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You are right Dave
if you ADD 128 & 128 you get flags S,V, Z , C flags set.
How ever if you AND, OR, EOR 128 + 128
Both AND & OR will only set S, N flag
While EOR will only set Z (not V as the last email was)
[a repeat of my first Email]
Footnote:
N is Negative
S is Signed
V is Two's complement overflow indicator
Z is Zero
C is Carry

have a nice day

----- Original Message -----
From: Dave Hylands
To:
Sent: Monday, July 24, 2006 1:43 AM
Subject: Re: [AVR club] now here is a "how come?"
Hi,

> The "how come" question comes that when you look at the data sheet
> (ATmega32) they say flags Z,N,V could be set by the result, (note there is
> no mention of flag S)

The AVR Instruction set data sheet indicates that S, V, N, and Z could
be modified, and that S = N xor V.

> How come I get flag S to be set and I can not set flag V?

The AVR Instruction Set data sheet quite explictly says that V will be
set to zero on the AND, OR, and EOR instructions.

Since S = N xor V, and V = 0, then S = N.

> (??? why would you want the "Two's complement overflow indicator" set with
> "AND" , "OR" , "EOR" instructions?? )
> Am I doing the wrong logic?

AND, OR and EOR are bitwise operations so logically there should be no
overflow, which is why V is set to zero.

V being set indicates a loss of information. AND, OR, EOR never lose
any information. The answer is always correct.

However, if you add 128 and 128 you get an answer of 0 with the V bit
set. Since 0 is the wrong answer, there was a loss of information and
the V bit will be set.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/



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Hi Eric,

On 7/23/06, Eric Richards wrote:
> You are right Dave
> if you ADD 128 & 128 you get flags S,V, Z , C flags set.
> How ever if you AND, OR, EOR 128 + 128
> Both AND & OR will only set S, N flag
> While EOR will only set Z (not V as the last email was)

So you're claiming that ANDing 0x01 and 0x02 will not set the Z bit?

I think you're wrong, at least based on the couple simple experiments
I've performed.

--
Dave Hylands
Vancouver, BC, Canada
http://www.DaveHylands.com/




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