bios8086: implement CHS disk i/o using int13h

Makefile

@@ -1,4 +1,5 @@
 FLOPPY = 360
+BIOS = 0xFA00
 
 PROGRAMS = $(patsubst %.asm,%.com,$(wildcard com/*.asm))
 DISTFILES = rdos.bin $(PROGRAMS)
@@ -10,6 +11,7 @@ QEMU_ARGS = $(addprefix --option-rom ,$(ROMS))
 
 NASM = nasm
 NASM_ARGS = -s -Ilib -Plib/nasmcomp.asm -w-macro-params
+NASM_ARGS += -DBIOS=$(BIOS)
 
 EMUL = utils/emul
 
@@ -46,6 +48,9 @@ rom/%.rom: rom/%.asm
 com/%.com: com/%.asm
 	$(NASM) $(NASM_ARGS) -o $@ $<
 
+bios%.bin: bios/%.asm
+	$(NASM) $(NASM_ARGS) -o $@ $<
+
 fdimage.img: boot/fat.bs $(DISTFILES)
 	mformat -C -i $@ -f $(FLOPPY) -B boot/fat.bs ::
 	mcopy -i $@ $(DISTFILES) ::