Doh. The disklabel is not 0x200 bytes of zeros, but it is 0x200 bytes long.

sys/boot/i386/boot2/boot1.S

@@ -139,7 +139,7 @@ main.4: 	xor %dx,%dx			# Partition:drive
 # ahead and load up the first 16 sectors (boot1 + boot2) from that.  When
 # we read it in, we conveniently use 0x8c00 as our transfer buffer.  Thus,
 # boot1 ends up at 0x8c00, and boot2 starts at 0x8c00 + 0x200 = 0x8e00.
-# The first part of boot2 is the disklabel, which is 0x200 bytes of zeros.
+# The first part of boot2 is the disklabel, which is 0x200 bytes long.
 # The second part is BTX, which is thus loaded into 0x9000, which is where
 # it also runs from.  The boot2.bin binary starts right after the end of
 # BTX, so we have to figure out where the start of it is and then move the

sys/boot/i386/boot2/boot1.s

@@ -139,7 +139,7 @@ main.4: 	xor %dx,%dx			# Partition:drive
 # ahead and load up the first 16 sectors (boot1 + boot2) from that.  When
 # we read it in, we conveniently use 0x8c00 as our transfer buffer.  Thus,
 # boot1 ends up at 0x8c00, and boot2 starts at 0x8c00 + 0x200 = 0x8e00.
-# The first part of boot2 is the disklabel, which is 0x200 bytes of zeros.
+# The first part of boot2 is the disklabel, which is 0x200 bytes long.
 # The second part is BTX, which is thus loaded into 0x9000, which is where
 # it also runs from.  The boot2.bin binary starts right after the end of
 # BTX, so we have to figure out where the start of it is and then move the