> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/man4/attimer.4.md).

# attimer.4

`attimer` — i8254 可编程间隔定时器（AT 定时器）驱动

## 名称

`attimer`

## 概要

本驱动是 x86 内核的必备组成部分。

以下可调参数可在 [loader(8)](/man/man8/loader.8.md) 中设置：

**`hint.attimer.X.clock`** 控制事件定时器功能的支持。将此值设置为 0 可禁用该功能。默认值为 1。

**`hint.attimer.X.timecounter`** 控制时间计数器功能的支持。将此值设置为 0 可禁用该功能。默认值为 1。

**`hw.i8254.freq`** 允许覆盖默认的计数器频率。同一值也可在运行时通过 `machdep.i8254_freq` sysctl 获取。

## 描述

本驱动使用 i8254 可编程间隔定时器（AT 定时器）硬件向内核提供一个时间计数器和一个事件定时器，并为系统扬声器生成声音。该硬件包含三个通道。每个通道包含一个 16 位计数器，按已知的、平台相关的频率递减。计数器可以工作在多种不同模式下，包括周期模式和单次模式。每个通道的输出具有平台定义的连线：一个通道连接到中断控制器，可用作事件定时器；一个通道连接到扬声器，用于生成声音；还有一个定时器保留供平台使用。

`attimer` 驱动使用单个硬件通道同时提供时间计数器和事件定时器功能。为使其成为可能，相应计数器必须以周期模式运行。因此，仅当禁用时间计数器功能时才支持单次事件定时器模式。

驱动提供的事件定时器与 CPU 电源状态无关。

## 参见

[apic(4)](/man/man4/apic.4.md), [atrtc(4)](/man/man4/atrtc.4.md), [eventtimers(4)](/man/man4/eventtimers.4.md), [hpet(4)](/man/man4/hpet.4.md), [timecounters(4)](/man/man4/timecounters.4.md)


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