Specifications

Understanding the Kernel Environmen

9-19

sor. Only interrupt requests at level 4 or higher are presented to the processor. All other

requests are ignored until the same processor lowers the interrupt level. A device driver

can be programmed to temporarily raise the processor Interrupt Priority Level (IPL) to

block undesirable interrupts. Thereafter, the driver lowers the processor’s IPL to its previ-

ous level.

The processor priority level adjustment functions are as follows (see spl(D3)):

splbase Block no interrupts to the processor (same as spl0)

spltimeout Block functions scheduled by itimeout and dtimeout

(see “Timing and Timeout Routines” for an explanation of

these routines)

spldisk Block disk device interrupts

splstr Block STREAMS interrupts

spltty Used by a TTY driver to protect critical code—spltty is

mapped to splstr

splhi Block all hardware interrupts, including the clock—should

be used sparingly

spln Block all interrupts at or below the value of n, where n

ranges from 0 to 8

spl0 Equivalent to splbase

spl8 Equivalent to splhi

splx Restore the processor’s interrupt priority level

Timing and Timeout Routines 9

The kernel provides the following timing and timeout routines for timing and timeout pur-

poses:

timeout(D3) Execute a function after a speciﬁed length of time

itimeout(D3) Execute a function after a speciﬁed length of time

dtimeout(D3) Execute a function on a speciﬁed processor at a speciﬁed

interrupt priority level after a speciﬁed length of time

untimeout(D3) Cancel previous timeout request

timeout(func, arg, interval) can be used in a character device driver in which interrupts

cannot be used to acknowledge a device operation or signal an event. It can be used as an

alternative to a busy wait to schedule a function after a reasonable amount of time. After

receiving a RESET ioctl command, a driver can schedule the completion of the reset

operation by scheduling a routine to do so after one second.