Shared memory between PDP-10 and a remote computer

gistfile1.txt

/* This is just pseudo code that happens to look a bit like C. */

/* The block of shared memory on the host looks like this. */
typedef struct {
  /* Only written by PDP-10.*/
  pid_t pdp10_pid;
  word_t pdp10_data;
  int pdp10_address;
  
  /* Only written by remote side. */
  pid_t remote_pid;
  word_t remote_data;
  int remote_nxm;
  char remote_core[128*1024];
  
  /* Written by both. */
  mutex_t lock[128*1024];
  word_t memory[128*1024];
} shared_memory_t;

void nxm(void)
{
  /* Raise NXM for this memory access. */
}

void doorbell(int address)
{
  pdp10_address = address;
  /* Signal remote side with USR1, wait for USR2. */
  kill(remote_pid, SIGUSR1);
  sigtimedwait(SIGUSR2);
  if(timeout) {
    remote_pid = 0;
    nxm();
  }
  if(remote_nxm)
    nxm();
}

void gimme(int address)
{
  /* Release lock on this core address. */
  doorbell(GIMME | address);
  if (pthread_mutex_timedlock(&lock[address], &timeout) == ETIMEDOUT) {
    remote_pid = 0;
    nxm();
  }
}

word_t read_remote(int address)
{
  word_t data;
  if(remote_pid == 0)
    nxm();
  if(remote_core[address]) {
    if (pthread_mutex_trylock(&lock[address], &timeout) == -1 && errno == EAGAIN)
      gimme(address);
    data = memory[address];
    return data;
  } else {
    doorbell(READ | address);
    return remote_data;
  }
}

void write_remote(int address, word_t data)
{
  if(remote_pid == 0)
    nxm();
  if(remote_core[address]) {
    if (pthread_mutex_trylock(&lock[address], &timeout) == -1 && errno == EAGAIN)
      gimme(GIMME | address);
    memory[address] = data;
  } else {
    pdp10_data = data;
    doorbell(WRITE | address);
  }
}

void handle_usr1(int sig)
{
  /* Remote side signaled PDP-10 with USR1, reply with USR2. */
  pthread_mutex_unlock(&lock[remote_address]);
  kill(remote_pid, SIGUSR2);
}

/* The remote handler looks like this. */
void handle_usr1(int sig)
{
  if (pdp10_address & GIMME)
    pthread_mutex_unlock(&lock[pdp10_address]);
  else if (pdp10_address & READ)
    pdp11_data = unibus_read(pdp10_address);
  else if (pdp10_address & WRITE)
    unibus_write(pdp10_address, pdp10_data);
  kill(pdp10_pid, SIGUSR2);
}