Kernel code always executes in one of two contexts:

• Process
• Interrupt

Now we understand that one can invoke kernel services by issuing a system call. When this occurs, the calling process runs the kernel code of the system call in kernel mode. This is termed process context – kernel code is now running in the context of the process that invoked the system call.

Process context code has the following attributes:

• Always triggered by a process (or thread) issuing a system call
• Top-down approach
• Synchronous execution of kernel code by a process

At first glance, there appears to be no other way that kernel code executes. Well, think about this scenario: the network receive path. A network packet destined for your Ethernet MAC address arrives at the hardware adapter, the hardware detects that it's meant for it, collects it, and buffers it. It now must let the OS know; more technically, it must let the Network Interface Card (NIC) device driver know, so that it can fetch and process packets as they arrive. It kicks the NIC driver into action by asserting a hardware interrupt.

Recall that device drivers reside in kernel-space, and therefore their code runs in Supervisor or kernel Mode. The (kernel privilege) driver code Interrupt service routine (ISR) now executes, fetches the packet, and sends it up the OS network protocol stack for processing.

The NIC driver's ISR code is kernel code, and it is has run but in what context? It's obviously not in the context of any particular process. In fact, the hardware interrupt probably interrupted some process. Thus, we just call this interrupt context.

The interrupt context code has the following attributes:

• Always triggered by a hardware interrupt (not a software interrupt, fault or exception; that's still process context)
• Bottom-up approach
• Asynchronous execution of kernel code by an interrupt

Technically, within interrupt context, we have further distinctions, such as hard-IRQs and softirqs, bottom halves, and tasklets. However, this discussion goes beyond the scope of this book.