[syslinux] Modify memdisk to mask/overlay start of hard drive

Alexander Foken alexander at foken.de
Sat Oct 22 13:20:19 PDT 2011

On 10/21/2011 08:37 PM, Erik Jensen wrote:
> I have a somewhat unique problem I am trying to solve that requires me
> to temporarily replace the first 63s of the main hard drive and boot
> the result. Basically, I am hoping it is possible to modify memdisk to
> serve requests for the first 63 sectors from a ram disk and pass other
> requests through to the hard drive. There will never be a read
> spanning the two, so I don't need it to be able to handle that
> complexity. Also, it will alway be masking the first 63s of the first
> hard drive, so it doesn't need to be configurable.
> How complicated would it be to modify memdisk to support this? Any
> advice on where to make the change? I do have some programming
> experience, but none in assembly.
> Thanks
That highly depends on the operating system that accesses the harddisk. 
DOS uses BIOS INT 0x13 (see <URL:http://www.ctyme.com/intr/int-13.htm>), 
so installing your own INT 0x13 handler or hacking the one installed by 
memdisk should be sufficient. Other systems like Linux, *BSD, Windows, 
install their own set of drivers once the system has been loaded. For 
those systems, you would need to install a modified INT 0x13 handler for 
the boot phase, and a modified system-specific driver once the system 
switches to its own drivers.


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