Added document to describe design of OS

OS-DESIGN.md

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+# Overview
+
+
+# System calls
+
+## System call list
+
+### map_anonymous(location, size, flags)
+
+
+
+Privilage level: user
+
+### unmap_anonymous(location, size)
+
+Privilage level: user
+
+### map_physical(location, size)
+
+Privilage level: system
+
+### unmap_physical(location, size)
+
+Privilage level: system
+
+### object_create(location, size, flags)
+
+Privilage level: user
+
+### object_aquire(id, location)
+
+Privilage level: user
+
+### object_release(id)
+
+Privilage level: user
+
+### send_pid(data, dest, flags)
+
+Privilage level: user
+
+### send_port(data, dest, flags)
+
+Privilage level: user
+
+### receive(buffer, flags)
+
+Privilage level: user
+
+### port_open(id)
+
+Privilage level: user
+
+### port_close(id)
+
+Privilage level: user
+
+### clone(entry, arg, stack, flags)
+
+Privilage level: user
+
+### signal_action(id, action, flags)
+
+Privilage level: user
+
+### signal_return()
+
+Privilage level: user
+
+### signal_raise(pid, id, arg)
+
+Privilage level: user
+
+### interrupt_action(id, action, flags)
+
+Privilage level: system
+
+### interrupt_return()
+
+Privilage level: system