# how to place two unsigned 16 bit values into a 32 bit unsigned value

Discussion in 'C++' started by pasamdivya@gmail.com, Nov 18, 2008.

1. ### Guest

Hi everyone,

I need to store an unsigned 16 bit number in the upper (MSB) and
another unsigned 16 bit number in the 16 LSB. Basically, I am trying
to store two unsigned 16 bit numbers into one unsigned 32 bit number.

So far, I have the following:

struct
{
unsigned int num1:16;
unsigned in num2:16;
} twoUnsigned16BitNums;

unsigned int num;

What I am having trouble with is to store the two 16 bit numbers into
the 32 bit number. I know one option is to use bit wise ANDs and ORs.
Any help on your part is greatly appreciated. Best Regards.

, Nov 18, 2008

2. ### James KanzeGuest

Re: how to place two unsigned 16 bit values into a 32 bit unsignedvalue

On Nov 18, 9:22 pm, wrote:

> I need to store an unsigned 16 bit number in the upper (MSB)
> and another unsigned 16 bit number in the 16 LSB. Basically, I
> am trying to store two unsigned 16 bit numbers into one
> unsigned 32 bit number.

> So far, I have the following:

> struct
> {
>    unsigned int num1:16;
>    unsigned in num2:16;
> } twoUnsigned16BitNums;

That may or may work (then memcpy'ing the results); check your
compilers documentation.

> unsigned int num;

> What I am having trouble with is to store the two 16 bit
> numbers into the 32 bit number. I know one option is to use
> bit wise ANDs and ORs.

That's the usual solution.

> Any help on your part is greatly appreciated.

Is "number1 << 16 | number2" what you're looking for? (Just
make sure that 32 bit arithmetic is used here. If number1 is an
unsigned int, and int's are only 16 bits, you'll have to cast it
to unsigned long first.)

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James Kanze, Nov 18, 2008

3. ### Bill DavyGuest

Depending on portability (compiler, processor, packing, etc) you may be able
to use a union. For VS on Intel

union

{

struct

{

unsigned a:16;

unsigned b:16;

} Half;

struct

{

unsigned ab:32;

} Whole;

} t;

t.Half.a = 0x1234;

t.Half.b = 0x5678;

t.Whole.ab is now 0x56781234

<> wrote in message
news:...
> Hi everyone,
>
> I need to store an unsigned 16 bit number in the upper (MSB) and
> another unsigned 16 bit number in the 16 LSB. Basically, I am trying
> to store two unsigned 16 bit numbers into one unsigned 32 bit number.
>
> So far, I have the following:
>
> struct
> {
> unsigned int num1:16;
> unsigned in num2:16;
> } twoUnsigned16BitNums;
>
> unsigned int num;
>
> What I am having trouble with is to store the two 16 bit numbers into
> the 32 bit number. I know one option is to use bit wise ANDs and ORs.
> Any help on your part is greatly appreciated. Best Regards.

Bill Davy, Nov 19, 2008
4. ### James KanzeGuest

Re: how to place two unsigned 16 bit values into a 32 bit unsignedvalue

On Nov 19, 9:24 am, "Bill Davy" <> wrote:
> Depending on portability (compiler, processor, packing, etc)
> you may be able to use a union. For VS on Intel

> union
> {
> struct
> {
> unsigned a:16;
> unsigned b:16;
> } Half;

> struct
> {
> unsigned ab:32;
> } Whole;
> } t;

> t.Half.a = 0x1234;
> t.Half.b = 0x5678;

> t.Whole.ab is now 0x56781234

Maybe. Trying to access it is undefined behavior, so there's no
way to tell.

--
James Kanze (GABI Software) email: