Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!amdcad!rpw3
From: rpw3@amdcad.AMD.COM (Rob Warnock)
Newsgroups: comp.arch
Subject: Re: 80486 vs. 68040 code size [really: how many regs]
Message-ID: <25662@amdcad.AMD.COM>
Date: 17 May 89 17:55:53 GMT
References: <950@aber-cs.UUCP>
Reply-To: rpw3@amdcad.UUCP (Rob Warnock)
Distribution: eunet,world
Organization: [Consultant] San Mateo, CA
Lines: 43

In article <950@aber-cs.UUCP> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:
+---------------
| This is a very good argument, so far, for an AMD 29k style very large
| register file, that becomes a statically managed first level memory, or for
| a SPARC style set of (less statically managed) windows...
+---------------

Hmmm... the 29k register windowing seems to have been misunderstood again...

On the Am29000, while the local registers (the 128 "windowed" or "stack
cache" regs) are statically *named* (on entry to a routine, lr0 = return
address, lr2 = first arg, etc.), the implied "first level memory" (when
the register set is considered as a stack cache) is *dynamically* managed
according to the instantaneous depth of the call stack by the routine entry
and exit assertions. That is, there is no predetermined correlation between
subroutine calls and spill/fill activity -- some regs are spilled (saved
to memory) when a new frame is opened if the cache is full, and regs are
filled (restored from memory) when a return is made to an upper-level
routine whose frame was spilled (i.e., stack cache is "empty"). Since
most frames are much smaller than the register file, there's *lots* of
hysteresis.

Also, the decrementing of the stack pointer (gr1) on routine entry
(to open a variable-sized frame of 2 to 126 regs) accomplishes a
dynamic remapping of which underlying physical registers the static
local register names will access. [The "mapping" is of course trivial:
bits <8:2> of "gr1" are added to the local register number (modulo 128)
to give physical register number.]

Since the overlapping windows [incoming args are in both the caller's
and callee's frames] may be of any size (2-126), the 29k's windows are
much "less statically managed" than the SPARC's. Even if the 29k had
the same number of registers available to the windowing mechanism
(instead of many more), it would still typically allow more routines'
windows to be in regs at once, reducing memory traffic.


Rob Warnock
Systems Architecture Consultant

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