Path: utzoo!attcan!uunet!husc6!purdue!gatech!amdcad!tim
From: tim@amdcad.AMD.COM (Tim Olson)
Newsgroups: comp.arch
Subject: Re: Execute instructions
Message-ID: <22417@amdcad.AMD.COM>
Date: 21 Jul 88 00:12:35 GMT
References: <11588@steinmetz.ge.com>
Reply-To: tim@delirun.amd.com (Tim Olson)
Organization: Advanced Micro Devices
Lines: 43
Summary:
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In article <11588@steinmetz.ge.com> davidsen@crdos1.UUCP (bill davidsen) writes:
| 
|   The old GE600 series had two instructions called XEC (execute) and XED
| (execute double). What these did was to fetch an operation from the
| target location and execute it (or them) while leaving the IC pointing
| at the XED instruction. Since the machine was able to apply IC
| modification to any operation, jumps relative to the original
| instruction were allowed.

Depending upon implementation, you could consider most RISC processors
that have delayed branches as having what amounts to the single
instruction execute, by putting a jump in the delay slot of another:

	.
	.
	jmp	$visit
	jmp	$continue
$continue:
	.
	.

This will cause the following pipeline execution (at least on the 29k):

Fetch		|   <$visit>	|  <$continue>	| <$continue+4>	|
Decode		|jmp $continue	|   <$visit>	|  <$continue>	|
Execute		|jmp $visit	| jmp $continue	|   <$visit>	|
Writeback	|		|		|		|

This has the effect of executing out-of-line the instruction at the
first jump address, then continuing with the stream starting at the
second jump address.

It may be possible to use this to help debugger breakpoint operation, by
moving the instruction at the breakpoint into another location, and
replacing it with the breakpoint instruction.  When you wish to
continue, just execute the replaced instruction out-of-line, and
continue with the next.


-- 
	-- Tim Olson
	Advanced Micro Devices
	(tim@delirun.amd.com)