... In Which It Is Revealed How An AHCI Bug Makes One's Insyde(s) Freeze

I found this code in Intel's AHCI Option ROM from the Insyde BIOS. It appears to be code to build the Translated Device Parameter Table (which is a slightly different¹ implementation than the one documented in the Enhanced Working T13 Draft 1126DT).

Psuedo code  (version: Serial ATA AHCI BIOS, Version iSrc 1.20_E.0019 07092009):

Function Create TDPT for drive
    Read partition table with INT13 0x201
    If read fails, or 0xAA55 signature isn't present, goto Calculate
    Get head,sectors from FIRST Partition table entry, Ending CHS values
    heads = head+1  (because of 255 limit in partition table)
    For each partition entry
        If BOOTABLE (entry[0] == 0x80) goto UsePartition
    EndFor
    For each partition entry
        if (entry[0] == 0) and (entry[4] != 0) goto UsePartition
    EndFor


Calculate
    Call CalculateCHS - using DPT and physical size
    if no need for translation, return GOOD
    Goto CreateTDPT with cylinders, heads, sectors


UsePartition:
    Read first sector of partition with INT13 0x4200
    If the word at offset 0x1A is less than 0x100,
      and the word at offset 0x18 is less than 0x40
    then
        set heads to the byte at offset 0x1A
        set sectors to the byte at offset 0x18
    fi
    tracksize = heads * sectors
    if tracksize == 0, Goto Calculate
    DWORD size = (DPT[heads]*DPT[sectors])*DPT[cylinders]
    WORD cylinders = size / tracksize <--- Bad!
    -- if the result is greater than 65536, a divide overflow occurs

    -- which isn't handled by the BIOSes

    if (cylinders > 1024) cylinders = 1024
    if ((heads == DPT[heads]) && (sectors == DPT[sectors])) return GOOD


CreateTDPT:
    WORD at DPT[8]  = DPT[0] - Save original cylinders
    BYTE at DPT[10] = DPT[2] - Save original heads
    BYTE at DPT[7]  = DPT[3] - Save original sectors
    WORD at DPT[0]  = cylinders
    BYTE at DPT[2]  = heads
    BYTE at DPT[3]  = sectors
    BYTE at DPT[5]  = 8 if heads greater than 8, otherwise 0
    BYTE at DPT[4]  = 0xA0
    BYTE at DPT[15] = SUM( DPT[0] .. DPT[14] )

So, what goes wrong? When it breaks, it starts with bad values from the partition table, and tries to fix it with values from the boot parameter block, if it finds "valid" numbers there for heads and sectors (that is, less than or equal to 0xFF and 0x3F, respectively). When these values aren't right,  due to full disk encryption, an operating system other than Microsoft Windows, or malicious intent:

• It uses the ending head/sector of the first partition to size the translation layer.
• Windows 7 with 100MB partition results in unexpected values for INT13, FUNCTION=8 (eg, 0x13 heads).
• It stores those values into the Translated Device Parameter Table..  and then some other code comes along and uses those values. While I can't find where those values are causing the exception, anything doing C/H/S translation will be unhappy.

Looking back at version Serial ATA AHCI BIOS, Version iSrc 1.20E (Gigabyte Desktop Motherboard), I found that it doesn't read from the BPB at all.  I speculate the extra read of the NTFS boot-sector was to workaround a problem on Insyde BIOS.   This version can be crashed if the two bytes in the partition table are small enough and will hang with error code 23. Award BIOS will function OK with the other unexpected values, but Insyde BIOS will still crash if it sees them.

Finally, one HP system with an Insyde BIOS has the latest(?) 'fixed' version (Serial ATA AHCI BIOS, Version iSrc 1.20_E.0024 12212009), which reads from both the partition table and the BPB, also adding  still more checks. Unfortunately, it seems as though someone messed up and added a further bug, as it doesn't actually use any of the values it reads, but rather discards them all.

New and improved UsePartition (Serial ATA AHCI BIOS, Version iSrc 1.20_E.0024 12212009):

Read first sector of partition with INT13 0x4200

if the word at offset 0x1FE is not equal 0xAA55

   and the byte at offset 0 is not equal 0xEB

   and the word at offset 0x1A is less than 0x100

then

    set heads to the byte at offset 0x1A

fi

if (tracks == 0) or ((sectors & 0x3F) == 0) Goto Calculate

-- New bug: Since sectors can be at most 0x3F from partition table

-- the newer version ALWAYS goes off to Calculate the CHS

if (sectors & 0xC0) == 0)  Goto Calculate

tracksize = heads * sectors

if tracksize == 0, Goto Calculate

DWORD size = (DPT[heads]*DPT[sectors])*DPT[cylinders]

WORD cylinders = size / tracksize <-- Uber dangerous

-- if the result is greater than 65536, a divide overflow occurs

-- which isn't handled by the BIOSes.

if (cylinders > 1024) cylinders = 1024

if ((heads == DPT[heads]) && (sectors == DPT[sectors])) return GOOD

Goto CreateTDPT

A year later and neither Acer nor Gigabyte are providing fixed BIOSes.

¹Expected Final TDPT Values from a 60GB SSD:

          WORD Logical Cylinders   0x400

    BYTE Heads               0xFF

    BYTE Sectors             0x3F

    BYTE Signature           0xA0

    BYTE HeadsAbove8Flag     0x08

    BYTE Ignored             0x00

    BYTE Physical Sectors    0x3F

    WORD Physical Cylinders  0x3FFF

    BYTE Physical Heads      0x10

    BYTE Ignored[4]          0x0

    BYTE Checksum            0x89