the disk is partitioned. Some will look for a valid partition table, and if they don't find one, they assume the disk is unpartitioned. Some will look for both a valid BPB and a valid partition table, and if they find neither, they refuse to boot the disk. Of course, there is no standard for what counts as "valid".bzt wrote:How does the BIOS know if the disk is unpartitioned (talking about non-GPT)?
Second, how do you distinguish an MBR from a VBR?
No, I assure you, it would hang during POST. It shows a loading bar on the screen during POST. If I used Windows to format an unpartitioned flash drive to FAT and changed the byte at offset 0xF to 0, the loading bar would stop partway.bzt wrote:It is more likely that you had a bad boot sector code that Dell BIOS refused to boot. And then when you put a BPB in it, you accidentally fixed the issue that was preventing the boot, but it wasn't for the BPB at all. Or when you've partitioned it, that override the entire boot sector replacing your bad code.
Sounds to me like a BIOS bug. I don't think it worth the effort to support that particular buggy BIOS.Octocontrabass wrote:doing the same with offset 0xF (heads) caused it to hang during POST.
Here:Octocontrabass wrote:Where did I say it doesn't?bzt wrote:First, you say that Dell BIOS checks for the BPB, then you say it doesn't. Which one is it then?
That's why I asked how does the BIOS know if the disk is unpartitioned?Octocontrabass wrote:(This only happens with unpartitioned drives, of course.)
Hold on right there. You said BPB is only checked if the disk is unpartitioned, therefore you cannot use the BPB check to determine if the disk is partitioned.Octocontrabass wrote:That depends on the firmware. Some will look for a valid BPB, and if they don't find one, they assume the disk is partitioned.bzt wrote:How does the BIOS know if the disk is unpartitioned (talking about non-GPT)?
Second, how do you distinguish an MBR from a VBR?
But how? Can you explain how to detect for an MBR partitioning table existence? (Other than it is in the first sector).Octocontrabass wrote:Some will look for a valid partition table
Code: Select all
/* return 1 if there's a partitioning table included, 0 otherwise */
int hasPartitioningTable(char sector[512])
{
/* I'm curious about this part */
}As I've said before it simply does not worth the effort to support such buggy BIOSes which violate the BBS specification. They are extremely rare, and now with UEFI taking over on real machines and BIOS soon will be exclusive to VMs it simply doesn't matter any more. (Plus this only happens if you choose "floppy emulation for USB" in the BIOS setup, so the solution on your Dell is just to uncheck that option. Sometimes called "use HDD emulation for USB" with reverse flag, but that's the same.)Octocontrabass wrote:Some will look for both a valid BPB and a valid partition table, and if they find neither, they refuse to boot the disk. Of course, there is no standard for what counts as "valid".
No, I said this particular Dell BIOS only uses the CHS geometry from the BPB for INT 0x13 AH=0x02 when the disk is unpartitioned. It may still examine the BPB when determining whether the disk is partitioned or not.bzt wrote:Hold on right there. You said BPB is only checked if the disk is unpartitioned, therefore you cannot use the BPB check to determine if the disk is partitioned.
It's different for each BIOS I've examined. Some look for exactly one "active" partition and assume any values other than 0 or 0x80 means no partition table is present. Some examine the CHS or LBA addresses to see if the partitions physically fit on the disk and don't overlap each other or the MBR. There are probably other methods in use, these are just the ones I was able to confirm through testing.bzt wrote:But how? Can you explain how to detect for an MBR partitioning table existence?
Because it maintains compatibility with DOS without forcing the user to change BIOS settings.bzt wrote:But take a step back, why is there a need for any BIOS to know if there's a partition on the disk?
On most of the PCs I've tested, there is no such setting.bzt wrote:play with your BIOS setup, there'll be a configuration which does not check for BPB nor partitions
I don't have room to set it up at the moment. I expect it'll work, unless you've intentionally crafted a BPB that makes it hang.bzt wrote:BTW, if you still have that old Dell, I would appreciate if you could give a try to boot BOOTBOOT on it and see if it works.