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Subject: Re: Descrambling

Re: Descrambling

From: BlueChip <>
Date: Sat, 09 Aug 2003 13:38:04 +0100

There is a big ANTI-assembler movement here, so if my personal experiences
here are worth anything, you are likely to get told to go back to C.

HOWEVER... *I* on the other hand, think assembler is perfectly acceptable
and would love to be running your code :)

SO... Even if your code is rejected, please make sure you put it somewhere
that the assembler freaks here can download it :) Maybe you could attach
it to your next email?

Real Programmers use GOTO
At 13:09 09/08/03 +0200, you wrote:
>Someone "complained" that the descrambling in rolo_load() was slow, so I 
>had a look at it. :) As divides on the SH-1 is fairly slow, having two per 
>descrambled byte explains a lot. So I first tried by replacing the two 
>divides with one multiply (basically use the "addr" calculation from 
>scramble.c instead). This halved the execution time.
>Then I rewrote the new version in assembler and halved the execution time 
>again. :) Moving it to the internal RAM cut another 20-25 percent. In 
>numbers, that means the execution time has gone down from 1.6 seconds to 
>0.3 seconds (32 ticks). There's one limitation though: the image can be at 
>most 256 kB large... Maybe that's one reason for the 200 kB limit imposed 
>by the Archos firmware?
>I also noticed that the code that really needed to be in the "top" RAM 
>wasn't much, so I moved that to the internal RAM too, so the .topcode 
>section could be removed. It means an additional 80 bytes of the internal 
>RAM is used (this includes the descramble code). Even if the ROLO stuff, 
>per definition, isn't executed much, I'd say it's worth it :) But what do 
>you think? Should I leave that in the patch?
>Also, before submitting it, I think I'll convert the checksumming to 
>assembler as well. It will only add a few bytes anyway, and only adds one 
>cycle per byte in execution time.
>Btw, while doing this, I noticed a potential bug in bitswap.S: a nop 
>should be added after the rts instruction. It isn't a problem with the 
>current code, but if it is changed, the bug could be triggered, I think...
>Magnus Holmgren
Received on 2003-08-09

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