Disk access

[thejpster]

In 5d885cd I took out disk access from the BIOS API. On reflection, I think that might have been a mistake. This is because:

• Some BIOSes will load the OS from disk, not find it in Flash. Therefore some BIOSes need disk access routines anyway.
• Putting disk access at the OS layer makes the OS care whether you have SDMMC interface or an SPI interface to your SD card.
• Putting disk access at the OS layer means the OS doesn't know how to read a Compact Flash card, or an IDE hard drive.

Basic enumeration of fixed and removable disks seems reasonable, along with functions to read/write 1 or more 512 byte sectors seems quite reasonable. Limiting to 2TB per device (2^32 sectors of 512 bytes each) also seems fine.

The downsides will be:

• Your BIOS will need a way (e.g. a BIOS set-up program) to specify where the SD card lives and how many you have. Press F10 on boot!

[thejpster]

Disk access has gone back into the BIOS API and so the book needs updating.