Makefile for building a bunch of .asm files into bootable DOS floppies

Makefile

#
# Builds all .asm files in src/ into .com files in build/
#
# For each built .com file in build/, a floppy image is generated in build/
# containing the program, as well as an autoexec.bat to run the program and
# shut down the computer on exit
#
# Requires a bootable DOS floppy image as a base (BASEFLOPPY)
#
# To have the autoexec.bat run the program under debug, use:
#
#   make DEBUG=1
#
# To run the generated floppy images, qemu may be used like this:
#
#   qemu-system-i386 -fda build/foo.img -boot order=a -nographic
#

ASM=nasm
MCOPY=mcopy
ZCAT=zcat

# should autoexec run the program with debug?
DEBUG?=0

# floppy to use as a base for creating the floppy - may contain useful
# stuff like a bootable dos and debug.com
BASEFLOPPY=../wread/dos622base.img.gz

BUILD_DIR := build
SRC_DIR := src
SRC_FILES := $(wildcard $(SRC_DIR)/*.asm)
COM_FILES := $(patsubst $(SRC_DIR)/%.asm,$(BUILD_DIR)/%.com,$(SRC_FILES))
IMG_FILES := $(patsubst $(SRC_DIR)/%.asm,$(BUILD_DIR)/%.img,$(SRC_FILES))

PROG_PREFIX=
ifeq '$(DEBUG)' '1'
PROG_PREFIX=debug 
endif

.PHONY: all clean
.PRECIOUS: $(BUILD_DIR)/%.com

all: $(IMG_FILES)

# bootable DOS floppy image
# requires the base image and mtools
$(BUILD_DIR)/%.img: $(BUILD_DIR)/%.com $(BUILD_DIR)/%.bat $(BUILD_DIR)/shut.com
	$(ZCAT) $(BASEFLOPPY) > $@
	$(MCOPY) -D o -i $@ $(word 2,$^) ::autoexec.bat
	$(MCOPY) -i $@ $(word 3,$^) ::shut.com
	$(MCOPY) -i $@ $< ::$(<F)

# autoexec.bat tailored to run our program
$(BUILD_DIR)/%.bat: $(BUILD_DIR)/%.com
	printf "@echo off\r\nprompt $$p$$g\r\n$(PROG_PREFIX)$(<F)\r\nshut" > $@

# build the program itself into a com file
$(BUILD_DIR)/%.com: $(SRC_DIR)/%.asm
	nasm -w+error -f bin -l $(BUILD_DIR)/$*.lst -o $@ $<

# small com program to shut down the computer by int 15h
# http://www.delorie.com/djgpp/doc/rbinter/id/08/14.html
$(BUILD_DIR)/shut.com:
	printf '\270\7\123\273\1\0\271\3\0\315\25' > $@

clean:
	rm -f $(BUILD_DIR)/*.com $(BUILD_DIR)/*.lst $(BUILD_DIR)/*.img