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

[Erik Jensen]

On Sat, Oct 22, 2011 at 1:20 PM, Alexander Foken <alexander at foken.de> wrote:
>
> 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.
>
> Alexander

Only the boot loader needs to see the overlay. Once the OS enters
protected mode, it doesn't matter.