LBACHS

[bloodhound23]

Is this lbachs function correct:

Code: Select all

LBACHS:;ax = lba
div BYTE[sectorspertrack]
push ax
mov [sector],ah
div  BYTE[numheads]
mov [cylinder],al
mov [head],ah
ret   

???

[pcmattman]

Try it and find out .

[Zenith]

But before you go and say 'it doesn't work!!!', you forgot to increment AX (LBA starts at 0, CHS starts at 0,0,1). Just add 'inc ax' to the beginning.

[bloodhound23]

The reason I asked was that I was slightly confused with the formulas.

[bloodhound23]

But before you go and say 'it doesn't work!!!', you forgot to increment AX (LBA starts at 0, CHS starts at 0,0,1). Just add 'inc ax' to the beginning.

thanks!

[pcmattman]

Also, I highly suggest doing such things as this in C and not in ASM as it's much easier to implement an algorithm such as the LBA->CHS conversion in C. Also, the compiler will automatically optimize your code (if -O0 is not set on the command line).

[bloodhound23]

Now my bootloader totals 73 bytes, I think that leaves me plenty of room to load stage2 now that I have my read and LBACHS done.

[bloodhound23]

Also, I highly suggest doing such things as this in C and not in ASM as it's much easier to implement an algorithm such as the LBA->CHS conversion in C. Also, the compiler will automatically optimize your code (if -O0 is not set on the command line).

It's a bootloader and when you only have 512 bytes I take it it's hard to write it in C. This isn't stage2 yet.
This is it now:

Code: Select all

LBACHS:;ax = lba
inc ax
div BYTE[sectorspertrack]
push ax
mov [sector],ah
div  BYTE[numheads]
mov [cylinder],al
mov [head],ah
ret

EDIT:

[octavio]

Also, I highly suggest doing such things as this in C and not in ASM as it's much easier to implement an algorithm such as the LBA->CHS conversion in C. Also, the compiler will automatically optimize your code (if -O0 is not set on the command line).

It's a bootloader and when you only have 512 bytes I take it it's hard to write it in C. This isn't stage2 yet.
This is it now:

Code: Select all

LBACHS:;ax = lba
inc ax
div BYTE[sectorspertrack]
push ax
mov [sector],ah
div  BYTE[numheads]
mov [cylinder],al
mov [head],ah
ret

EDIT:

The first error is the useless push ax instruction
the second is that this code is very limited,enought for a floppy but not for
a hard disk.the formula is ok.
this is the code i use:

Code: Select all

;eax=lba address
edx=0 ebx=w[esi+info\sectors_x_heads] div ebx ;lba->chs
  xchg edx,eax div b[esi+info\n_sectors] inc ah
;ah=sector number
;al=head
;edx=cylinder

And C is much easier than Assembly ... unless you are a assembly programmer.

[bloodhound23]

why for a hardisk? Maybe because I don't use the max cylinder info?

EDIT: push ax is left over from the old code I had that I revised
EDIT2: It's not hard to code it in C, just hard to keep it at 512 bytes.

[octavio]

why for a hardisk? Maybe because I don't use the max cylinder info?

EDIT: push ax is left over from the old code I had that I revised

Because the numbers are very small. After the first division
al=heads*cylinders but you need at least 16 bits just for the cylinder.
ej: a 4GB disk has 8912 cylindrs 15 heads and 63 sectors.
Also with modern hard disk you will have to use lba instead of chs.

[bloodhound23]

How do I use lba? Bios uses CHS.

[bloodhound23]

I truly don't get it after going through the INT list.

[bloodhound23]

Anyway would there be a problem if I used this to boot from USB?

[bloodhound23]

I just looked at the grub stage 1 source. Helps like you wouldn't belive