On Wed, 15 Feb 2006, Phillip Susi wrote:
> linux-os (Dick Johnson) wrote:
>> If the disc is a modern disk, and the BIOS is modern as well,
>> it won't care. For instance, if we attempt to seek to cylinder
>> 10, sector 10, and there are only 9 sectors, then the supplied
>> head number is incremented, the sector to be read becomes 1
>> (dumb ones based), and everything is fine. If the head number
>> can't be incremented, it wraps to 0. Problems occur if the BIOS
>> has been set to "physical" mode for access. In this mode, the
>> CHS are absolute and "you can't get there from here." In the
>> physical mode, you can't have more than 1024 cylinders because
>> they need to fit into 10 bits.
>>
>> As long as the BIOS is set for LBA, the boot sequence should not
>> care.
>>
>
> Are you sure? Do all bioses perform this auto correction? I would have
> thought that they would simply fail the request because you asked for a
> sector or head that is outside the valid range. Even if some bioses
> will accept illegal values and auto translate, I doubt that they all do.
>
> And what if you error in the other direction? If the MBR lists a LOWER
> number of heads than the bios thinks there is? In that case you're
> going to ask the bios for a larger cylinder number, and it will happily
> read a sector from the disk that is further from the start than you
> intended.
Heads start at 0. Sectors start at 1. Cylinders start at 0.
A "lower head" than allowed would be 0xff so the BIOS wouldn't
know it was "lower". The BIOS doesn't look at the MBR for
normal read/write access! Only while booting does it
read the first sector of the master boot record (MBR) into
the appropriate physical place (0x7c00). Then it checks to see
if there is an 0xaa55 as the last word in the sector. If so,
it executes code starting at offset zero. Modern BIOS don't
even check the "boot flag" because it may be wrong, preventing
a boot.
Now, during the boot sequence, the BIOS via INT 0x13 or 0x40
will be called upon to read data into memory from various
offsets on the media. If the offsets are calculated in the
same way that they were calculated when the disk was initialized
as a boot disk, then everything is okay. The calculations of
offsets do not require the same C/H/S phony variables! One
only has to follow the correct rules. The rules are that
heads increment from 0, as do cylinders, and sectors start
at 1. Also "sectors" must be 512-byte intervals even though
the physical media may have 16 kilobyte sectors. Given
these rules, there are zillions of ways for one to arrive
at the correct offset. The interpretation will be correct
IFF the number of cylinders are extracted first, then the
number of heads (tracks), then the number of sectors, always
using the largest number that will fit into the BIOS
registers used to make that access.
In the case of "large media" access, the cylinders are
set to 0xffff. This triggers additional logic that invents
a new virtual sector length to accommodate.
The following is the __entire__ boot code for an IBM/PC
compatible BIOS! Constant "DISKS" is 0x13.
;
ALIGN 4
INT_19H:
STI
PUSH DS
PUSH ES
XOR DX,DX ; Get a zero
MOV DS,DX ; Set segments
MOV ES,DX ; DS = ES = 0
MOV CX,256 ; The IBM/AT bios clears 256 WORDS
MOV DI,7C00H ; Boot location
XOR AX,AX ; Get a zero
REP STOSW ; Clear that area.
XOR DX,DX ; Reset any floppy disk
XOR AX,AX ; Reset Disk subsystem
INT DISKS ; Ignore any error
MOV AX,0201H ; Read one sector
MOV BX,7C00H ; DS:BX points to buffer
MOV CX,1 ; First sector
XOR DX,DX ; First floppy disk
INT DISKS ; Disk vector
JC SHORT FAIL ; Can't read it
IPL: CMP WORD PTR [BX+1FEH],0AA55H
JNZ SHORT FAIL ; IPL bad
DB 0EAH ; Jmp FAR
DW 7C00H ; Offset
DW 0 ; Segment
FAIL: POP ES ; Restore segments
POP DS
IRET
Cheers,
Dick Johnson
Penguin : Linux version 2.6.13.4 on an i686 machine (5589.66 BogoMips).
Warning : 98.36% of all statistics are fiction.
_
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