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From: cmcmanis@stpeter.Eng.Sun.COM (Chuck McManis)
Newsgroups: comp.sys.amiga.tech
Subject: Re: Whats wrong with self Modifying Code?
Keywords: religion, gurus, whats up?
Message-ID: <138523@sun.Eng.Sun.COM>
Date: 7 Jul 90 05:18:54 GMT
References: <1990Jul6.201328.24660@csmil.umich.edu> <1990Jul6.201743.24777@csmil.umich.edu>
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There isn't anything "wrong" with it per se as long as the system
supports it. When asked in context to the Amiga there are a couple
of issues :

Issue : How do you get reliable self modifying code execution
	when there is an instruction cache present ? Let's take
	for example the code that goes something like :
		mov	#JumpTable, D0	; Load D0 with address of table
		add	D1, D0		; Calculate offset
		mov	D1, #JumpTarget	; Store in in the Jump instruction
		.dw	4efch		; opcode for a jump immediate
	JumpTarget:
		.ds	4		; 4 byte jump address
	JumpTable:			; Table of addresses. 
		.dl	function1
		.dl	function2
		.dl	function3
	In this example, We allow the parameter passed in D1 to be used
	as an offset into a jumptable. The final jump instruction is 
	self modified, and the routine branches appropriately. Now on
	an instruction cache machine, all of the instructions get 
	fetched into the cache. When the 'mov D1,#JumpTarget' instruction
	happens, while it changes the version in memory, the cache
	is _not_ changed. Poof! The code breaks on 68020 and 68030
	Amigas with the instruction cache turned on. Generally the
	68K family is flexible enough in its addressing modes that 
	you can accomplish what you would have done one way, another
	"legal" way that doesn't involve self modifying code. This
	allows your program to continue to function on high end Amigas.

Issue : What do you do when a version of LoadSeg makes the code hunk
	of your program "execute-only" on the MMU?
	Using the same example above, when an MMU is present it is
	possible (and probable) that at sometime in the future the
	MMU will "protect" your code from getting stomped on by making
	the memory it is running in "execute/read" only. When that is
	the case the "mov D1,#JumpTarget" instruction generates a
	CPU exception. This exception will be treated by LoadSeg as
	either a runaway process or some other valid reason to shut
	down your task. Suddenly you are hosed again.

So the bottom line isn't that it is morally wrong, simply that it 
won't work on some legitimate Amiga systems and is thus "illegal"
in terms of having full Amiga compatibility. If one chooses to use
self modifying code in a commercial or even freeware Amiga program
they should be sure to spell out clearly on either the package or
in a README file that the code will not work reliably on a 68020 or
68030 system. 


--
--Chuck McManis						    Sun Microsystems
uucp: {anywhere}!sun!cmcmanis   BIX: <none>   Internet: cmcmanis@Eng.Sun.COM
These opinions are my own and no one elses, but you knew that didn't you.
"I tell you this parrot is bleeding deceased!"