Getting the integervalue of bits inside an unsigned int

Question

I want to write a kernel module which reads out a register, (return value is saved in an unsigned int) and then read the the bits 16 to 22 from this variable and the convert it to an integer number. The conversion is no problem. But getting the bits out in the first place is the problem. As example I have this 2 values in hex:

0x88290000d
0x005a0a00d

and from this 2 values I want the bits 16 to 22 as integer, any ideas how I can implement that in my kernel module ?

Start by [reading a good beginners book](http://stackoverflow.com/questions/562303/the-definitive-c-book-guide-and-list). It should have chapters or sections about the bitwise operators. — Some programmer dude, Jun 20 '17 at 11:06
assuming you number the bits from least significant `0`, shift right by 16 places and apply a mask filtering those (now lowest) 7 bits ( `(val>>16) & 0x7fU` ) — , Jun 20 '17 at 11:08
Where does the nine-digit value `0x88290000d` come from, a `uint64_t`? — Weather Vane, Jun 20 '17 at 11:10
@FelixPalmen this way i already tried it, and the result is can't be. Weather Vane yes — Martin, Jun 20 '17 at 11:12
@WeatherVane : could be a 36 bit address... (32 bits x86 use max 36bit addresses) — joop, Jun 20 '17 at 11:12
"*the result is can't be*" -- what? :o I don't understand this, please create an [MCVE](https://stackoverflow.com/help/mcve) demonstrating what you tried. — , Jun 20 '17 at 11:12
the result is negativ, and there shouldn't be a negativ value — Martin, Jun 20 '17 at 11:13
@MartinWalzl If you mask with 0x7F, the result is never negative, even if you use `int` instead of `unsigned int`. — Sergey Kalinichenko, Jun 20 '17 at 11:14
the result is printed via console command "var tail -f /var/log/syslog" — Martin, Jun 20 '17 at 11:15
`(0x88290000d >> 16) & ((1llu << (22-16+1)) - 1)` or other variants. — chux - Reinstate Monica, Jun 20 '17 at 13:41

Sergey Kalinichenko · Accepted Answer · 2017-06-20T11:20:05.283

2

Here is how you extract bits 16 through 22, inclusive (7 bits):

Read the number from the register into unsigned int reg = ...
Shift reg to the right by 16, so bit 16 is at the least significant position: reg >>= 16
Mask the number with 0000000001111111₂, which is 0x7F: reg &= 0x7F

Note: The above counts bits starting from zero (the traditional way of numbering bits).

edited Jun 20 '17 at 11:20

answered Jun 20 '17 at 11:13

Sergey Kalinichenko

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OP's `0x88290000d` may easily not fit as in `unsigned int reg = 0x88290000d;` – chux - Reinstate Monica Jun 20 '17 at 13:37
This Answer worked perfect. But now i have a other problem, i did the same twice with 2 different values, i printed everything out and worked. Then I removed the debug printk's and the value was 0, after putting them in again i had the correct value, you have any ideas on that too ? – Martin Jun 20 '17 at 13:43
@MartinWalzl When printing a value changes the way the program works, it means there is undefined behavior in your code. Post a separate question with details on what you did and how you printed the values, or run your code through [valgrind](http://valgrind.org/) if it is available on your platform. – Sergey Kalinichenko Jun 20 '17 at 13:47

score 0 · Answer 2 · answered Jun 20 '17 at 12:05

You can define a mask that is active for only 7 bits(16 bit to 22 bit). So your mask will be defined as.

mask=0x007F0000   //Your mask
var2=0x00270f00   //Variable which you want to mask
result=mask & var2 // bitwise-and operation to mask the value
result>>=16       // right shift the value to get only this value.

Hope this helps :)

Getting the integervalue of bits inside an unsigned int

2 Answers2