diff --git a/build/Makefile b/build/Makefile
index 9440b07..86bfeef 100644
--- a/build/Makefile
+++ b/build/Makefile
@@ -156,12 +156,10 @@ clobber : clean
 
 
 # generic rule for assembling a .s into an 0405 a.out format.
-# XXX make fixaout.py overwrite a.out instead of write b.out
 .SUFFIXES : .s .0405
 .s.0405 :
 	@$(AS) $<
 	@../tools/fixaout.py
-	@rm a.out
-	@mv b.out $@
+	@mv a.out $@
 	@echo done $< $@
 
diff --git a/tools/fixaout.py b/tools/fixaout.py
index 2de9e1e..ce5ac90 100755
--- a/tools/fixaout.py
+++ b/tools/fixaout.py
@@ -28,4 +28,4 @@ def write(fn, d) :
 d1 = words(read('a.out'))
 hdr = d1[:8]
 d = [0405, 12+hdr[1], 0, 0, hdr[4], 0] + d1[8:]
-write("b.out", unwords(d))
+write("a.out", unwords(d))