What does: subi r24,lo8(-2) do?

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theFloe
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Posted by theFloe: Mon. Sep 27, 2004 - 11:51 PM
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Hi,

I'm not experienced with the assembler for avr's but i'm doing my best to translate some code from asm to win-avr C.
What does this do?
subi r24,lo8(-2)

subi is a substract command and r24 should be decrement by lo8(-2)
What value gives lo8(-2)?
What does lo8 do? Isn't that the low 8 bit of an 16bit word like low()?
Please tell me what this line does, thanks

Hava a look at my web page -> http://www.tobiscorner.at.tf

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Tools, Atmel Studio (AVR-related)
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SteveN
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Posted by SteveN: Tue. Sep 28, 2004 - 12:19 AM
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Hi,

The AVR does not have an add immediate instruction but it does have a subtract immediate. If I understand it correctly what you have is the same as adding 2 to r24.

I don't know why lo8 is there, I am not a C person.

Regards,
Steve

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EW
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Posted by EW: Tue. Sep 28, 2004 - 03:35 AM
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lo8() and hi8() are indeed macros (or probably more precisely, builtins) for the GNU assembler (that works with the GCC compiler) and IIRC, it's to get the low byte and hi byte of a 16-bit variable.

Eric Weddington
www.linkedin.com/in/ericweddington/

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glitch
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Posted by glitch: Tue. Sep 28, 2004 - 04:43 AM
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EW wrote:
lo8() and hi8() are indeed macros (or probably more precisely, builtins) for the GNU assembler (that works with the GCC compiler) and IIRC, it's to get the low byte and hi byte of a 16-bit variable.

Constant, not variable.

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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theFloe
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Posted by theFloe: Tue. Sep 28, 2004 - 07:01 AM
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Thank you all!

Hava a look at my web page -> http://www.tobiscorner.at.tf

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EW
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Posted by EW: Tue. Sep 28, 2004 - 03:44 PM
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glitch wrote:
EW wrote:
lo8() and hi8() are indeed macros (or probably more precisely, builtins) for the GNU assembler (that works with the GCC compiler) and IIRC, it's to get the low byte and hi byte of a 16-bit variable.

Constant, not variable.

Oh, thanks glitch! :oops:
I program in C so I'm a little unsure of some things in the GNU assembler.

Eric Weddington
www.linkedin.com/in/ericweddington/

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Bingo600
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Posted by Bingo600: Sat. Oct 2, 2004 - 01:57 PM
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Just to set things clear (for me :wink: )

If -2 is the same as 0xFFFE (Word based)

Would it not be the same as 

subi r24,0xFE

Witch is "addi r24,2"

/Bingo

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glitch
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Posted by glitch: Sat. Oct 2, 2004 - 04:21 PM
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indeed it is, in fact that's what the compiler changes it to. It's just easier to understand what subtracting -2 from an element does, instead of subtracting 0xFE.

Writing code is like having sex.... make one little mistake, and you're supporting it for life.

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