[syslinux] some issues with modules/syslinux

[H. Peter Anvin]

On 01/15/2012 03:28 PM, Bernd Blaauw wrote:
> Dear Syslinux bootloader list,
> 
> I'm running into a few issues concerning Syslinux modules here, hoping
> you can answer one or more of the following questions:
> 
> * Where's "ifmemdsk.c32" ? 4.05, 4.06pre1 and latest 4.10pre seem to
> lack it. Only the "ifmemdsk.c" file, which is likely sourcecode. Not
> seeing any "ifmemdsk.cbt" legacy comboot module.
> 

ifmemdsk isn't a COM32 module, it's a DOS program.

> * Do COM32 modules have their own memory requirements? Menu.c32 fails on
> a 2MB QEMU system, same for CHAIN.C32, yet Isolinux (and Syslinux) load
> fine if no com32 is loaded. Memdisk also loads fine (only tested with
> 360K floppy image).
> I haven't tried 1MB QEMU (hangs on win32 due to either QEMU or SeaBIOS
> or bug) nor 0MB system (Coreboot with Cache-As-Ram in theory).

Yes, like anything else they do.

> * There doesn't seem to be a fallback to text-only mode for the UI
> parameter in case of low memory situations. Physical systems have 4MB
> already at the very least, but loading an ISO into RAM followed by
> booting it can get you into situations where there's just enough memory
> to load isolinux.bin/cfg but not any COM32 module, thus also not GUI.
> Thus, hang. (and, relying on localboot 0x00/0x80 instead of CHAIN.C32).
> Trying with 3MB or more seemed to work just fine

The problem is that by the time you have loaded the module you're
already out of memroy.  The fallback pretty much is the CLI.  You're
talking about memory sizes that hasn't been seen on mainstream hardware
for 20 years.

> * is there a working version of memdisk.c32 [http://reboot.pro/14468/] ?
> COM32 modules seem to be version-dependant and trying this with 4.05
> ended up with "not a COM32R image" so I guess it's written for 4.04.
> 
> * How's Syslinux influencing gate A20? And MEMDISK? They both seem to
> result in different A20 access methods being displayed by various memory
> managers under FreeDOS. One of them actually manages to crash
> unfortunately.

Syslinux opens the A20 gate and leaves it open.  MEMDISK has to flip it
as necessary to access high memory, depending on the exact mode the
machine is in.

	-hpa