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Re: BIOS limits on CD boot ?

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"James Harris" <james.harris.1@gmail.com> wrote in message 
news:mm3qte$20v$1@dont-email.me...
>
> The relevant sources and how I understand they fit together are:
>
> * ISO 9660 which descibes the basic layout of a data CD: 2k sectors, 8.3 
> file names which have a highly restricted character set - apparently to 
> match MS-DOS - such as no lower case. I think ISO 9660 has no standard use 
> for the first 16 real (i.e. 2k) blocks are reserved, i.e. the first 32k. 
> As a consequence that's a wild west area a bit like the space between MBR 
> and first volume on a hard disk.
>
> * The first part of the standard is real sector 16 (i.e. immediately after 
> the above area). It is the first of a series of 2k volume descriptors. 
> There are different types of volumes, one of which is a boot record and 
> another of which is a terminator which marks the end of the series.
>
> * Rock Ridge is an extension to provide POSIX permissions and to let ISO 
> 9660 files have better names - up to 255 chars and lower case etc.
>
> * Joliet is also to provide better names (with 16-bit chars) but 
> apparently via a separate directory tree. Being separate its entries may 
> not match one-to-one with the ISO 9660 files.
>
> * CDs are recorded in sessions. Each session subsumes info from the 
> previous one in that it at least provides a new and updated set of 
> structures and directories etc. I think that when reading a CD something 
> has to know to start from the beginning of the last session rather than 
> the beginning of the medium. I don't know how different BIOS calls deal 
> with that.
>
> * BIOSes will assign a standard 8-bit id to an optical disc. It seems that 
> the one booted from should always be 0x80 if emulating a hard disk, 0 if 
> emulating a floppy, and 0x81 or above if booting without emulation.
>
> * There are various BIOS extension interrupt calls for CD reads. IIRC I 
> saw int 0x13-48 (get drive parms) mentioned as working with CDs, and 0x4a 
> to 0x4d specifically mention CD ROM boot.
>
> * El-Torito is a standard for making CD's bootable. That's a fun doc in 
> that all numbers therein are in hex. Where it says 800 it means 2k, for 
> example. I'll stick to decimal.
>
> El-Torito says that the boot record must be in real sector 17 (i.e. just 
> after the volume record in sector 16) of the last session on the disc. 
> This 2k boot record points to a boot catalogue which is a series of 
> 32-byte entries, each describing a way to boot from that disc.
>
> Ways to boot are: floppy emulation, hard disk emulation, and native (i.e. 
> non-emulated). The latter allows the BIOS to load more than 0.5k of boot 
> code. The type of boot seems to be controlled by the boot medium type 
> which can be any of three types of floppy, hard disk, or none.
>
> That's about all I know. As I say, someone may be able to add to that and 
> there is lots of good info on osdev.org and Wikipedia.

From what I remember, you pretty much got it spot on.

Ben

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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

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