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b1528932



Joined: 21 May 2010
Posts: 287
b1528932
i have 2 boot devices:
- pata cdrom
- floppy

when i boot from floppy, im able to replace my floppy image, i have control over A:\
when i boot from cd, i have no control over a:\. It contain some programs, wich came i dont know from where.


Could you explain me this behaviour?
Post 14 Jan 2011, 15:32
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edfed



Joined: 20 Feb 2006
Posts: 4238
Location: 2018
edfed
i have the same problem on my aspire one, when it boots on usb, it don't see anything else than A: and B: as the same usb pen.

even other usb drives are not recognised, ssd is absent, and SDcard too.

but bios shows them in the boot devices.

i think it is due to the bloatness of the modern PC world, they add features over features, instead of restating from scratch.

it is the reason why manufacturers can sold hw so cheap, and build new versions so fast.

it is elementary economy, applied to boot process. Very Happy
Post 14 Jan 2011, 16:07
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Dex4u



Joined: 08 Feb 2005
Posts: 1601
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Dex4u
Its very simple, to boot from cd you emulate a floppy or hdd, so all int 13h for floppy are pass to the floppy image on cd.

To make a cd bootable you useally need to use a floppy img and that where the apps are.
You may beable to see them with winimage ?
Post 14 Jan 2011, 19:26
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b1528932



Joined: 21 May 2010
Posts: 287
b1528932
i dont get it. Why i emulate? Isnt hd and cd on same bus? Meaning its working in same manner? CD have bootsector, there is no real diffrence between cd and hdd and floppy
Post 14 Jan 2011, 20:01
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Mike Gonta



Joined: 26 Dec 2010
Posts: 238
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Mike Gonta
b1528932 wrote:
i dont get it. Why i emulate? Isnt hd and cd on same bus? Meaning its working in same manner? CD have bootsector, there is no real diffrence between cd and hdd and floppy
Here is the reference document.
“El Torito” Bootable CD-ROM Format Specification

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Post 14 Jan 2011, 22:01
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edfed



Joined: 20 Feb 2006
Posts: 4238
Location: 2018
edfed
hem...
and for normal CDs like good old audio cd?
how does it work?

is there a boot sector?
something like this? because, if you look well, CD ROM is a SCSI device, using IDE port on a PATA ribbon cable, with a compatibility to audio CDs, then, i doubt about the floppy emulation...
because if you look well, you see that a CD rom is in fact a hard disk, with some lba adressing, N bytes per sectors, N sectors. for a total of 700MB, or 74 minutes of a symphony, readable even on a very basic CD player.

it is that strange thing that lead to confusion i think.
Post 14 Jan 2011, 22:04
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Mike Gonta



Joined: 26 Dec 2010
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Mike Gonta
b1528932 wrote:

when i boot from cd, i have no control over a:\. It contain some programs, wich came i dont know from where. Could you explain me this behaviour?
'El Torito' Bootable CD-ROM Format Specification wrote:
If the CD-ROM boots as the A drive the systems normal A drive will become the B drive. If the system has a
B drive it will become inaccessible.
"El Torito" Bootable CD-ROM Format Specification

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Post 14 Jan 2011, 22:07
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me239



Joined: 06 Jan 2011
Posts: 200
me239
This may sound stupid, but wouldn't a CD-R not be changeable no matter what the circumstances. If that's case.
Post 14 Jan 2011, 23:30
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b1528932



Joined: 21 May 2010
Posts: 287
b1528932
Ok, b is actually there. But i bet i checked agnist this scenario before posting...

Quote:
and for normal CDs like good old audio cd?
how does it work?

is there a boot sector?

isnt there?
cd/dvd sector is something 23XX bytes large, and i can use either all of bytes for data, or reserve space for error correction data. When mounting audio cd i will just have information about tracks where data is stored, no filesystem.

And i do have a question wich been bothering me for some time.
using bios int 13 i can read data from drive. How does actual data transfer is managed?
Can i transfer 'at once' no less than 1 sector? Is the sector some sort of atom in nonvolatile media?

Quote:
CD ROM is a SCSI device, using IDE port on a PATA ribbon cable

what? it doesnt make sense to me. Could you be more precise? For me, scsi and pata are just cable types and interfaces used to transmit data. And now your telling me that 'scsi' is a generic device... wtf!
Post 15 Jan 2011, 08:29
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ManOfSteel



Joined: 02 Feb 2005
Posts: 1154
ManOfSteel
b1528932 wrote:
Can i transfer 'at once' no less than 1 sector?

If you poll the disk using the in instruction, probably. But using the BIOS's int 0x13? No, I don't think so.

b1528932 wrote:

edfed wrote:
CD ROM is a SCSI device, using IDE port on a PATA ribbon cable

what? it doesnt make sense to me. Could you be more precise? For me, scsi and pata are just cable types and interfaces used to transmit data. And now your telling me that 'scsi' is a generic device... wtf!

ATAPI devices speak SCSI on a command/response-level.
In the beginning, there were only SCSI CD-ROM devices, which were very expensive. So they started to make ATA CD-ROM devices. They practically emulated SCSI devices and just gave them ATA interfaces and cabling.
Post 15 Jan 2011, 14:01
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b1528932



Joined: 21 May 2010
Posts: 287
b1528932
So scsi is not just cable, its also a language.
Is it true, that even usb device might speak scsi so OS will show it as a cdrom or something else?
Post 15 Jan 2011, 18:15
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ManOfSteel



Joined: 02 Feb 2005
Posts: 1154
ManOfSteel
b1528932 wrote:
So scsi is not just cable, its also a language.

It's not a language. It's an interface. And like all interfaces it has commands that make an interaction possible. Just like ATA has its own commands.

b1528932 wrote:
Is it true, that even usb device might speak scsi so OS will show it as a cdrom or something else?

The computer (BIOS) itself can see it as different possible devices. For example, if I were to enter my machine's BIOS boot devices settings, I would see the USB pendrive I just inserted listed as an internal disk right under my 2 real internal disks.
Post 15 Jan 2011, 19:52
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