some FAQs - DiskOnChip, CompactFlash, IDE stuff.

I received a couple of questions - in private email so I won't quote it here, but I do want my answers to go to the list so hopefully I don't get asked again in the near future...

The questions were basically "is it possible to write a CompactFlash MTD driver" and "Why does the MTD driver for the DiskOnChip exist, since it supports INT 13h BIOS calls and works like a true IDE drive?"

The MTD drivers are for driving memory devices, including flash. Most of the code is for driving standard NOR flash chips attached (fairly) directly to the CPU's memory bus.

CompactFlash devices are not flash, as far as the computer is concerned - they _really_ are IDE drives, and electrically you cannot tell otherwise.

You may think they have flash chips inside them - we couldn't possibly comment. They may as well be alien technology which allowed the manufacturer to shrink a normal disk drive so it fits inside that package and stop you from hearing the whirring sounds.

CompactFlash devices are driven by the existing IDE drivers in Linux. They can operate in 'True IDE' mode, or they can operate as PCMCIA IDE devices - mostly the latter is used. (CompactFlash is also used as a term for a slightly smaller form factor of PCMCIA socket. It has nothing to do with actual flash in that case either :)

CompactFlash devices do all the wear levelling internally. Like FTL and NFTL, they have a kind of pseudo-filesystem which they use for emulating a block device with normal-sized (512 byte) sectors. But they do it entirely internally, and you cannot see inside.

The DiskOnChip devices are _not_ IDE devices. They are real NAND flash devices, connected to the CPU's memory bus via a clever ASIC - and the computer actually has to deal with the flash chips directly - the emulation of a hard drive is done in software, not in hardware as in the case of CompactFlash. Hence they're driven by a special device driver which is part of the MTD system.

The DiskOnChip devices come with their own special BIOS extension to drive the hardware (and the translation layer on the flash) and pretend to be a block device via the standard INT 13h DOS interface. But once Linux is running, that BIOS extension is useless. They have nothing to do with IDE. _Any_ block device has to provide an INT 13h interface to DOS, including SCSI drives and ramdisks.

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dwmw2

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