prep(8): document search for 9LOAD
 [rsc] --rw-rw-r-- M 1328934 glenda sys 13874 Mar 28 12:55 sys/man/8/prep
	/n/sourcesdump/2006/0328/plan9/sys/man/8/prep:600,609 - 
	/n/sourcesdump/2006/0329/plan9/sys/man/8/prep:600,610
	  .I bfile
	  to make the disk a boot disk.
	  It gets loaded by the BIOS at 0x7C00,
	- reads the root directory into address 0x7E00, and looks at
	- the first root directory entry.
	- If that file is called
	- .BR 9LOAD ,
	+ reads the first sector of the
	+ root directory into address 0x7E00, and looks for
	+ a directory entry named 
	+ .BR 9LOAD .
	+ If it finds such an entry, 
	  it uses
	  single sector reads to load the file into address 0x10000 and then
	  jumps to the loaded file image.
	/n/sourcesdump/2006/0328/plan9/sys/man/8/prep:610,616 - 
	/n/sourcesdump/2006/0329/plan9/sys/man/8/prep:611,617
	  The file
	  .B /386/pbslba
	  is similar, but because it uses LBA addressing (not supported
	- by all BIOSes), it can access more than the first 8.5GB of the disk.
	+ by older BIOSes), it can access more than the first 8.5GB of the disk.
	  .PP
	  .I Mbr
	  installs a new boot block in sector 0 (the master boot record)