corecode has written:
>On Thu, 20 Dec 2001 14:2:16 +0800 Leslie Jackson <[EMAIL PROTECTED]>
>wrote:
>
>> Now i think i've got a basic knowledge about the BIOS data area.
>> But reading the code of boot0.s, i got another question as in the
>> following code:
>>
>> main.5: incw dx # Next item
>> addb $0x10,bl # Next entry
>> jnc main.3 # Till done
>>
>> The partition table in MBR only has 4 items, why check for 16 times??
>
>it doesn't check for 16. it adds 0x10 to bl and if bl overflows (prolly
>the end of the partition table. as sizeof(ever entry) == 0x10 and the 4th
>ends at 0x1fc another (a 5th) increment will overflow bl and result in a
>set carry flag.
>
Thanks for your insight! And i've got the point.
Before the loop around on the partition table, the register %dx has been assigned
the value $0x7c2(That is: 0x600 + 0x1be + 4) as in the code:
movw $(partbl+0x4), %bx
So 4 items of the partition table has been checked, %bl has the value $0xf2.
And the following increment(addb $0x10, %bl) will result in a carry flag.
Am i right??
Thanks a lot.
BTW: the more i read it(boot0.s), the more i find it's worth reading.
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