T-Kernel Specification (1.B0.02)

Chapter 2
Concepts Underlying the T-Kernel
Specification
2.1 Meaning of Basic Terminology
1. Task, invoking task
The basic logical unit of concurrent program execution is called a “task”. Whereas the programs
assigned to one task are executed in sequence, programs assigned to different tasks can be executed
in parallel. This concurrent processing is a conceptual phenomenon, from the standpoint of applications;
in actual implementation it is accomplished by time-sharing among tasks as controlled
by the kernel.
A task that invokes a system call is called the “invoking task”.
2. Dispatch, dispatcher
The switching of tasks executed by the processor is called “dispatching” (or task dispatching). The
kernel mechanism by which dispatching is realized is called a “dispatcher” (or task dispatcher).
3. Scheduling, scheduler
The processing to determine which task to execute next is called “scheduling” (or task scheduling).
The kernel mechanism by which scheduling is realized is called a “scheduler” (or task scheduler).
Generally a scheduler is implemented inside system call processing or in the dispatcher.
4. Context
The environment in which a program runs is generally called “context”. For a context to be
called identical, at the very least the processor operation mode must be the same and the stack
space must be the same (part of the same contiguous area). Note that context is a conceptual
matter from the standpoint of applications; even when processing must be executed in independent
contexts, in actual implementation both contexts may sometimes use the same processor operation
mode and the same stack space.
5. Precedence
The relationship among different processing requests that determines their order of execution is
called “precedence”. When a higher-precedence process becomes ready for execution while a lowprecedence
process is in progress, as a general rule the higher-precedence process is run ahead of
the other process.
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