5.13 Limit Checking

5.13 Limit Checking

1. Used to prevent a process from accessing memory locations outside segments allocated to that process

   • in enforced when a process needs to access a memory location in a given segment and checks the location is within the segment
     • Limit checking prevents programming errors or malicious programs crashing the OS or overwriting sensitive infomation

2. the limit check compares the value of an offset with the size of the segment

   • The size of a segment is stored in the limit field of the segment descriptor and the G (granularity) bit
     • G Bit = 0
       • If the G bit is clear the size of the segment is the size of the limit field
         • The limit is a 20 bit field
           • segment cannot exceed 2/20 =1 MB
     • G Bit = 1
       • When the G bit is set the size of the segment is multiplied by 2/12
         • the size of the segment cannot exceed 2/32 = 4GB