What is the symbol of the `mux` chip in C?

Question

I was studying logic gates when I came to know that each logic gate was already defined in C. For example, for the AND logic gate, the symbol is &. For OR, it is |. But I could not find a symbol for the MUX chip.

So, if there is a symbol for MUX, could someone tell me? If there isn't, could someone tell me how to replicate a MUX chip in C?

Bysshe

How exactly would you expect a MUX operator to work? How would you supply its inputs? – Helminthology

@Dmitri, I would make a function in which I will pass it's inputs. – Bysshe

Answer 1

Please be advised that C operates at a much higher level of abstraction than logical gates, so making such comparisons might lead to confusions. That said, the closest you might come to a demultiplexer (I'll start with that since it's simpler) is the left shift operator:

a << b

This expression, assuming that a and b are int expressions, will produce a new int whose bits are the bits of a shifted to the left b times. For example, if a is 0100011011010110 and b is 3, the result will be 0011011010110000. Now, if a is either 0 or 1 and you interpret the resulting integer as being a bus, this corresponds to a demultiplexer.

A multiplexer/selector can be implemented by the right shift operator >>, which shifts bits to the right. However, the result must be &'ed with 1 in order to clear any other bits than the one you were interested in:

(c >> b) & 1

This effectively selects the bit at index b (starting at the least significant bit) from c.

Hardie

So you mean to say, that you cannot make a MUX in C? (By the way I didn't downvote or upvote anyone's answer till now.). – Bysshe

Looks like we all got one except for one answer that immediately got upvoted. I'll go ahead and cancel out the downvote you got. – Elgar

do you even know what a mux is? – Item

@AshishAhuja: What I'm saying is that the question doesn't make sense because a MUX is a digital circuit, not a programming construct. However, if you interpret int values as buses, you can make something that has similar effect - but when your program is executed, the operation will most likely not be executed by an actual MUX (except in the sense that MUXes are a part of the Arithmetic-Logic Unit of your processor, but this is, as mentioned, on a much lower level). – Hardie

@Adam: Thanks; same to you. – Hardie

@AasmundEldhuset, yeah I wanted to make a virtual MUX which will not be executed by an actual MUX. Just virtual. – Bysshe

@AshishAhuja: Then, I believe that my suggested expressions will do what you want (as long as your bus doesn't have more bits than an int, typically 32). – Hardie

@Jasen, yes I know what a MUX is. It is a chip which selects one of several analog signals (or maybe digital), and forwards the input into a single line. – Bysshe

...I, on the other hand, mixed up the definitions of multiplexer and demultiplexer; a correction is coming shortly. – Hardie

mux is like CD4519, analogue switches are different. – Item

One more downvote - can the downvoters please explain why? – Hardie

Answer 2

Closest is the conditional operator: ? :

eg:

 x ? b : a 

if x is 0 you get a if it's 1 (or anything else) you get b

This operator works on entire values, like || && == and ! do. It does not operate on bits as ^ ~ & and | do.

There is no direct equivalent for a multi-input mux. but you can fake one using an anonymous array, eg:

 ((int[]){a,b,c,d,})[x]

but many people frown on constructions of that form.

if you need a bitwise mux you'll need to build it from the bitwise operators eg:

 a ^ (( b ^ a ) & x)

Item

Answer 3

C has four bitwise operators:

• AND, &, as in a & b
• OR, |, as in a | b
• XOR, ^, as in a ^ b
• NOT, ~, as in ~a

There is no MUX operator.

Be careful about your phrasing. These are called bitwise operators, and are analogous to logic gates applied to all bits in an integral type. In C logical operators are different.

Elgar

So is there a way to make a MUX chip using these? – Bysshe

@AshishAhuja a MUX isn't something you'd normally code at the C level. The same task is usually accomplished in several other ways, such as a conditional (if, switch or ?:), and array lookup, bit packing, or others. It all depends on what you want to use your MUX for. Remember that C is not a hardware language, it's a high level language. – Elgar

thanks for informing me that, but if you want to do it, what is wrong. I am doing this as I was learning circuits and hardware. And, I didn't want to do soldering, as the last I did soldering on a circuit, I burned one of my finger. – Bysshe

soldering without burning fingers is an important skill. not a skill I have fully mastered, I count it a success if I have no blisters the next day. – Item

Answer 4

The |, &, and ~ operators are bitwise operators. They work in parallel on individual bits in the operands. There is no corresponding bitwise operator for a multiplexer. The ternary operator:

output = cond ? a : b

comes close, but the selector operand is treated as a single bit, and not a vector of bits (that is, all output bits come from a or all output bits come from b, you can't have some of the output bits come from a and some come from b). To get a true bitwise multiplexer, where the selector is a vector that selects individual bits from a or b, you can implement it the way you would build one from discrete logic gates:

output = (cond & a) | (~cond & b);

Here, a 1 in a cond bit allows the corresponding bit from a to pass, and blocks the corresponding bit from b (because b is masked with the inverse condition). A 0 in a cond bit blocks the corresponding bit from a, and allows the corresponding bit from b to pass. The two masked values are bitwise OR'd together, so a bit of output is either the corresponding bit from a of b, depending upon the state of the corresponding bit in c.

Daune