backtick in verilog numeric constant

Question

So I was writing some verilog in quartus and wondering why the heck it was misbehaving.

I eventually discovered the problem was some constants where I had inadvertantly used a backtick instead of a single quote. For example I had 32`hdeadbeef instead of 32'hdeadbeef.

What I find surprising is that this compiled, did I find a bug in quartus? is this some obscure bit of verilog syntax? if so what does it mean?

Answer 1

I have just been reading the IEEE standard about macro's and defines. There is nothing in there which says that the macro name must be known. (In your case `hdeadbeef would not match any define).

However I can imagine them not defining that as that would be too far fetched.

Using common sense I would say it is a bug in the parser.

---

Speculating:
The following is allowed:

`ifdef this_variable_is_not_defined

So maybe the code for that got also used for `this_macro_does_not_exist

• Peter Green: It does seem quite plausible that it is treating the unknown macro as an empty string and then treating the number that was supposed to represent the width as a straight decimal constant of default width.
• Oldfart: That is what I was assuming. Just tried it in Vivado but there I get an error message.
• Peter Green: I haven't tried Vivado but I have noticed in the past that Quartus is a lot less strict about Verilog than Modelsim is.
• dave_59: The LRM says "After a text macro is defined, it can be used in the source description by using the (`` ` ``) character, followed by the macro name." I understand this to mean "before a text macro is defined, it cannot be used ..." Otherwise, a simple misspelling of a compiler directive, like `celdefine would go unnoticed. This is the only rational behavior.

Answer 2

This is most likely a bug and should have been an error . The backtick ` is only used in with compiler directives

• Dave Tweed: Yes, and one of those directives is `define <name> -- and when you want to use <name> in a statement, you say `<name>. Not a bug.
• dave_59: @DaveTweed that assumes someone already has the statement `define deadbeef in their code, which I doubt
• Dave Tweed: No, it doesn't. It's perfectly OK to refer to an undefined symbol, which expands to nothing.
• dave_59: Not true. You should get an undefined compiler directive or macro. Try it.