Channel Communications

Using shared variables to communicate between threads requires that the user abides by self-imposed protocol conventions.

Typical patterns are:

• always ready,
• simplex guard with reader always faster than writer,
• four-phase handshake,
• two-phase handshake.

Some protocols cannot be pipelined, some degrade throughput when pipelined and others are designed for it

Some approaches completely ban shared variables and enforce use of channels (Handel-C and the main Bluespec dialect). »(LINK: Handlec.pdf)

Bluespec infers channel-like behaviour from user syntax that looks like conventional reads and writes of shared variables.

Handel-C uses explicit Occam/CSP-like channels ('!' to write, '?' to read):

   // Generator (src)     // Processor          // Consumer (sink)
   while (1)              while(1)              while(1)
   {                      {                     {
     ch1 ! (x);             ch2 ! (ch1? + 2)      $display(ch2?);
     x += 3;              }                     }
   }

Using channels makes concurrency explict and allows synthesis to re-time the design.

In both languages, handshaking wires internal to a module may disappear during synthesis.