Question

Question: Repeat Exercise 2.43, but this time use ll/sc to perform an atomic update of the shvar variable directly, without using lock() and unlock(). Note that in this problem there is no variable lk.

Short answer

The MIPS assembly code to implement the given C code:

try: ll $t0, 0($a1)

slt $t1, $t0, $a2

bnez $t1, skip

mov $t0, $a2)

sc $t0, 0($a1)

beqz $t0,try

skip:

Step-by-step solution

Step 1:Determine theMIPS branch instructions and parallelism

Parallelism can be achieved by MIPS assembly language through a pair of special instructions. The instructions load linked and store conditional are used in sequence. If the contents of the memory location specified by the load linked are changed before the store conditional to the same address occurs, the store conditional fails.If the store conditional succeeds, it changes the value of the register to 1. Then the load linked will return the initial value. This sequence implements the atomic change on the memory location.

Determine the MIPS assembly code to implement the given C code.

Given C code:

lock(lk);

shvar=max(shvar.x);

unlock(lk);

It is given that: use ll/sc to perform an atomic update of the shvar variable directly, without using lock() and unlock().

MIPS code for the given C:

try: ll $t0, 0($a1) #load linked will load the value from memory and monitor the memory location to check if any process performs write to it

slt $t1, $t0, $a2 #set to less than will check the values of $t0 and $a2, if $t0 is less then $a2, then $t1 will be set to 1

bnez $t1, skip # check for the value of $t1, if the value is not equal to zero , then will branch to skip function

mov $t0, $a2 # value of $a2 will be moved to $t0

sc $t0, 0($a1) #store conditionals will store the value to the memory, if there is no writes to the monitored memory address.

beqz $t0,try #Checks the value of $t0 , if the value is zero , then the branch will move to try function.

skip: