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Overflow not detected

Note: This was originally posted on 6th September 2010 at http://forums.arm.com

Hi all,
I'm quite new in arm assembly programming, and I went into a odd behaviour.

I'm going to multiply two numbers with the mul instruction;
the code is:

ldr      r0, =0x26000000
ldr      r1, =0x10
muls r2, r1, r0
bvs    somewhere

The result from the the mul istruction is (0x2600 0000 * 0x10 = ) 0x2 6000 0000.
Now, i expected the V bit going to one, but none of the CPSR bits is setted.
Why is happening this? How can I detect overflow in such operations?
I'm using the gnuarm compiler, and working with 32-bit registers.

Thanks in advance,
Stefano.

Peter Harris over 12 years ago

Note: This was originally posted on 6th September 2010 at http://forums.arm.com

The mul instruction only sets the N and Z flags; C and V are not set.

I think you would need to use UMULL (unsigned) or SMULL (signed) to produce a 64-bit result, and detect the overflow manually based on the value of the top 32-bits.

If you are writing assembler you are probably going to need access to the ARM Architecture Manual which will help you with this kind of issue. You can register for access here (follow the PDF link):

[url="http://infocenter.arm.com/help/topic/com.arm.doc.ddi0406b/index.html"]http://infocenter.arm.com/help/topic/com.a...406b/index.html[/url]

Cheers,
Iso
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Scott Douglass over 12 years ago

Note: This was originally posted on 7th September 2010 at http://forums.arm.com

Why is happening this? How can I detect overflow in such operations?
I'm using the gnuarm compiler, and working with 32-bit registers.

There's a blog entry "next door" that discusses this [url="http://blogs.arm.com/software-enablement/detecting-overflow-from-mul/"]http://blogs.arm.com/software-enablement/d...rflow-from-mul/[/url].
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Stefano Cillo over 12 years ago

Note: This was originally posted on 7th September 2010 at http://forums.arm.com

There's a blog entry "next door" that discusses this [url="http://blogs.arm.com/software-enablement/detecting-overflow-from-mul/"]blogs.arm.com/.../url].

I'm sorry, I didn't notice that.
However, thanks for replying: that's what I needed.
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