Question

List two challenges an operating system faces when passing parameters between user and kernel mode (e.g., consider the differences between passing parameters via procedures in the same process versus between processes/applications). Describe how an operating system can overcome them.

Answer 1

A system is said to be in user mode if the operating system is running a user application. If the system is booted and some preserved instructions are executed for operating system then it is said to be in Kernel mode.

Two challenges an operating system can face when passing parameters between user and kernel mode are:

1. The numbers of parameters passed should be limited to six.

2. The size of the parameters should fit within the size of the registers. In other words, size of the parameters should not exceed the size of the registers.

To overcome such challenges, before a system call, the parameters has to be saved in registers. A system call is basically a program requested by the kernel. On the other hand, in User mode, the CPU is allowed to access some portion of memory and also less peripheral devices. But in Kernel mode, the CPU has privilege to access the memory as well as peripheral devices (additional devices). Thus to avoid the parameter passing challenges, the CPU may shift from User mode to Kernel mode. To switch from User mode to Kernel mode, a TRAP instruction has to be executed, which will result the CPU to switch to Kernel mode. Before this switching, the previous instruction locations have to be saved so that after completion of the work in Kernel mode, the user's remaining work can be continued.