$($a$)$ In this question, you may invent your own instruction set as long as it is reasonable

for your program. You should also comment your program properly.

The City Observatory is building an automatic system to issue bad weather

warning to the citizen. There is a rainfall meter and a wind speed meter to

measure the amount of rainfall $($in $mm)$ and the wind speed $($in $\frac{m}{s}).$ The

control and status register of rainfall meter $($RFCSR$)$ has the following format:

Bit $0$ Error bit, the rainfall meter is in error if bit$=1$

Bit $1$ Ready bit, bit $=1,$ the rainfall meter is ready

Bit $2$ Data available, bit$=1,$ data available in RFBR$,$ automatically

cleared when RFBR has been read

Bit $3$ Set this bit to start reading the rainfall meter.

Similarly, the control and status register of windspeed meter $($WSCSR$)$:

Bit $0$ Error bit, the windspeed meter is in error if bit$=1$

Bit $1$ Ready bit, bit $=1,$ the windspeed meter is ready

Bit $2$ Data available, bit$=1,$ data available in WSBR$,$ automatically

cleared when WSBR has been read

Bit $3$ Set this bit to start reading the windspeed meter.

The buffer registers for rainfall meter and widspeed meter are RFBR and WSBR

respectively. The value return is the multiple of $0\mathrm{.}1mm$ and $0\mathrm{.}1\frac{m}{s}.$ For example,

if $5$ is read on the rainfall meter, the rainfall is $0\mathrm{.}5mm.$

The $7-$segment LED display is used to display the weather condition, It is controlled

by the buffer register LEDBR. The value $($from $0$ to $9)$ written to LEDBR will be

displayed on the LED. If the value $10$ is written, a hyphen $(-)$ will be displayed.

Write an assembly language program that uses Programmed $\frac{\text{I}}{O}$ to read the

rainfall and windspeed meters continuously. $[10]$

If there is an error in either meter, display a $"-"$ on the LED. Otherwise,

$($b$)($i$)$ The above program uses programmed I$/$O$.$ If interrupt$-$driven I$/$O is used

instead, describe what the process will be $($no program needed$),$ including

how the CPU services the interrupt, information saving and return. $[3]$

$($ii$)$ With reference to the program in part $($a$),$ if interrupt driven I$/$O is used,

which part of the program will not be needed anymore. $[2]$