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javascriptbit-manipulationarraybuffer

Bitwise operators in Javascript and in general

I am working on parsing some binary files, I have them opened and in an ArrayBuffer.

In the particular file structure I am reading, there are a number of bits which are boolean and I can check whether they are checked with:

(flag & 1) != 0; // bit 0 
(flag & 2) != 0; // bit 1 
(flag & 4) != 0; // bit 2

etc.

However, I am having trouble getting the values of the bits followed. They span over multiple bits (for example bits 4-6) and consist of an integer value from 0-7.

How are multiple bits read like that? I understand that this isn't as much of a JavaScript question than that of how bits and bitwise operators work.

Solution

  • Assuming you want 4-6 bits from a byte like this:

    76543210
 ^^^

    You would construct a bit mask like this:

    0x70

    which means:

    01110000

    And then you would & that with the number and shift to right 4 times:

    ( byte & 0x70 ) >> 4
//Number between 0-7