Please explain this makefile

[jason7007]

Hi Gurus,
I am very new to OS dev, and very curious about building my own os.
Recently I was studying the code Christopher Giese on his Cosmos 10 OS but I got stucked on his makefile, I could not understand it. I already learned about DJGPP and Nasm.
Can somebody teach me or explain this Makefile line by line to me.
I would appreciate it so much.
Thank u very much in advance.


# defines
.SUFFIXES: .asm .x .exe
MAKEFILE=makefile
# uncomment the next line to rebuild everything if this makefile changes
#MAKEDEP=$(MAKEFILE)
# chose ELF, COFF, or PE in the next two lines
LSCRIPT   =ldscript/coffkrnl.ld
INCDIR   =inc
AFLAGS   =-f coff -dUNDERBARS=1 -i$(INCDIR)/
CFLAGS   =-g -Wall -W -O2 -I$(INCDIR) -fno-builtin -nostdinc
LFLAGS   =-g -T$(LSCRIPT)
LIBC   =tinylib/libc.a
OBJS   =krnl/kstart.o krnl/main.o krnl/video.o krnl/kbd.o krnl/loader.o \
   krnl/time.o krnl/paging.o krnl/syscalls.o krnl/debug.o krnl/mm.o \
   krnl/tasks.o
DELETE   =deltree /y

# targets
all:   krnl.dsk

clean:
   $(DELETE) *.dsk *.lst *.sym *.x *.com krnl\*.o util\*.exe

realclean:
   make -f $(MAKEFILE) clean
   make -C tinylib -f $(MAKEFILE) clean
   make -C apps -f $(MAKEFILE) clean

# implicit rules
.c.exe:
   gcc -s -O2 -o$@ $<

.c.o:
   gcc $(CFLAGS) -c -o$@ $<

.asm.o:
   nasm $(AFLAGS) -o$@ $<

# dependencies
util/rdsk.exe: util/rdsk.c $(MAKEDEP)

krnl/kstart.o: krnl/kstart.asm $(MAKEDEP)

krnl/main.o: krnl/main.c $(MAKEDEP)

krnl/video.o: krnl/video.c $(MAKEDEP)

krnl/kbd.o: krnl/kbd.c $(MAKEDEP)

krnl/loader.o: krnl/loader.c $(MAKEDEP)

krnl/time.o: krnl/time.c $(MAKEDEP)

krnl/paging.o: krnl/paging.c $(MAKEDEP)

krnl/mm.o: krnl/mm.c $(MAKEDEP)

krnl/tasks.o: krnl/tasks.c $(MAKEDEP)

# explicit rules
load.com: load.asm $(MAKEDEP)
   nasm -f bin -o$@ $<

$(LIBC): tinylib/$(MAKEFILE)
   make -C tinylib -f $(MAKEFILE)

krnl.x: $(LIBC) $(OBJS) $(LSCRIPT) $(MAKEDEP)
   ld $(LFLAGS) -o$@ $(OBJS) $(LIBC)
   objdump --line-numbers --source $@ >krnl.lst
   nm --line-numbers $@ | sort >krnl.sym
   strip $@

krnl.dsk: util/rdsk.exe krnl.x $(MAKEDEP)
   make -C apps -f $(MAKEFILE)
   echo krnl.x >x
   echo apps/hello.x >>x
   echo apps/echo.x >>x
   echo apps/time.x >>x
   echo apps/tetris.x >>x
   echo apps/tetris.x >>x
   echo apps/invade.x >>x
   echo apps/protect.x >>x
   echo apps/protect.x >>x
   echo apps/protect.x >>x
   echo apps/protect.x >>x
   echo apps/protect.x >>x
#   echo apps/protect.x >>x
   util\rdsk.exe -o $@ @x
   del x

[AR]

Which particular part do you want explained?

The "XYZ = ABC" creates an environment variable named "XYZ" and stores "ABC" in it. "$(XYZ)" returns the contents of the environment variable.

"clean:" declares a make target (eg. command line "C:\>make clean"), the things after the colon on the same line are "dependencies". Dependencies are built before the object in question (you usually have an "prog.exe: $(BUILDOBJS)" where BUILDOBJS declares all the object files required to construct the binary), you can also have recursive targets (The ".c.o" things) which basically mean "if the file has extension X then do this to make it into extension Y" (eg. "make C into O").

You can also write recursive makefiles (The further "make" commands within the makefile) but I won't go into the details of that for the moment.

[This was essentially a crash course, feel free to ask more specific questions]

[jason7007]

Where did you get this knowledge? from DJGPP manuals or DOS batching manual?

Thank you any way.
I hope more explanation on other lines.

[Ushma]

http://www.gnu.org/software/make/manual/make.html

Enjoy.