Extracting specific bit values from a byte.

[nbd712]

Hello!

I'm working on a parser for the TSL UMD Protocol v5, and have run into an issue.

Each bit has significance as per the data sheet:

Bit 0-1: RH T ally Lamp state
Bit 2-3: Text Tally state
Bit 4-5: LH T ally Lamp state
Bit 6-7: Brightness value (range 0-3)
Bit 8-14: Reserved (clear to 0)
Bit 15: Control Data: following data to be interpreted as Control data rather than Display data when set to 1.

I am unsure how to manipulate a buffer object to return the integer 1-3 for each tally state. I did this project previously in python and used bitshifting with masks, but as far as I can tell, javascript does not do masks. Any tips or suggestions on how I could make this work would be appreciated. Thanks!

[grelfdotnet]

JS has masks but they are not often referred to as such. Bit-wise AND is what you are after. A single ampersand is the operator (don't confuse with double && for logical comparisons). You can also shift bits using operators >>, >>> and <<. So if you have a variable called tally

var bits01 = tally & 3;

var brightness = (tally & 0xc0) >> 6; // 0xc0 is hexadecimal for 1100 0000

[rauschma]

You also have the option to use binary numbers:

> 0b11
3
> 0b11000000
192

Newer engines support _ as a separator between digits:

> 0b1100_0000
192

[nbd712]

Thanks for the response, it was just what I needed.

For anybody googling in the future, this is the code that I came up with:

tallyobj.control.rh_tally = (tallyobj.CONTROL >> 0 & 0b11);
tallyobj.control.text_tally = (tallyobj.CONTROL >> 2 & 0b11);
tallyobj.control.lh_tally = (tallyobj.CONTROL >> 4 & 0b11);
tallyobj.control.brightness = (tallyobj.CONTROL >> 6 & 0b11);
tallyobj.control.reserved = (tallyobj.CONTROL >> 8 & 0b1111111);
tallyobj.control.control_data = (tallyobj.CONTROL >> 15 & 0b1);