C: Bit mask question

[Karl Auer]

On Thu, 2009-04-30 at 13:19 +0200, Bas Jacobs wrote:
> Consider a control register, which includes flags and bitfields. When
> writing such a register you often see constructions like register =
> flaga | flagb | flagc; etc. In a library those flags are then defined
> as #define flaga (1 << 6) and #define flagb (1 << 13) (for instance).
> In my library there is also a definition for a bitfield, which looks
> something like this: #define bitfieldA (0x0F << 16) for a 4bit
> bitfield located at bits 19 .. 16. Lets say that I want write 0x6 into
> this field, how do I do this (using the macro of course)?

Assuming C, you bitwise OR the desired value with the existing value,
just as you do with single-bit values.

#define bitfieldA (0x0F << 16)
#define mybits    (0x06)
#define myflags   (bitfieldA | mybits)

Alternatively if it is bitfieldA you want changed:

#define bitfieldA ((0x0F << 16) | 0x06)

Uncompiled, untested, but roughly like that.

Regards, K.

PS: Try not to use reserved words like "register" as variable names :-)

PPS Try to make your macro names look like macro names and not like
ordinary variables. So "FLAGA" rather than "flaga"...

PPPS: "16", "0x06" and "0x0F" are all candidates for getting the

-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Karl Auer (kauer at biplane.com.au)                   +61-2-64957160 (h)
http://www.biplane.com.au/~kauer/                  +61-428-957160 (mob)

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