10-26-2015
The IBM PC has had a pulse generator/timer from the beginning (which also used to drive the PC speaker.) In the olden days, before LSI, it was its own separate chip attached to the data bus. It was very flexible, but generally configured to generate hardware interrupts at 18ms intervals. This hardware interrupt is an IRQ, pretty much hardwired into the CPU, causing it to stop whatever it's doing and jump to a hardwired location in RAM. The OS can do whatever it pleases with that interrupt by putting code in that location.
The hardware is going to be very different on non-PC architectures but the idea is the same, a pulse generator wired to the CPU.
If you're really curious about historical PC architecture, and have a bent towards electronics, I suggest
The 8088 book. The 8088 as much as anything else is responsible for why the PC is the way it is.
Last edited by Corona688; 10-26-2015 at 04:21 PM..
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LEARN ABOUT FREEBSD
attimer
ATTIMER(4) BSD Kernel Interfaces Manual ATTIMER(4)
NAME
attimer -- i8254 Programmable Interval Timer (AT Timer) driver
SYNOPSIS
This driver is a mandatory part of x86 kernels.
The following tunables are settable from the loader(8):
hint.attimer.X.clock
controls support for the event timer functionality. Setting this value to 0 disables it. The default value is 1.
hint.attimer.X.timecounter
controls support for the time counter functionality. Setting this value to 0 disables it. The default value is 1.
hw.i8254.freq
allows overriding the default counter frequency. The same value is also available at run-time via the machdep.i8254_freq sysctl.
DESCRIPTION
This driver uses i8254 Programmable Interval Timer (AT Timer) hardware to supply the kernel with one timecounter and one event timer, and to
generate sound tones for the system speaker. This hardware includes three channels. Each channel includes a 16 bit counter which decreases
with a known, platform-dependent frequency. Counters can operate in several different modes, including periodic and one-shot. The output of
each channel has platform-defined wiring: one channel is wired to the interrupt controller and may be used as event timer, one channel is
wired to the speaker and used to generate sound tones, and one timer is reserved for platform purposes.
The attimer driver uses a single hardware channel to provide both time counter and event timer functionality. To make this possible, the
respective counter must be running in periodic mode. As a result, the one-shot event timer mode is supported only when time counter func-
tionality is disabled.
The event timer provided by the driver is irrelevant to CPU power states.
SEE ALSO
apic(4), atrtc(4), eventtimers(4), hpet(4), timecounters(4)
BSD
May 26, 2014 BSD