XML CO631 Anonymous Questions and Answers Keyword Index

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Keyword reference for replicator

Question 60 (2004):

Hi, could you just give an explanation of how the following code works (taken from "sort_pump.occ"):

    PROC sort (CHAN OF BYTE in, out)
      [total - 2]CHAN OF BYTE c:
      PAR
        cell (in, c[0])
        PAR p = 1 FOR total - 3
          cell (c[p - 1], c[p])
        cell (c[total - 3], out)
    :

Thanks.

Answer 60:

This code creates a pipeline of processes (of `total - 1' cells) -- see slides 5-17 and 5-18. The two end cells are catered for specially (since these plug into the external environment -- via `in' and `out'). The cells between these are created using a replicated PAR. If you substitute a constant value for `total' (say 10), then expand and flatten the PAR replicator, it becomes a bit clearer:

    [8]CHAN OF BYTE c:
    PAR
      cell (in, c[0])

      cell (c[0], c[1])        -- PAR p = 1 FOR 7
      cell (c[1], c[2])        --   cell (c[p - 1], c[p])
      cell (c[2], c[3])
      cell (c[3], c[4])
      cell (c[4], c[5])
      cell (c[5], c[6])
      cell (c[6], c[7])

      cell (c[7], out)

Keywords: sort-pump , par , replicator

Question 29 (2004):

Just a quick question regarding sort_pump.occ. In the window process, does the following fill the buffer array with end markers?

  SEQ
    SEQ i = 0 FOR buff.max
      buffer[i] := end.marker
    ptr := 0

If so, what is the point of doing so?

Also, what does ptr mean? Is it some kind of field to keep track of the index of the array? Thanks.

Answer 29:

Yes. The statement "buffer[i] := end.marker" is replicated buff.max times in SEQuence, each with a different value for the control variable i (starting with zero, in this case, and incrementing by one each time).

We have to put something in the buffer since what is there initially gets pumped into the sorter as we type characters. We need something visible for display -- so zeroes (ASCII nulls) would not work. End markers work (they are mapped to visible dots for display) and they are harmless -- just ignore them (as does the lay.out process does if it sees consecutive markers!).

Note that in occam, there is no auto-initialisation of variables to default values (such as zero).

The answer to your last question is yes -- see the code labelled display buffer and update buffer, further down in the window process.

Keywords: replicator , sort-pump

Question 95 (2003):

I was having problems putting the processes together using a replicator and was wonder if the code in the secure.college PROC create 6 philosophers instead of 5 as its is from 0 to 5

Answer 95:

Replicators in occam are as they seem: replication `FOR 5' means you get 5 replications.

By default, the replicator value increments by one, with the first replication having the `start' value. The general syntax for a replicator in occam is:

SEQ|PAR|ALT|IF  name  = start  FOR  count

KRoC/Linux additionally supports ``STEP  stride'' for replicators.

Keywords: replicator