The Spanish government is considering introducing charges for the use of motorways. One possible mechanism is that detector stations can be set up at regular intervals along all motorways; as a vehicle passes the detector station its details are logged and a road charge is recorded. At the end of each month the vehicle owner can be sent a bill for his or her motorway usage.
Each vehicle will require an interface device which interrupts an on-board computer when a detector station requests its details. The computer has a word size of 16 bits and has memory-mapped I/O, with all I/O registers 16 bits in length. Interrupts are vectored, and the address of the interrupt vector associated with the detector station is $8 \# 60 \#$. After an interrupt has been received, a read-only input register located at $8 \# 177760 \#$ contains the basic cost of using the current stretch of motorway (in euros). The hardware priority of the interrupt is 4 . The computer software must respond to the interrupt within a period of 5 seconds, and pass its ownership details to the detector station via the interface device. It does this by writing to a bank of five control and status registers located at 8\#177762\#. The structure of these registers is shown in Table 14.1.
TABLE CAN'T COPY.
The bank of 'control and status' registers (CSR) is write-only.
Write an Ada task to interface with the detector station. The task should respond to interrupts from the interface device and be responsible for sending the correct vehicle details. It should also read the data register containing the cost of the road usage, and pass the current total cost of the journey to a task which will output it on a visual display unit on the vehicle's dashboard. You may assume the following is available:
package Journey..Details is
Registration_Number : constant String(1..8) :=
"....... ";
type Travel_Details is
(Business, Pleasure, Overseas_Tourist,
Police, Military, Emergency_Service);
for Trave1_Details use
(Business $\Rightarrow 1$, Pleasure $\Rightarrow 2$,
Overseas_Tourist $\Rightarrow 3$, Police $\Rightarrow 4$,
Military $\Rightarrow 5$, Emergency_Service $\Rightarrow$ 6) ;
subtype Security_Code is Integer range 0 .. 2047;
function Current_Journey return Travel_Details;
function Code return Security_Code;
end Journey_Details;
package Display_Interface is
task Display_Driver is
entry Put_Cost (C : Integer);
-- prints the cost on the dashboard display
end Display_Driver;
end Display_Interface;
Assume that the compiler represents the type Character as an 8-bit value and the type Integer as a 16-bit value.