Which BIOS function can read/writes USB solid disk? How BIOS boot USB disk?

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If the PC's BIOS supports it (and I assume we're talking about a traditional PC, and a traditional - aka non-EFI - BIOS here), then the USB disk (which might, after all be a real rotating disk too) will be accessible through the usual Int 13h functions. And it boots the same way.

That's about the same answer I gave to this question, in another group.

sk?

How can I know its disk number, which will be passed to BIOS in DL when an int 13h function is called ?

disk?

It's not very consistent, unfortunately, but if you have the emulation turned on in your BIOS, it's most commonly a hard disk. In some BIOS's it's configurable - you can set the BIOS to emulate either a floppy or HDD. If it's the boot device, it'll be 0x00 or 0x80. And remember there can be more than one.