Cirrus Logic CS485 Uživatelský manuál stáhnout pdf (Strana 40)

CS485G Spring 2015 40

(a) Monolithic: many functions, all linked together.

(b) Other: object-oriented, layered, dynamically loaded modules

but with some object-based data structures, layers for utilities

such as memory management, and dynamically loaded mod-

ules, mostly for device control.

6. The kernel code runs only when

(a) A process requests assistance via a system call.

(b) A process executes an invalid instruction.

7. The kernel executes in a privileged mode that lets it execute any

instruction, access any memory location, access any device.

8. Processes execute in user mode, which limits the instructions, mem-

ory access, and device access.

44 Exception handling

1. Hardware exceptions are of two types:

(a) interrupts are caused asynchronously by devices; examples are

clock interrupts and device-completion interrupts. Some are

unrecoverable, such as power fail.

(b) traps are caused by faults during executing an instruction, such

as division by zero or attempt to access memory in an invalid

way. They are also intentionally caused by processes in order to

invoke a system call.

2. When an exception occurs:

(a) The CPU saves the current state (at least the PC, usually other

information as well such as the current processor state) in a

standard place (often on the stack).

(b) The CPU changes to privileged mode.

exception, typically through a jump table called an interrupt

vector). That location is in the kernel.

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Cirrus Logic CS485 Uživatelský manuál Strana 40