(14)是系统的第一道防线，用以防止非法数据和非法用户的进ok；它一般装在(15)上以保护一个子网。

(89)

(88)

(86)

(87)

(84)

User　programs　interact　with　the　kernel　through　a　set　of　standard(71).　They　request　services　to　be　provided　by　the　kernel.　Such　services　would　include　accessing　a　file:　open　close,　read,　write,　link,　or　execute　a　file;　starting　or(72)accounting　records;　changing　ownership　of　a　file　or(73);　changing　to　a　new　directory;　creating,(74),　or　killing　a　process;　enabling　access　to　hardware　devices;　and　setting　limits　on　system　resources.　Unix　is　a　multi　-user,　multi　-tasking　operating　system.　You　can　have　many　users　logged　into　a　system　simultaneously,　each　running　many　programs.　It's　the　kernel's　job　to　keep　each　process　and　user　separate　and　to　regulate　access　to(75),　including　cpu,　memory,　disk　and　other　L/O　devices.

(83)

(82)

(81)

(79)