Re: bit fields and data structure

From:

Maxim Yegorushkin <maxim.yegorushkin@gmail.com>

Newsgroups:

comp.lang.c++

Date:

Mon, 17 Nov 2008 01:48:53 -0800 (PST)

Message-ID:

<d9dbb4b3-642d-4b17-9575-807a9a8eaddb@v5g2000prm.googlegroups.com>

On Nov 17, 1:53 am, ma740988 <ma740...@gmail.com> wrote:

I often peruse source that use memory mapped registers as a
configuration and/or command and status interface between embedded
software and FPGA logic. In doing so you're given a register name
accompanied by descriptions and a location in memory for reading/
writing data.

For ex:.

Revision Register - Location 0x0000
[31...15] spare
[14...00] number

Control Register - Location 0x0001
[31..3] -- Spare
[2] -- Address Line 3
[1] -- Address Line 2
[0] -- Address Line 1

To describe these registers in source one could do:

  struct RevisionRegister {
    unsigned short number : 16;
    unsigned short spare : 16;
  };

struct ControlRegister {
  unsigned int Spare : 29;
  unsigned short AddressLine3 : 1;
  unsigned short AddressLine2 : 1;
  unsigned short AddressLine1 : 1 ;

} ;

The trouble with the composite type ControlRegister is sizeof
(ControlRegister) is not 32 bits.

It is because bit-fields of different type can not be merged. Fix:

    struct ControlRegister {
      unsigned Spare : 29;
      unsigned AddressLine3 : 1;
      unsigned AddressLine2 : 1;
      unsigned AddressLine1 : 1 ;
    } ;

That said, its not possible to
receive ControlRegister in it's current form from an end user and
overlay it at the appropriate location. The fundamental issue
surrounds the use of bitfields and implementation defined nature of
POD types. The question: What are some of the - if you will - tips
for dealing with this issue?

Another issue is that bit-field bit order can be little or big-endian
depending on the platform and the compiler. A portable way to read
mapped registers is to read the register, convert it from the byte
order of the hardware that exposes that register into the byte order
of the cpu (if byte orders are the same this is a noop), and then
apply mask by using bitwise and operator to extract the interesting
bits. This way you don't depend on the bit-field bit order.

--
Max