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OriginalGriff wrote: Good grief, have I really been here 9 years? 
In computer years that's what, forever? 
Wait, I've been on CP longer than Griff?!
But I only have 0.6% as much rep as you do... granted I only have about 1.6% as many messages. Which must mean that my comments are slightly less than 50% as useful. I better step it up!
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In over 14 years, I have...
- Written a couple of CP articles
- Posted much sage advice and some drivel (or is that the other way around?
 ) on CP - Grown older, but not necessarily wiser
Ad astra - both ways!
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Is the PC term for "Sugar Daddy" "Glucose Guardian"?
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AntiTwitter: @DalekDave is now a follower!
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if it's a PC term is there a keyboard shortcut?
there's sure a lot of keystrokes in "Glucose Guardian." (and yes, I used cut & paste.)
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That was a sweet thought, so ignore those who condom it.
"Somehow, Wales always enters into these things - worries about sheep developing tooth decay . . . "
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| "The difference between genius and stupidity is that genius has its limits." - Albert Einstein | | "If you are searching for perfection in others, then you seek disappointment. If you are seek perfection in yourself, then you will find failure." - Balboos HaGadol Mar 2010 |
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I have been designing my little 8 bit computer and now have a memory map that should be adequate.
16k ROM
8k RAM (basic RAM if no paged RAM is present, later fixed RAM that should not be paged away)
6k Video RAM
2k I/O registers
32k Paged RAM (32k pages out of up to 16MB)
Nice, huh? NOT!
The 2k for the I/O registers would allow 16 bytes memory space for each of up to 128 I/O devices. No need to overdo it, but I'm going to need about 16 of them. The problem is that at least six of the planned devices will need interrupts. I don't want to poll for keystrokes, mouse events, bytes coming from the IDE port, bytes coming from the serial port, sound chips having empty buffers...
There is an interrupt controller IC, but it will hog up a 4k area in the memory map (which I don't have) at a fixed place where it really hurts (in the 32k paged RAM). This will not help.
Now what will I do? I could simply use an AND gate with enough inputs to unite the IRQs from all devices. When any one of them goes low, the output of the gate will go low and trigger the interrupt.
Nice and well. The processor will call its interrupt routine - and now what? It does not know which device to service. Any ideas?
I have lived with several Zen masters - all of them were cats.
His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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Use your AND gate, and a also wire the interrupts via a latch to a byte in the "device 0" memory space (you may need a latch, it's been a while since I did this kind of thing).
Interrupt fires, read the byte, clear it, you know the interrupt source(s) from the bit pattern.
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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Good idea. Interrupt priorities could be done in the interrupt routine, but I have to be careful if another interrupt occurs while I'm still servicing one. The interrupt controller would have dealt with all that, but that's really not an option.
I have lived with several Zen masters - all of them were cats.
His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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I was thinking the same as OG, 16 IRQ's needs only 4 bits (so the byte could handle up to 256 IRQ's - enough for the future.)
Could be wrong but back in the day I sort of remember when the Z80 caught an IRQ it disabled further interrupts till you executed a particular instruction (or was it a special variant of the RET instruction?) - something like that? (And there may have been 1 or 2 specific IRQs that weren't disabled). Was a loooong time ago, could be wrong, may have been a dream.
Perhaps your CPU has something similar?
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I thought about a 16 -> 4 multiplexer (they are cheap enough), but you can't respond to multiple interrupts that way, so if two come in while you are processing one with interrupts disabled you will lose one or the other when you reenable. A pair of eight bit latches doesn't take a lot more room, and is more flexible.
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
AntiTwitter: @DalekDave is now a follower!
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Brings back memories. Do you have a processor that automatically disables further interrupts when the first interrupt occurs, or is an instruction to disable interrupts? If the former, one workaround I had to implement once was to check at the end of the interrupt handler if any other bits had been set and route them in the code. If the latter, you could either write your IRQ handlers to be really really safe, or you could write an IRQ handler that queues the interrupt and have a separate (does your processor do threading?) "loop" (ugh, but again, been there, done that) that services the queue.
Heck, if you can create a separate thread, don't even use the IRQ line, just poll the latch. As evil as that sounds, it actually works quite well, IIRC. If your processor doesn't support threads, then, ummm...not a great solution.
Anyways, that's my 2c from what I remember having to deal with in a previous lifetime.
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Interrupts are disabled when the processor acknowleges an interrupt and are reenabled when you return from the interrupt routine. Threads would be great, but the processor does not support them. I would need some kind of timer interrupt to schedule and switch threads. This is becoming circular.
I have lived with several Zen masters - all of them were cats.
His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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CodeWraith wrote: This is becoming circular.
Otherwise known as "stack overflow" Yup, that about sums up the complexity of interrupts. Simple concept, yet probably where a good 50% (if not more) of my debugging would go into when I used to write assembly.
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are you a wizard?
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I hunt Balrogs every day!
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I knew it! 
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I suppose then you also have the robe, pointy hat, eat a lot and never bathe.
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Yes, No, Yes, No comment.
Gandalf bathed - how do you think he became Gandalf the White? Eating Tide Pods?
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Ah, but on Discworld only he Archchancellor bathed (even then a few times only),
and that worried the rest of the wizards a great deal.
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It worried 'em even more when they found out the bathroom was a Bloody Stupid Johnson creation!
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
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Look over the pond: Chlorox bleach.
I have lived with several Zen masters - all of them were cats.
His last invention was an evil Lasagna. It didn't kill anyone, and it actually tasted pretty good.
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OriginalGriff wrote: I hunt Balrogs every day!
Lucky for us you haven't actually encountered any. 
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I dunno - I think there are quite a few in QA!
Bad command or file name. Bad, bad command! Sit! Stay! Staaaay...
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I now have a less scary backdrop!
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