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Broken decoder for 16/32 ISAs


I have been debugging a problem in the generated decoder for the
simulators.  Here is the scenario, involving an ISA with a mix of 16
and 32 bit instructions (and lsb0? set to #f, so most significant bit
is bit 0).

The 16 bit instructions are laid out like so:

	     +---------+---------+
	     | insn16  |         |
	     +---------+---------+
	     0        15

And the 32 bit instructions are laid out like so:

	     +-------------------+
	     |	    insn32       |
	     +-------------------+
	     0			31

The (-gen-decode-bits) function computes, amonst other things, the
amount to shift a sequence of bits to the right, whereby they are then
masked and examined by the decoder.

For the architecture I've briefly described above, I believe the logic
in utils-sim.scm is wrong:

  (shift (- (if lsb0?
		(- first bits -1)
		(- (+ start size) (+ first bits)))	<----
	    pos)))

The line indicated is used to compute the shift value when lsb0? is
#f.  Even for 16 bit instructions, the shift value needs to be at
least 16 to get at the bits of insn16 (see above).

The `size' variable seems to be passed in by callers, but it's unclear
how this value is calculated or what it is meant to be in a variable
length ISA.  Shouldn't size really be the sizeof(insn) here?  I'm a
bit out of my depth in this part of cgen, but any suggestions would be
much appreciated.

For now, I'm using the patch below.

Ben


Index: utils-sim.scm
===================================================================
RCS file: /cvs/cvsfiles/devo/cgen/utils-sim.scm,v
retrieving revision 1.15
diff -u -r1.15 utils-sim.scm
--- utils-sim.scm       2000/12/04 18:34:37     1.15
+++ utils-sim.scm       2001/05/31 12:47:54
@@ -546,7 +546,7 @@
                            ; FIXME: Need to handle left (-ve) shift.
                            (shift (- (if lsb0?
                                          (- first bits -1)
-                                         (- (+ start size) (+ first bits)))
+                                         (- (+ start 32) (+ first bits)))
                                      pos)))
                     (string-append
                      " | ((" val " >> " (number->string shift)


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