Add a description about how HDX should be used when preparing a NetBSD-bootable

disk using AHDI partitioning. (From Udo Erdelhoff).

distrib/notes/atari/prep

@@ -7,6 +7,10 @@ therefore advised that you:
 
 	Back up the partitions you are keeping.
 
+If NetBSD has a disk of it's own, you can delay the partitioning until
+the installer requests you to do it. This means that you can safely skip
+the rest of this section.
+
 If NetBSD has to share the disk with another operating system, you must
 take care of partitioning your harddisk before installing NetBSD; creating
 space for at least root, swap and /usr partitions and possibly at
@@ -16,6 +20,66 @@ Warning: The AHDI partioning function erases all partions on your harddisk
          even if they are not changed!
          I know this is rather stupid, but don't say I didn't warn you.
 
+If you want to use an AHDI partitioning sceme and you want to be able to boot
+directly into NetBSD, there are some constraints on the partition layout.
 
-If NetBSD has a disk of it's own, you can delay the partitioning until
-the installer requests you to do it.
+As you might know; every hard disk has a "root sector" that contains
+information about the size of the hard disk and the partitions on the hard
+disk. The root sector can only contain the neccessary data for four
+partitions. Nobody thought that this limitation would cause any problems.
+After all, 640 KByte should be enough.  As hard disk grew, it was neccessary
+to define more than four partitions. In order to be more or less compatible
+with the old format, a new type of partition entry was defined: XGM partions. 
+
+An XGM partition is a "look over there" sign: Another root sector can be
+found at the start of the XGM partition. This root sector contains the
+remaining real partitions.  And this is the big mystery: Partitions defined
+in the root sector of the hard disk are called "primary partitions",
+partitions defined in the root sector of an XGM partition are called
+"extended partitions".
+
+The bootblock will only work if the first NBD partition is a primary
+partition. This is not a limitation of NetBSD but a limitation of TOS/AHDI:
+You can only boot from primary partitions.
+
+If you are creating your partitions with HDX, you'll have to be very careful
+to fulfill this rule. HDX has some very strange ideas when it comes to
+extended partitions. Fortunately, you can edit this stuff: The
+"Edit partition scheme of the unit" dialog box has a button label "expert".
+This button is inactive unless you have defined more than four partitions.
+Click on it *after* you have defined the sizes of the partitions.
+
+A new dialog box appears on the screen. The left side contains two blocks of
+partitions: The upper block always contains the first four partitions, the
+lower block contains the last three partitions. If you have defined less than
+7 partitions, some fields of the lower block will contain the string "unused".
+Some of the partitions will be displayed in reverse video: These are the
+extended partitions.
+
+The right side contains six possible ranges for the extended partitions. It
+is not possible to define your own range, you will have to use one of the
+schemes offered by HDX. To quote from Ghostbusters: Choose and die.
+The default scheme used by HDX is the first scheme: Extended partitions start
+with the second partition and end with the second to last partition. If you
+have defined 7 partitions, partitions #2 to #5 will be extended partitions,
+while partitions #1, #6 and #7 will be primary partitions.
+
+You can move the extended partition range by clicking on one of the buttons
+on the right side of the dalog box. Try to find one where your first NetBSD
+partition is a primary partition. Golden rules:
+    * If the disk contains no GEMDOS partitions, don't use AHDI. Let NetBSD
+      handle it alone.
+    * If the disk contains one GEMDOS partition, make it partition #1 and
+      start the extended partition range at partition #3. This allows you
+      to boot from both the GEMDOS and the NetBSD partitions.
+    * If the disk contains two GEMDOS partitions, use partitions #1 and #2
+      for GEMDOS, partition #3 for NetBSD-root. Start the extended partition
+      range with partition #4.
+    * If your disks contains three or more GEMDOS partitions, you are in
+      trouble. Try using partitions #1 and #2 as the first two GEMDOS
+      partitions. Use partition #3 as the first NetBSD partition. Start the
+      extended partition range with partition #4. Put the other NetBSD
+      extended partition range.
+
+Good luck, you'll need it...
+