[riot-devel] bitfeilds

Juergen Stuber juergen at jstuber.net
Sun Oct 30 10:55:35 CET 2016


On Fri, 28 Oct 2016 10:03:17 +0200
Juergen Stuber <juergen at jstuber.net> wrote:
> 
> When you use shift and mask you usually do a single access for
> all fields of a register.

Note that you shouldn't do it in two assignments
(I'm seeing this in cpu/stm32l1/periph/gpio.c):

    port &= ~mask;
    port |= (new_value << shift);

This will result in bigger code and the first assignment will write a
spurious value to the register, which might cause problems.

    port = (port & ~mask) | (new_value << shift);

is better. Or

    port = (port & ~mask)
         | (new_value1 << shift1)
	 ...
         | (new_valueN << shiftN);

for multiple fields.


Grüße
Jürgen

-- 
Jürgen Stuber <juergen at jstuber.net>
http://www.jstuber.net/
1B78 A579 E159 2A85 67BB  1314 C083 224B 0F9C DA21
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