setting a specific bit

[slacker]

how would i go about setting an individual bit...on the keyboard controller i want to set a certain bit to turn on a LED light without messing with the other bits.

[Mr_Spam]

ive also been thinking of some LEDS handeling algorythems... is there a way to read the status of which LEDS are on or off? then you could toggle that bit to change without chaning the state of the other leads.

[distantvoices]

You keep an global unsigned char-variable in your keyboard driver module: uchar_t led_state

it has 8 bits. we need the lower three:

000: everything off.

001: scroll on = 1
010: caps lock on = 2
100: num lock on = 4

say you want to set num lock:

led_state+=4.

to set caps lock: led_state+=2.

Now you have the following bit-mask:110

say you want to switch off num lock:

led_state-=4.

Now the bitmask looks like this: 010.

got it?

[Pype.Clicker]

i wouldn't use '+' and '-' to manipulate a bitfield, if i were you: trying to set bit 1 with x+=2 will modify bit 2 if bit 1 was already set ...

It's cleaner to use bitwise operators like '&' and '|'

x|=2 : sets bit 1, other bits unchanged
x&=(~2) : clears bit 1, other bits unchanged.

[distantvoices]

*gg*

It's a good method with bitwise operators, you are right.

this ~-operator, it INVERSES the bits, isn't it? Thus, ~2 would look like 101 in binary, and a field, say 110&101 will result in 100 - > bit 010 is erased. Did I understand this right?

[Pype.Clicker]

yes, its roughly this, except that ~2 will not be 101,but rather 11111101 (0xFD) if the expression is a char-sized, 0xFFFD if the expression is short-sized and 0xfffffffd if it's an int.

To know how such operators work, you must learn their truth tables. For instance,

1&1 == 1, 1&0==0, 0&1==0, 0&0==0.
--> a 0 in x will clear the corresponding bit in a&x,
a 1 in x keeps the bit from a.

1|1 == 1, 1|0==1, 0|1==1, 0|0==0.
--> a 1 in x will set the corresponding bit in a|x

1^1 == 0, 1^0==1, 0^1==1, 0^0==1.
--> a 1 in x will toggle the corresponding bit in a^x

[distantvoices]

truth tables:

AND (a&b) or (a&&b):
a and b result

1 1 true
1 0 false
0 1 false
0 0 false

OR(a|b) or (a||b):
a or b result
1 1 true
1 0 true
0 1 true
0 0 false

exclusive OR:
a XOR b result
1 1 false
1 0 true
0 1 true
0 0 false

Implication: also expressed as !a|b (NOT a OR b)
a implies b result
1 1 true
1 0 false
0 1 true
0 0 true

I didn't like the surreal experiments the teachers/professors did with these things, but I like using it in real nice nifty programs. These constructs can shorten else lenghty if structures.

[Mr_Spam]

ah the good old truth tables.... did thoes last year in math class. I finally have a use for them!

[Perica]

..

[Peter_Vigren]

Wait for the keyboard to accept commands. Send the command 0xED (put next byte into the output port) to port 0x60. After the keyboard once again is ready to accept a command, send the LED byte.

The 3 lesser bits (0:2) are the LED bits and the above must be zero.

[slacker]

i wouldn't use '+' and '-' to manipulate a bitfield, if i were you: trying to set bit 1 with x+=2 will modify bit 2 if bit 1 was already set ...

It's cleaner to use bitwise operators like '&' and '|'

x|=2 : sets bit 1, other bits unchanged
x&=(~2) : clears bit 1, other bits unchanged.

what does the ~ do? i never learned about it....
and what does the &= and |& do?

[Curufir]

Well my C is a little hazy, but as far as I remember a tilde is the equivalent of a NOT gate.

So to run this through with only 8 bits you have:

Code: Select all

x&=(~2) : clears bit 1, other bits unchanged

You end up doing:

Code: Select all

x & 11111110

Which rather effectively clears the first bit.

Presumably the C compiler is smart enough to use the same number of bits in the 2 as in X, so all other bits will be left unchanged by the AND. This saves you the hassle of specifying a precise number to AND with at compile time (Magic numbers and relying on type size is bad practice).

Forgot this in the original post. +=, -= , |=, &= etc are all abbreviations for common mathematical/logical operations.

Eg x+=1 actually means x=x+1

The rest follow exactly the same form.

Standard C stuff, but as I mentioned I'm a little rusty on it, so if I got something wrong please correct the mistake.

[Whatever5k]

You end up doing:

Code: Select all

x & 11111110

Actually that's

Code: Select all

x = x & 11111101

Remember: 2 is 10 in binary and ~ just inverses the bits (1 gets to 0 and other way round).

x &= y <equal> x = x & y
x | = y <equal> x = x | y

same with other operators (+,-,/, etc.)

[Curufir]

Good catch Abless, serves me right for posting in the middle of the night .