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Detailed Flowcharts

Appendix D

D17

Figure D.20

SLEEP and W

AKEUP Interaction

PROCESS A PROCESS B PROCESS

SLEEP

A Previous

Wakeup

Later

WAKEUP

NOTE: Sequence of processor

execution

is indicated by circled

numbers.

10090–I

The SLEEP and WAKEUP subroutines are always used in connection

with some type of indivisible process interlock. Indivisibility is achieved

on many processors (as on the Z80) by disabling processor interrupts. For

this reason, SLEEP and WAKEUP assume that interrupts are off when

they are called. They will always return with interrupts off.

When one process calls SLEEP, the results is a return from a call to

WAKEUP by another process. When a process calls WAKEUP, the result

is a return from a call to SLEEP by another process. An interrupt

subroutine that calls WAKEUP is viewed as a subroutine of the

interrupted process.

Figure D.20 shows an example of interaction between SLEEP and

WAKEUP. In this example, process B work up process A some time ago.

Now at 1, when A goes to sleep, actual execution resumes after the

wakeup call in B at 3 and 4. Some time later, process C (at an interrupt,

for example) calls WAKEUP at 5. Execution flow proceeds to the

instructions at 8 following the call to sleep in process A. The next time A

calls SLEEP, the WAKEUP call in C will terminate.

This is not the only possible implementation of SLEEP and WAKEUP. A

second alternative implementation would allow a process to call

SLEEP and WAKEUP

1 2 ... 222 223 224 225 226 227 228 229 230 231 232 ... 238 239

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