Hard disk real estate is divided into many small storage units called sectors (also called as blocks). Each sector is 512 bytes in size. Note that this is completely different from cluster size. Cluster size is a result of the FAT file system. Sectors are fixed at 512 bytes and can actually be considered as the minimum allocation unit. But because of the sheer number of allocation units that would need to be tracked, it's impossible to use the sector size as the cluster size. So, a cluster will usually consist of multiple sectors. However, that's a story for another time. :)

The original IDE protocol allowed for the transfer of a single sector of data (not cluster) per interrupt. This means that if the cluster size of a partition on your hard disk is 16KB, reading a single cluster from that partition will require at least 32 interrupts to complete! This transfer method is called single sector transfer and as you can see, it takes up a lot of resources.

Realizing this, hardware engineers looked for a solution. Since a cluster normally consists of several sectors, the minimum number of sectors transferable by the operating system per data request would be the number of sectors per cluster. This is because the operating system reads and writes data in clusters, and not sectors. Usually, several clusters are transferred at one go. So, it makes sense to transfer multiple sectors using only a single interrupt. After all as stated earlier, the operating system transfers data not in sectors, but in clusters (which translates into multiple sectors).

Hence, the birth of the multiple sectors transfer mode or more popularly known as block mode transfer. By allowing up to 64KB of data to be transferred per interrupt, this feature speeds up hard disk access and reduces overhead. This feature can be toggled through the BIOS option called IDE Block Mode or its many other incarnations like IDE HDD Block Mode and Multiple Sectors Transfer. However, not everyone will be able to use this transfer mode.

Example of a BIOS option for the IDE Block Mode feature (boxed in red)

Requirements

First of all, the motherboard chipset must support such a feature. To be more specific, the IDE controller that's integrated with practically every shipping chipset. If the integrated IDE controller does not support IDE Block Mode, you can circumvent that problem by using a separate IDE controller that supports IDE Block Mode instead. Since every chipset now supports IDE Block Mode transfers, this is only necessary if you are using a motherboard with an old IDE controller that does not support IDE Block Mode transfers.

There also ought to be a BIOS option that allows you to toggle the setting on or off. Although it's not necessary for that BIOS option to be there, most motherboard manufacturers include it for troubleshooting purposes (more on this later). In motherboards that don't come with such a BIOS setting, the feature will have been hardwired as enabled or most likely disabled.

Why disabled? Well, it makes sense for the manufacturer to disable it by default instead of enabling it. Without a BIOS toggle, it would be impossible for the user to disable the setting if the manufacturer hardwired it as enabled. This means in the event of an incompatibility with the multiple sector transfer mode, it would be impossible to use the onboard IDE controller. Thus, it's safer for the manufacturer to disable IDE Block Mode if they don't include the BIOS option to toggle its operation.

Finally, all the hard disks in the system must support IDE Block Mode. This is because while you can select whether to enable or disable IDE Block Mode, you still can't enable it for one IDE channel and disable it for another. Currently, enabling IDE Block Mode in the BIOS means enabling the feature for the onboard IDE controllers and thus all the IDE drives running off them; and vice versa.

Since all current hard disks support IDE Block Mode, that shouldn't be a problem for most people. But if you own an old hard disk that does not support IDE Block Mode however, there's still no need to worry. As far as I'm aware of, the hard disk will still function normally. Of course, it now appears that the smart thing to do would be to enable IDE Block Mode if there's a BIOS option for it since the hard disk will still work whether it actually supports IDE Block Mode or otherwise. Though this may be true for most users, there may be a critical issue with this feature that you will need to be aware of.