BSD-UNIX
university of california
bell labs
Time-sharing

clock algorithm / second chance algorithm
   circularly linked list
   walk incircle, check ref of pages (if set clear)
   if reach page wi/out reference, means it wasn't accessed so kill it
   second chance

  concurrency issues...

block devices - hard drives -
character devices - keyboard, mouse, /dev/null
sockets - pipes (named, unnamed), network socket ex. tcpip based ....

modem  = character device
freebsd = complete & free...

MOCK
//carnagie mellon
where all processes are in user space

microkernel root;

pg 125 dinosaur
   ports in mach (like mailbox)

NUMA

xNU = X (not unix) kerenel for mac
   freebsd but for mac
   from freebsd, took:
         process model,
        network stack,
       virtual file system

  from Mach, took kernel