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I've just found my first processor bug. No, not on one of the big guys like ARM or Intel but still a decent find on the Rabbit family of processors. The bug seems to have been there from the earliest days and to find it 20 years later is quite unexpected.
I've always said that I like programs you debug with an oscilloscope and hardware bugs you find with a test program, but this is the first bug on silicon and not at board level.
The feeling is quite ecstatic  (what a big nerd am I!)
Mircea
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But isn't this like "It must be a compiler bug!"
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Almost, but I have a hierarchy of esteem where at the bottom are software hacks (like me), compiler writers are some levels above and chip designers are almost at the top.
To find a bug so high up the food chain is exhilarating
Mircea
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Most probably not. Test again and again ask yourself again and again is it really the processor and not your side. Most probably you will find it is your side, this from experience from my side 
[Edit]
Otherwhise you can name the bug
modified 27-Mar-21 21:01pm.
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Quote: Most probably not. Test again and again ask yourself again and again is it really the processor and not your side.
I did and I'm fairly sure this is not the case. Filed a bug report and we'll see what happens. I don't have any qualms admitting my errors so I promise I'll post the "denouement" 
Mircea
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Good luck then 
modified 27-Mar-21 21:01pm.
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Mircea Neacsu wrote: I don't have any qualms admitting my errors so I promise I'll post the "denouement"
And if it turns out you were right, will we have to call you Mircea cel Mare?
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Quote: And if it turns out you were right,
Unfortunately that's not the case. It was just my misreading of documentation as @Randor pointed out[^].
Anyway, as they say, the journey is more important than the destination and this one was a fun one
Mircea
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Awesome!
Have you verified the "offense" on multiple samples of the chip?
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Yes, on 3 generations of the chip: Rabbit 2000, 3000, 4000.
Mircea
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A sand crab? Or maybe a sand flea? Cool.
modified 17-Feb-21 15:14pm.
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Never found a processor bug, but not all USART chips were the same: one brand that the purchase dept found cheap only worked async, and failed horribly with synchronous data. Yeuch, but that was a git to find - my dev hardware and half the production stuff had the "good chips".
I remember finding my first compiler bug though: A Fortran problem on a GEC 4070 which generated the wrong machine code if the same variable was used in an IF and assigned in the next line of code. Joy when I worked out what it was and reported it to the ops manager. Who just said "yeah, we know about that one..." and left me feeling very flat.
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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As Griff was saying:
Quote: Joy when I worked out what it was and reported it to the ops manager. Who just said "yeah, we know about that one..." and left me feeling very flat.
I worked it out what it is and here is my result.
Z80 processor has/had an instruction DJNZ that decrements register B and jumps if not zero. This is bread and butter of most small loops in Z80 assembly. It also has a set of "alternate" registers BC', DE' HL' but rather few instructions operate on those alternate registers. To compensate for this, Rabbit introduced an ALTD prefix that can be added to most instructions to make them operate on the alternate registers. Now what happens with the instruction ALTD DJNZ? Well, it takes the content of B register, decrements it and stores it in B' register making it a fairly useless instruction.
Of course, this is not documented in the manual and you have to figure it out for yourself.
Anyway, it left me feeling very flat
Mircea
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The alt registers (or "shadow" registers) I always kept for interrupts (or NMI when we used that) so we could work faster - memory access to stack the registers is a load slower than EXX which was only 4 T states, compared to a minimum of 11 (plus any memory wait states) for a PUSH.
With a 4MHz processor, you shaved T states where you could!
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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Looks documented to me. The ALTD flag only changes the destination.
Mircea Neacsu wrote: it takes the content of B register, decrements it and stores it in B register
I believe what is actually happening here is that the ALTD instruction is setting the destination of the DJNZ operation to one of the "alternate" registers which leaves the B register unchanged.
Best Wishes,
-David Delaune
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You are right; I should have paid more attention. However, it seems to me that it drastically reduces the usefulness of the instruction.
Mircea
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Mircea Neacsu wrote: You are right; I should have paid more attention.
Too bad you didn't find a cpu bug. ![Rose | [Rose]](https://codeproject.global.ssl.fastly.net/script/Forums/Images/rose.gif)
Mircea Neacsu wrote: However, it seems to me that it drastically reduces the usefulness of the instruction.
Of course, more registers are almost always better!
Best Wishes,
-David Delaune
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Quote: Too bad you didn't find a cpu bug.
Well the chase was exiting 
Mircea
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Mircea Neacsu wrote: chase was exiting
Your chase might be exiting but with an altd djnz instruction your loops will never be exiting. 
However with this pedantic joke this thread has become much more exciting.
Best Wishes,
-David Delaune
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@Mircea
We have some of those boards integrated as part of industrial control systems, anything specific we need to worry about?
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No, I don't think so. The only possible damage is to my ego but I'm a big boy and I can take it
I was programming a particularly tight loop to read from an SPI device and thought I could use ALTD DJNZ instruction to shave some cycles and use the alternate B register. It turns out the ALTD prefix in this case is useless as I explained in a previous message.
However, an instruction not doing what I expected can hardly be called a bug. No one has yet implemented the RPM (Read Programmer's Mind) instruction.
Mircea
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Hi Paul have you got AOMEI working with your Surface ? I can't get it working it fails with cannot create bitmap - Macrium works though
"I didn't mention the bats - he'd see them soon enough" - Hunter S Thompson - RIP
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He has now assumed the role of Agent 47.
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Ah ... my external drives are all Micro-B connectors, and I just never get around to buying a USB-C adapter cable.
It's not a priority for me as I don't keep anything serious on it - but it'd be a pain to reset to factory and then reload everything. Hmm. I should get a cable ... and a USB-C thumb drive to boot from ...
Thanks for the reminder ...
I just installed the standard version, and it runs. I added the SD as the destination, and it seems to be working fine, 6% already.
So, yes - "it works for me".
"I have no idea what I did, but I'm taking full credit for it." - ThisOldTony
"Common sense is so rare these days, it should be classified as a super power" - Random T-shirt
AntiTwitter: @DalekDave is now a follower!
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