Difference between system call and kernel call in Minix/Microkernel

Question

Conceptually, whats the difference in Minix between a System call and a Kernel call?

I kind of understand this difference in an Operating System like Linux, but what about in a microkernel like Minix? Are both types of calls the same?

Answer 1

Implementation wise, system calls are the same in a microkernel vs a monolithic kernel - the system call traps to the kernel and the kernel handles the request.

The big different is what the system calls are for - let's take the file system as an example.

In a monolithic kernel most services are implemented inside the kernel, including the file system. So to open a file, do operations on a file, a system call is required.

In a microkernel, most services are implemented at user-level and what requires a system call really depends on how the system is architected. In a Minix-like system where services are componentised, a file system request is made to a file system server. This still involves a system call - but the system call is an IPC message to the file system server encoding the required operation, rather than a request to the kernel for a specific operation. The kernel simply facilitates the message transfer.

As I stated though, this depends on the system architecture. Microkernel based systems can have an architecture where the file system is included in the process accessing it, so no system calls are required for most file operations (but you will likely need some for your storage driver).