Re: 3 byte variables [feature request]

Keep in mind that, even with a "real gcc implementation", that code would
still be very inefficient due to weaknesses in the MSP430 CPUX instruction
set.  Getting the upper 4 bits into and out of a 20-bit register is a
computationally complex operation.  The best approach I've found that
doesn't require a word-aligned memory location involves a 5-bit
rotate-through-carry.  Casts between 32-bit and 20-bit values on the MSP430
are extremely expensive.


On Wed, Jun 5, 2013 at 3:22 PM, David Brown <[email protected]>wrote:

>
> 24-bit integers would have to be 16-bit aligned for the cpu to be able to
> load them efficiently, those negating any storage benefits.  So to be able
> to store them efficiently, you would effectively have a struct containing
> three 8-bit bytes (or an array of three bytes).  I think the best you could
> do here is to make a C++ class containing these three bytes, with
> conversion operations to and from int20_t types.  It should be possible to
> do this with code that is almost as efficient as a real gcc implementation.
>
> mvh.,
>
> David
>
>
>
> On 05/06/13 16:09, Peter Bigot wrote:
>
>> Best to submit this to the gcc project; it's certainly not something that
>> would be added through mspgcc.
>>
>> int20_t was an enhancement I thought useful since the MSP430 CPUX
>> architecture enables it, but I don't have reason to believe other MSP430
>> toolchains (including the upcoming RH GCC) will recognize it.  Native
>> support for it on CPUX still requires 32-bits when it's written to memory,
>> though, and that wouldn't change since its inherent in the way the
>> instruction set works.
>>
>> Peter
>>
>>
>> On Wed, Jun 5, 2013 at 8:53 AM, kuldeep dhaka <[email protected]>**
>> wrote:
>>
>>  while working on a program i faced a problem.
>>> i need only 20 bit , and having a 32 bit only waste more memory(12 byte),
>>> 24bit would much be better.
>>> i need a uint24_t variable but gcc dont support it so i have to start
>>> with
>>> uint32_t.
>>> it would be very nice if gcc support such variables.
>>> i tried a little program and found that gcc (on i686) dont support it.
>>>
>>> http://embeddedgurus.com/**stack-overflow/2009/06/three-**byte-integers/<http://embeddedgurus.com/stack-overflow/2009/06/three-byte-integers/>
>>>
>>>
>

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