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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 157
126
PROCESSES AND THREADS
CHAP. 2
The TSL Instruction
Now let us look at a proposal that requires a little help from the hardware.
Some computers, especially those designed with multiple processors in mind, have
an instruction like
TSL RX,LOCK
(Test and Set Lock) that works as follows. It reads the contents of the memory
word
lock
into register
RX
and then stores a nonzero value at the memory address
lock
.
The operations of reading the word and storing into it are guaranteed to be
indivisible—no other processor can access the memory word until the instruction is
finished. The CPU executing the
TSL
instruction locks the memory bus to prohibit
other CPUs from accessing memory until it is done.
It is important to note that locking the memory bus is very different from dis-
abling interrupts. Disabling interrupts then performing a read on a memory word
followed by a write does not prevent a second processor on the bus from accessing
the word between the read and the write.
In fact, disabling interrupts on processor
1 has no effect at all on processor 2.
The only way to keep processor 2 out of the
memory until processor 1 is finished is to lock the bus, which requires a special
hardware facility (basically, a bus line asserting that the bus is locked and not avail-
able to processors other than the one that locked it).
To use the
TSL
instruction, we will use a shared variable,
lock
, to coordinate
access to shared memory. When
lock
is 0, any process may set it to 1 using the
TSL
instruction and then read or write the shared memory. When it is done, the process
sets
lock
back to 0 using an ordinary
move
instruction.
How can this instruction be used to prevent two processes from simultaneously
entering their critical regions? The solution is given in Fig. 2-25.
There a four-in-
struction subroutine in a fictitious (but typical) assembly language is shown. The
first instruction copies the old value of
lock
to the register and then sets
lock
to 1.
Then the old value is compared with 0.
If it is nonzero, the lock was already set, so
the program just goes back to the beginning and tests it again. Sooner or later it
will become 0 (when the process currently in its critical region is done with its crit-
ical region), and the subroutine returns, with the lock set. Clearing the lock is very
simple. The program just stores a 0 in
lock.
No special synchronization instruc-
tions are needed.
One solution to the critical-region problem is now easy. Before entering its
critical region, a process calls
enter
region
, which does busy waiting until the lock
is free; then it acquires the lock and returns. After leaving the critical region the
process calls
leave
region
, which stores a 0 in
lock
.
As with all solutions based on
critical regions, the processes must call
enter
region
and
leave
region
at the cor-
rect times for the method to work. If one process cheats, the mutual exclusion will
fail. In other words, critical regions work only if the processes cooperate.
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