Project ForgeThe P and V operations on counting semaphores, where s is a counting semaphore, are defined as follows: P(s): s=s-1; if s<0 then wait. V(s): s=s+1; if s<=0 then wakeup a process waiting on s. Assume that Pb and Vb, the wait and signal operations on BINARY semaphores, are provided. Two binary semaphores xb and yb are used to implement P(s) and V(s) as: P(s): Pb(xb); s=s-1; if(s<0){Vb(xb); Pb(yb);} else Vb(xb). V(s): Pb(xb); s=s+1; if(s<=0) Vb(yb); Vb(xb). The initial values of xb and yb are respectively
- A0 and 0
- B0 and 1
- 1 and 0
- D1 and 1
Explanation
xb protects mutual exclusion for the shared counter s, so it must start AVAILABLE: xb=1. yb is used purely to block waiting processes until explicitly signaled, so no process should be released prematurely: yb=0. This is the standard construction for implementing a counting semaphore from binary semaphores.
Source: GATE CS 2008 (2008)
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