Syslinux - Jun 2009 - [PATCH] [memdisk] Add some DriveShiftLimit logic

Let's try that patch again...  Here's a better one, instead.  I tested
with:

qemu -kernel memdisk -initrd memdsk.hdd -append pause -hda one.hdd -hdb
two.hdd -hdc three.hdd -hdd four.hdd

On each HDD image, I had a file (and volume label) to identify the HDD
with the DOS 'dir' command.  The expected behaviour was met; I wound up
with DOS drive letters:

C: memdsk.hdd
D: one.hdd
E: two.hdd
F: three.hdd
G: four.hdd

Always room for improvement, but seems to work.  I probe with INT 13h,
AH=15h.  One can enable a little debug message by #defining
DBG_DSKPROBE.

Enjoy!

- Shao Miller
-----Original Message-----
From: Miller, Shao 
Sent: Monday, June 01, 2009 16:48
To: For discussion of Syslinux and tftp-hpa
Subject: [PATCH] [memdisk] Add some DriveShiftLimit logic

Instead of MEMDISK always inserting itself and shifting all BIOS drives
up by
one, we are pursuing logic whereby MEMDISK's "installer" stage
will:

1. Decide which BIOS drive numer we desire
2. Probe for a contiguous range of already-present BIOS drives starting
with our desired number
3. Save the top-most drive number of this contiguous range
4. Install the INTerrupt hooks
5. Our INTerrupt code will only shift drive numbers within this range

Why?  As an example, suppose that we have BIOS drives 0x00, 0x80, 0x81,
0xE8.  If we wish to hook our MEMDISK disk as the new 0x80, we shift the
previous 0x80 to become 0x81, we shift the previous 0x81 to become 0x82,
but we leave 0xE8 alone, because it's outside of the contiguous range we
probed for starting at 0x80.  In this way, any system code already
installed and relying on drive 0xE8 will be left sane.  Previously,
drive 0xE8 would have been shifted to become 0xE9, possibly breaking
such system code. This is just an example.

This patch adds a DriveShiftLimit field to store the top-most drive
numer that we are interested in shifting.  The drive probing logic is
not fully implemented in this patch, but the "stage is set" for
further
refinement.

- Shao Miller
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