Chapter 1

Figure 2.6
The kernel executive contains support for device drivers so that a user program can issue a
request to device driver code running kernel-side, in either the kernel executive or kernel
server. Such requests typically initiate a device operation, or tell the driver that the
requesting program is waiting for something to happen on the device.
Drivers also process device interrupts and then tell the user (or kernel) program that an
earlier request is complete. Interrupt handling works at two levels.
� First-level handling is done by an interrupt service routine (ISR). ISRs must be short,
and can't do very much, because they could occur at any time, even in the middle of a
kernel server operation. Usually, they simply acknowledge the device that raised the
interrupt and then set a flag to request the kernel to run a delayed function call (DFC)
for second-level processing.
� The kernel schedules the DFC when it is in a more convenient state – immediately, if
user-mode code was executing when the interrupt occurred; otherwise, when the kernel
would have otherwise crossed the privilege boundary back to user code. DFCs can use
most kernel APIs. DFCs typically do a small amount of processing and then post to a
user thread to indicate that an I/O request has completed.
2.8 Timers
The kernel supports a tick interrupt that is used to drive round robin scheduling of equal
priority threads. It runs at 64 Hz on ARM, unless that frequency is inconvenient for specific
hardware, and at 10 Hz on the PC-based emulator.
The tick interrupt can also be accessed (via User::After() and RTimer::After()
function calls) by user programs. The tick interrupt suspends during power-off, so that if you
request a timer to expire after 5 s, and then turn the machine off 2 s later, the timer event will
occur 3 s after you turn the machine back on again – or even later, if you immediately turn
the machine back off!
The kernel also supports a date/time clock, which you can access using User::At() and
RTimer:At(). This timer expires at exactly the time requested. If the machine was turned
off when the timer expires, it is turned on. This behavior makes it a suitable time for alarms,
as the system will power up and can then alert the user.
2.9 Memory
System memory is managed by the memory management unit (MMU).