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alexh · 16 June 2009, 14:49

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.

------------

Now while the IDE (ATA5 and lower) command set supports an 8-bit sector count, very few operating systems use the maximum value. Windows (and most other operating systems) tend to use 64 or 128 sector commands. I know this because I discovered at work several years ago that PATA hard drives have their cache optimised for 64 sectors while SATA were optimised for 128 sector commands.

Back to the plot:

The second magic number touted for MAX transfer "0xFE00" is 127 * 512. I am not sure exactly why this value would be more compatible than 0x1FE00, but I imagine it has to do with the fact poor quality IDE devices do not expect sector counts above 128 sectors.

You pretty much guarantee "sector count" compatibility with any IDE device with 0xFE00 and I cannot believe that it is slower.

Anyhow, hope that has explained things, rather than confused things.

16 June 2009, 14:49	#6
alexh Thalion Webshrine Join Date: Jan 2004 Location: Oxford Posts: 14,354	IDE & Max Transfer 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. ------------ Now while the IDE (ATA5 and lower) command set supports an 8-bit sector count, very few operating systems use the maximum value. Windows (and most other operating systems) tend to use 64 or 128 sector commands. I know this because I discovered at work several years ago that PATA hard drives have their cache optimised for 64 sectors while SATA were optimised for 128 sector commands. Back to the plot: The second magic number touted for MAX transfer "0xFE00" is 127 * 512. I am not sure exactly why this value would be more compatible than 0x1FE00, but I imagine it has to do with the fact poor quality IDE devices do not expect sector counts above 128 sectors. You pretty much guarantee "sector count" compatibility with any IDE device with 0xFE00 and I cannot believe that it is slower. Anyhow, hope that has explained things, rather than confused things. Last edited by Jope; 04 December 2010 at 10:16.