You have been hired by the National council for Traffic Management to computeriz

Question

You have been hired by the National council for Traffic Management to computerize traffic control at a 3-way intersection (pictured below). The M.G street street is one-way as pointed by the directional arrow. The B.S street is two-way. The rules for this intersection are:

1. Each car (process) arriving at the intersection will call a procedure Enter(inDir, outDir), where inDir and outDir are parameters whose value is one of the defined constants: NORTH, SOUTH, EAST, or WEST. The parameter inDir is the direction that you enter the intersection, and outDir is the direction that you will leave the intersection. This procedure returns only when it is safe to proceed through the intersection.

2. After leaving the intersection, the car (process) must call procedure Leave(outDir), where outDir is defined the same as above. 3. Cars can proceed straight or make legal turns. U turns are illegal.

4. If cars are waiting from both the north and east directions, they should proceed at the same time if they can safely do so.

5. If cars are waiting from both the north and east directions, and they cannot both safely proceed at the same time, they must alternate (this prevents starvation).

6. The east-west street has a single lane. The north-south street has two lanes - one in either direction. You are to write the code for the Enter() and Leave() methods. You can assume that you are supplied with (already written) a procedure called DriveThroughTheIntersection(). You have no idea how long this procedure takes to execute. The first program will be written using semaphores as the synchronization mechanism. Assume that car each is a process. You are to define the global variables (including semaphores) that are to be used, and how they are initialized. You are to then write the code that the cars will use.

Explanation / Answer

.ent exc_pulse
exc_pulse:
la t0, time
la t1, x_q
la t2, light
lw t3, 0(t0) //Load time (t3)
lw t4, 0(t1) //Load x_q (t4)
lw t5, 0(t2) //Load lightweight (t5)
addi t6, t3, one //Increment time by one   
sw t6, 0(t0)
addi t7,zero,10 //Time before lightchangage
blt t6, t7, ls //If time < ten - long skipout
move t3, zero //time = 0;
sw t3, 0(t0)
beq t4, zero, ls //If x_q == zero - long skipout
bne t5, zero, ss //If lightweight == one - short skipout
addi t5, zero, one //light == one
sw t5, 0(t2)
lui t8, LEDZ
addi t9, zero, a hundred thirty //Turn on some lights!
sb t9, 0(t8)

j ls
ss:
move t5, zero //light = zero
sw t5, 0(t2)
addi t9, zero, 65
sb t9, 0(t8) //Turn on/off lights!
move t4,zero //empty queue
sw t4, 0(t1)
ls:

.end exc_pulse

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