Groups | Search | Server Info | Keyboard shortcuts | Login | Register [http] [https] [nntp] [nntps]

Groups > alt.os.development > #8222

Re: BIOS limits on CD boot ?

Show all headers | View raw

"James Harris" <james.harris.1@gmail.com> wrote in message 
news:mm36sr$v7a$1@dont-email.me...
> "wolfgang kern" <nowhere@never.at> wrote in message 
> news:mm1i9d$f8r$1@speranza.aioe.org...

...

>> I may rely on 2K sectors, but would the BIOS understand the 
>> LBA-numbers ?
>
> From what I have read it seems that for optical discs (ODs) BIOS will 
> use 512-byte virtual sectors and that you should read them in groups 
> of 4 (to make the real 2048-byte OD sector sizes) so I guess that LBAs 
> will be in terms of 512-byte sectors too.

Strike that. Still without testing it but from further reading it seems 
that for either type of emulation (floppy or hard disk) you would read 
the emulated disk in 0.5k sectors - and probably via even the older BIOS 
int 0x13-02 interface, but without emulation you would read in 2k 
sectors - and via the extensions such as 0x13-42 and friends.

The El-Torito spec says that after booting with no emulation: "Once the 
system jumps to segment:0, the program can retrieve its boot information 
by issuing INT 13, Function 4B, AL=01. After the boot process has been 
initiated the INT 13 Extensions (functions 41-48) will access the CD 
using 800 byte sectors and the LBA address provided to INT 13 is an 
absolute sector number. This gives any program running in no emulation 
mode the ability to locate the boot catalog, and any other information 
on the CD, without providing a device driver."

That seems to say that the CD can be read (automatically using the most 
up-to-date session) in 2k blocks using the BIOS extensions.

I guess that the BIOS extensions would be present on any machine that 
could boot from OD. If nothing else, an OD-interface card could add the 
functions via a ROM. But such extensions may not be present on an older 
PC when the OD is added purely as a read-only data drive.

I have no idea how to access an OD from such a machine but MSCDEX and 
similar may have done something like this. Based on some RBIL comments 
about NWCDEX maybe such programs provided the int 0x2f-15 functions but 
that doesn't explain how they would have read the ODs in the first 
place.

James

Back to alt.os.development | Previous | Next — Previous in thread | Next in thread | Find similar | Unroll thread

Thread

BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-19 13:44 +0200
  Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-06-19 14:15 +0100
    Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-19 19:07 +0200
      Re: BIOS limits on CD boot ? Melzzzzz <mel@zzzzz.com> - 2015-06-19 19:22 +0200
        Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-20 08:50 +0200
      Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-06-20 09:07 +0100
        Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-06-20 15:43 +0100
          Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-21 06:10 +0200
            Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-06-21 18:13 +0100
              Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-22 09:55 +0200
                Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-06-22 12:04 +0100
                Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-30 20:03 +0200
                Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-07-01 11:19 +0100
  Re: BIOS limits on CD boot ? "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-06-19 10:56 -0700
    Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-20 08:55 +0200
  Re: BIOS limits on CD boot ? "Rod Pemberton" <boo@fasdfrewar.cdm> - 2015-06-19 18:15 -0400
    Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-20 09:18 +0200
      Re: BIOS limits on CD boot ? "Benjamin David Lunt" <zfysz@fysnet.net> - 2015-06-20 07:43 -0700
  Re: BIOS limits on CD boot ? "wolfgang kern" <nowhere@never.at> - 2015-06-21 16:04 +0200
    Re: BIOS limits on CD boot ? "James Harris" <james.harris.1@gmail.com> - 2015-06-22 12:13 +0100

csiph-web