supporting mixed 16/32-bit ISA's
Peter.Targett@arccores.com
Peter.Targett@arccores.com
Tue Jan 22 03:57:00 GMT 2002
I've had a look through the CGEN mail archive and came across the mail
below.
I'm particularly interested in CGEN's ability to describe mixed 16/32
bit ISA's. We have a new ISA at ARC which has a truely intermixed
16/32 instruction set - basically, can I describe the ISA in CGEN? The
32-bit instructions (and long immediates that can form part of an
instruction) are actually stored half-word endianized. Would this also
be a problem?
Thanks in advance for any advise,
Peter.
--
peter.targett@arccores.com
---------------------------------------------------------------------
Broken decoder for 16/32 ISAs
To: cgen at sources dot redhat dot com
Subject: Broken decoder for 16/32 ISAs
From: Ben Elliston <bje at redhat dot com>
Date: Thu, 31 May 2001 23:08:27 +1000 (EST)
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 bit)) <----
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.
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