i386/63628: patch to i.386 master boot record to allow boot from USB Flash drive

[Alan Robson]

>Number:         63628
>Category:       i386
>Synopsis:       patch to i.386 master boot record to allow boot from USB Flash drive
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    freebsd-i386
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          change-request
>Submitter-Id:   current-users
>Arrival-Date:   Mon Mar 01 20:20:12 PST 2004
>Closed-Date:
>Last-Modified:
>Originator:     Alan Robson
>Release:        5.2 Release
>Organization:
>Environment:
FreeBSD planb.local 5.2-RELEASE FreeBSD 5.2-RELEASE #0: Sun Jan 11 04:21:45 GMT 2004     root at wv1u.btc.adaptec.com:/usr/obj/usr/src/sys/GENERIC  i386

>Description:
For some reason, the BIOS will not load a partition boot record from a flash USB "thumb" drive using the Cylinder, Head & Sector values in the MBR in an int13/02 BIOS call. Instead and LBA call should be used
  
>How-To-Repeat:
install the minimal FreeBSD installation to a USB Memory stick from the distribution CD and try to boot from it. The boot always fails, even when the bios supports booting from a USB device.
   
>Fix:
Modify the mbr.s file to try LBA as the first resort, then fall back to CHS if that fails. My system works with the following diff -u patch to mbr.s, I have no way to test it on other equipment.
 
--- mbr.s.origional     Sun Feb 29 20:58:05 2004
+++ mbr.s       Sun Feb 29 21:20:40 2004
@@ -89,9 +89,7 @@
                movw 0x2(%si),%cx               # Load cylinder:sector
                movw $LOAD,%bx                  # Transfer buffer
                cmpb $0xff,%dh                  # Might we need to use LBA?
-               jnz main.7                      # No.
                cmpw $0xffff,%cx                # Do we need to use LBA?
-               jnz main.7                      # No.
                pushw %cx                       # Save %cx
                pushw %bx                       # Save %bx
                movw $0x55aa,%bx                # Magic
>Release-Note:
>Audit-Trail:
>Unformatted: