i had remembered the first time i bought a pc, the tech said that i should use separate ide cable for hdd and cd drives. but sometimes i see pc with hdd and cd rom sharing one ide cable.

when is it possible to share ide cable for hdd and cdrom? but when is it not recommended? ;)

My understanding is...

There's no problem sharing a cable; it's sharing the controller that the cable is connected to that's the problem.
CD-drives are slower than HDD's.
Both drives on the controller must take turns; they don't work at the same time.
If you put a slow drive on the same controller as a fast drive, then the time the fast drive must wait, whilst waiting for the slow drive to complete its task, has a significant impact upon its performance.

The ONLY time I will allow a system to share the ide cable/channel with a cd-rom or other slow device
is When NOTHING else is available ..
IE: the first ide channel is full and you need to add a 3rd HDD and a CD_rom

Other wise Like Sylvander said it creates problems with performance

Yep, I'm the same as Rick, I NEVER put a hard drive and CD ROM on the same cable unless I have no choice, for perfrmance reasons. The IDE controller can work no faster than the slower device and the CD ROM is slower even than the older ATA 33 drives...it wll slow down the system, at least during drive accesses. I don't think graphics or sound are affected, but disk access certainly is.