Originally Posted by alexh

I was replying to someone over at Amiga.org about MAX TRANSFER settings for IDE controllers (such as A1200/A600 Gayle IDE) giving them the standard "try 0x1FE00 and if that doesnt work try 0xFE00".

While writing I accidentally worked out the logical reason behind these "Magic numbers".

• IDE commands (ATA5 or lower) have "sector count" field which is 8-bits.
• SCSI commands (SCSI10) have a "transfer size" field which is 16-bits.

For those who do not know, a sector is the smallest individual amount you can access on a Hard drive and has always been (until recently) 512 bytes. And "sector count / transfer size" is the amount to be transferred in a single command.

I noticed that "0x1FE00" is 255 * 512, and that 255 is the largest value that can be represented by 8-bits. So by using "0x1FE00" it prevents the driver (scsi.device) creating malformed IDE commands with sector counts larger than 8-bit.

I am positive this is the logical answer as to why MAX TRANSFER should be "0x1FE00" for IDE devices.------------