gnupic: Re: [gnupic] Re: gpsim register question
Subject:
Re: [gnupic] Re: gpsim register question
From:
Robert Pearce ####@####.####
Date:
31 Dec 2006 15:35:58 +0000
Message-Id: <RSwKqNKeQ9lFFwUN@daniel.huneausware.local>
On Sat, 30 Dec 2006, Maxim Wexler ####@####.#### wrote :
>aargh, did it again! Confusing the tab with the caps lock 8(
>
Easily done!
>Anyway
>
>COUNT1 equ 20h
>
>Now when I step through the code and pass the decfsz command 20h in the
>register viewer fills up with FFh from 00h.
>
I'm not entirely sure I understand you here, but let's see if my
interpretation is right.
You're single-stepping through the code until you get to the subroutine
"Delay". Then when you single step over the line
Loop1 decfsz COUNT1,1
you see the RAM cell at 0x20 go from 0x00 to 0xFF
Is that right?
If so it's exactly what I'd expect.
>Where does that 255 come from?
It's one less than the zero that was in there before.
> There's no mention anywhere in the previous
>code.
No, but there's already a zero in there, and you've run an instruction
that subtracts one from that value.
> And besides, according to the pdf for DECFZ f,d f can't be more that
>127.
Ah, perhaps here is the confusion. The 'f' in that mnemonic refers to
the _address_ of a RAM cell, and the instruction coding only has 7 bits
for that. Hence 'f' can only be 0..127 - you have defined it as 0x20.
The instruction decrements the _contents_ of the RAM cell at address
'f', which in your case are currently undefined, and GPSim defaults to
all zero.
>
>Another thing: In the breadboard window only porta0 to porta5 become
>outputs. 6&7 remain inputs. Why, after being sent all zeros?
>
That would depend on the processor type. Some have restrictions on
whether certain pins can be outputs - for example, the 12C5xx range
allocate GPIO3 to the MCLR pin, and force it to always be an input
regardless of the state of TRIS.3
--
Rob Pearce http://www.bdt-home.demon.co.uk
The contents of this | Windows NT crashed.
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