Trouble with bit fields

[Ney André de Mello Zunino]

Hello.

I have been having some trouble dealing with bit fields. The following
is a simple program that demonstrates it.

#include <iomanip>
#include <iostream>

struct instrucao_i
{
unsigned short opcode: 6;
unsigned short rs: 5;
unsigned short rt: 5;
unsigned short immediate: 16;
};

int main()
{
instrucao_i i = { 0x24110064 };
std::cout << std::hex;
std::cout << "opcode: " << i.opcode << '\n';
std::cout << " rs: " << i.rs << '\n';
std::cout << " rt: " << i.rt << '\n';
std::cout << "immed.: " << i.immediate << '\n';
}

Here is the binary representation of the 32-bit word being used to
initialize /i/:

0010 0100 0001 0001 0000 0000 0110 0100

Since the /opcode/ field is 6 bits long, it should be equal to the first
6 bits of /i/, i.e., 001001, which is 9 in decimal. However, this is the
output I get with both VC++ 7.1 and BCC32 5.5.1 on Windows:

D:\Temp>teste
opcode: 24
rs: 0
rt: 0
immed.: 0

Could anybody shed some light on this subject?

Thank you very much,

--
Ney André de Mello Zunino

[Ron Natalie]

"Ashes" <av************ **@nospam.hotma il.com> wrote in message

I believe that the reason the struct has to be reversed to get the byte
ordering you need is Microsoft compiler specific.

Bitfields are pretty worthless if you care about the packing. It's implementation
dependent everywhere. I had no fewer than 4 different definitions of a bitfield
back when I was using it to pick out bits of a network packet. I finally just gave
up and used masks and shifts instead.

[Thomas Matthews]

Ron Natalie wrote:

"Ashes" <av************ **@nospam.hotma il.com> wrote in message

I believe that the reason the struct has to be reversed to get the byte
ordering you need is Microsoft compiler specific.

Bitfields are pretty worthless if you care about the packing. It's implementation
dependent everywhere. I had no fewer than 4 different definitions of a bitfield
back when I was using it to pick out bits of a network packet. I finally just gave
up and used masks and shifts instead.

Actually, the problem isn't so much related to the platform
but with the compiler. I've had different compilers change
the ordering and also screw up on accessing the bit fields.
I just save myself the agony and use masks and shifts. This
is more portable.

--
Thomas Matthews

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[Ney André de Mello Zunino]

Thomas Matthews wrote:

Actually, the problem isn't so much related to the platform
but with the compiler. I've had different compilers change
the ordering and also screw up on accessing the bit fields.
I just save myself the agony and use masks and shifts. This
is more portable.

Ok, I think I've read enough about those nasty bit fields. I guess I
will just join most of you and go the way of bit shifting. Nevertheless,
I must say I would rather see compilers improve in this regard than
have a unreliable feature lying around.

Thanks to all,

--
Ney André de Mello Zunino