Execution inside a process [VHDL]

Question

I have, for example, this code snippet:

p1 : process (clk)
begin

if (a = '1') then
    a <= 0;
end if;

if (b = '1') then
    b <= 0;
end if;

end process p1;

Which if-sentece is executed first? a or b? I know that process is a concurrent statement but I don't know if within a process the execution is secuential or concurrent.

Thanks!

Answer 1

This code does not describe a state machine, so the concepts of sequential statements or program execution don't apply here. The correct answer to your question is "無."

The sequential execution of a process will create a state machine only if there are wait statements, which are not allowed in processes with sensitivity lists. The order of statements is relevant for resolving signal assignments (last assignment to a signal wins), and for conditions based on variables (where assignments are visible to the following statements in the same process).

The two conditional statements only depend on signal values, only modify signal values and have no interdependencies, so their effects are concurrent even though they are technically part of a sequence. If you synthesize this design, you end up with completely independent circuits.

In a bigger scope, the entire process is optimized out, as there is only a single place driving a and b, and the only value ever assigned is 0.

You cannot have other concurrent statements or processes drive a and b either, as that would cause a conflict.

Answer 2

Sequentially. All statements in VHDL proecess are executed one after, like if a process is a conventional SW program.

I like to tell about VHDL program as a network of processes, which execute in parallel to each other. It is exactly like modern SW program consists of threads at the top level. Yes, threads are also concurrent but statements within them are not. The difference why VHDL is simpler because all communications are hardwired (you cannot dynamically decide which process to send info or receive from) and computation is synchronized every clock (so that you know what the other processes are doing, unlike asynchronous SW processes). To be fair, synchronization is not mandatory. You can write asynchronous processes (synchronizing on signal different from the common clock), but normally you have synchronous processes in one clock domain because it makes life so much easier.