grus4dos : map --mem option

map --mem option
https://github.com/chenall/grub4dos/blob/master/README_GRUB4DOS.txt#L3136 

***                  The CDROM emulation (virtualization)               ***
*****************************************************************

The CDROM emulation is sometimes called ISO emulation. Here is an example:

map  (hd0,0)/myiso.iso  (hd32)
map  --hook
chainloader  (hd32)
boot

if myiso.iso is not contiguous and you have enough memory, add a --mem option:

map  --mem  (hd0,0)/myiso.iso  (hd32)
map  --hook
chainloader  (hd32)
boot

Note: (hd32) is a grub drive number equivalent to (0xA0). If a virtual drive is
specified with a drive number greater than or equal to 0xA0, then it will be
treated as a cdrom (i.e., with 2048-byte big sectors).

Like normal disk emulations, the CDROM emulation also (mainly) works with
real-mode OSes. After a protected-mode OS kernel (such as
WinNT/2K/XP/VISTA/LINUX) gains control, the OS would have no ability to access
the virtual CDROM through BIOS int13.

DOS/Win9x users may google for ELTORITO.SYS and use it in CONFIG.SYS as a
device driver for the virtual cdrom.

Example usage of eltorito.sys in CONFIG.SYS:

device=eltorito.sys /D:oemcd001

Corresponding MSCDEX command which can be placed in AUTOEXEC.BAT:

MSCDEX /D:oemcd001 /L:D


Due to some bugs found in eltorito.sys, the driver could fail to load. If you
encounter such problems, then you may replace (hd32) with (0xFF) for the
virtual cdrom drive number and try again.