Fix to make making a bootable cd possible again.

ow that the image has grown beyond the 1.44M that fits on a floppy.
(previously, the floppy emulation mode was used for cd's.)

the boot cd now uses 'no emulation mode,' where an image is provided on
the cd that is loaded and executed directly. this is the boot monitor.

in order to make this work (the entry point is the same as where the
image is loaded, and the boot monitor needs its a.out header too) and
keep compatability with the same code being used for regular booting, i
prepended 16 bytes that jumps over its header so execution can start
there.

to be able to read the CD (mostly in order to read the boot image),
boot has to use the already present 'extended read' call, but address
the CD using 2k sectors.

boot/Makefile

@@ -36,9 +36,16 @@ rawfs86.o:	rawfs.c rawfs.o
 	-cmp -s rawfs.o rawfs86.o && ln -f rawfs.o rawfs86.o
 
 boot:	boothead.s boot.o bootimage.o rawfs86.o
-	$(LD86) -o $@ \
+	$(LD86) -o bootexec \
 		boothead.s boot.o bootimage.o rawfs86.o $(LIBS)
-	install -S 8kb boot
+	install -S 12kb bootexec
+	# This is code that is executed when used on a bootable
+	# CD, as its entry point is the start of the file then.
+	# It jumps over the a.out header into the part of the
+	# code in boothead.s where the code knows it's booting
+	# from CD if entered there.
+	( printf '\xeb\x3e              ' ; cat bootexec ) >boot
+	chmod 755 boot
 
 edparams.o:	boot.c
 	ln -f boot.c edparams.c
@@ -112,5 +119,5 @@ $(BIN)/edparams:	edparams
 
 clean:
 	rm -f *.bak *.o
-	rm -f bootblock addaout installboot boot masterboot jumpboot edparams 
+	rm -f bootblock addaout installboot boot masterboot jumpboot edparams
 	rm -f dosboot boot.com mkfile mkfile.com