bit wise manipulation on a 32-bit integer

Question

I am trying to read a 32 register,modify its first 8 bits[BIT7:BIT0] and write back its value. Does the code below achieve that?

reg_val = register_read(register_object);
reg_val = ((reg_val & 0xffffff00) | new_value));
register_write(register_object,reg_val);

Also is it the most efficient way to achieve that.any suggestions or comments are appreciated.

Too many right parentheses, but the obvious intent is correct (with the stipulation identified by EboMike that new_value < 256 :) — Ray Toal
– Ray Toal, Commented Aug 2, 2011 at 4:49

mlp · Accepted Answer · 2011-08-02 04:47:49Z

3

Unless new_value is guaranteed only 8 bits wide you should ensure it:

reg_val = ((reg_val & 0xffffff00) | (new_value & 0xff));

Also is it the most efficient way to achieve that?

Any compiler worth its salt will translate that into The Right Thing.

answered Aug 2, 2011 at 4:47

mlp

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Comments

EboMike · Accepted Answer · 2011-08-02 04:47:04Z

2

Assuming that new_value is guaranteed to be <256, and assuming that these are unsigned integers, then yes, this is the proper way to do it.

answered Aug 2, 2011 at 4:47

EboMike

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snap. And ensuring unsigned was something I forgot - good point.