semaphores

I got some questions about semaphores.

here is a reader/writer algorithm.

Semaphore x =1, y=1,z=1,w=1,r=1;
int nread=0,nwrite=0;

Reader:

Code:

for(;;) 
{
    P(z); P(r); P(x);
    if (++nread == 1)
        P(w);
    V(x); V(r); V(z);
    READ
    P(x);
    if (--nread == 0)
        V(w);
    V(x);
}

Writer:

Code:

for(;;) {
    P(y);
    if (++nwrite == 1)
        P(r);
    V(y);
    P(w);
    WRITE
    V(w);
    P(y);
    if (--nwrite == 0)
         V(r);
    V(y);
}

I dont understand why so many semaphores are needed. what if I take away sempaphore z and P(z) and V(z)? Will reader take precedence over writer?


[code][/code]tagged by Salem