Re: [MiNT] Pexec() patch in FireTOS

[Lonny Pursell <atari@bright.net>]

Hello,

I'm quite aware of all of this. This is what plagued GFA, about 150+
occurrences of exactly this and needless to say rather tedious to fix, but I
did it. ;)

I dis-assembled memset() and applied the same patch firetos does and built a
small custom library for PureC, now when I link a codec or any other PureC
source I get a clean version of memset(). I sent vido some test files, still
awaiting results. 

-- 
Lonny

on 8/26/15, 2:57 AM, markus@mubf.de wrote:

> Hi Lonny,
> 
> I think nobody can tell if this patch will make it work, but at least it
> will take you a good deal forward.
> 
> The background is that the Pure C libs use a feature/quirk in the 68k
> architecture to speed up the code a bit.
> 
> They avoid a slow (on 68000 only) lsl.w #8,xxx (a multiplication by 256)
> by pushing a byte to the stack and fetching it back as a word.
> 
> The m68k requires a word-aligned stack. Motorola engineers decided to
> implement a 'self healing stack' feature that silently extends a byte
> pushed to the stack into a word to keep the alignment.
> Which I found always ugly btw, since it makes the - otherwise pretty and
> straightforward - instruction set unorthogonal, making the A7 register
> behave differently than any other address register - in my opinion it
> should just crash in such case.
> 
> (Ab-)using that feature, you can pop this extended byte back as a word,
> effectively resulting in a left shift by 8 bits. On mc68000 that lacks
> the barrel shifter later chips had (which made shift execution times
> independend of shift size), this is much faster than the equivalent
> (clean and slow) lsl.w instruction.
> 
> Coldfires can deal with an unaligned stack (it just makes them a little
> slower), so Freescale removed that 'self healing stack' feature from the
> instruction set. If you push a byte and pop a word on Coldfires, you get
> the low byte of whatever was on the next higher stack address in the
> high byte of the popped word, but even worse, you damage the next higher
> stack contents resulting in a crash sooner or later.
> 
> Bad decisions always get punished. It just takes a while, sometimes
> 
> Cheers,
> Markus
> 
> P.S.: this is something the cf68klib (the m68k emulation library used in
> FireTOS) cannot catch and fix on the fly as it does for other
> incompatible m68k code. The cf68klib relies on exceptions fired by
> incompatible instructions. As the instruction stream of Pure C's
> memset() implementation is perfectly valid code on Coldfire as well (it
> just yields different results), there is no exception to catch. Thus,
> FireTOS intercepts Pexec() and explicitly scans for the specific byte
> pattern of Pure C's memset() library routine to apply a patch. This
> fixes the issue for Pure C compiled programs, but other code (that has a
> different byte pattern but also uses this 'trick') will still fail.

-- 
Lonny Pursell    http://www.bright.net/~gfabasic/