This program should read the values of an array one at a time, and based on
the value in the array location, perform some arithmetic operation on values on
a stack.
The following are numeric values of arithmetic operations:
- 0 = push number,
number is in next location
- 1 = add top number to
second number
- 2 = subtract top
number from second number
- 3 = multiply top
number by second number
- 254 = display top
number + newline
- 255 = exit
Here are a few small arithmetic script examples:
SCRIPT1 = 0, 100, 0 200, 1, 0, 3, 3, 0, 10, 2, 254, 255
The script is translated into this: push 100, push 200,
add, push 3, multiply, push 10, subtract, display, exit.
After push 100, push 200 are evaluated, the stack looks like
this:
| |<- sp
|-----|
|-----|
| 100 |
|-----|
The add instruction pops the top two numbers, adds them
together, and pushes the result. After the first add is evaluated, the
stack looks like this:
| | <- sp
|-----|
| 300 |
|-----|
When the example script given above is run, it displays the
single number 890.
Your assembly language program must be general enough to run any script
specified in the data area of your program.
To run the script, read the first value in the script. Based on that value,
call a subroutine. The ``0'' subroutine reads the next value in the script and
push it on the stack, and then returns. The ``1'' subroutine pops 2 values from
the stack, adds them, pushes the result, and returns. Etc.
You should designate one register, say r12, as your script array
index. This register will always point to the current element in the array of
script numbers (instructions).
The main body of the program is a loop which:
- reads the value from the
script array at the location indicated by r12, and
- calls the appropriate
subroutine based on that value.
It does this repeatedly until subroutine ``255'', the exit
subroutine, is executed.
Hints:
-you will need to design a “case statement”
- a display function which converts from a number to a
string ascii codes which can be printed.
to convert a hex digit to ascii
AREA asciit, CODE,
READONLY
SWI_WriteC EQU &0
SWI_Exit EQU &11
Mask EQU
0x0000000F
ENTRY
start
LDR r0, Digit ;load
digit
LDR r1, Result ;address
for result
CMP r0, #0xA ;is
the number < 10 decimal
BLT Add_0 ;
then branch
ADD r0, r0, #"A"-"0"-0xA
;add offset for 'A' to 'F'
Add_0 ADD r0, r0, #"0" ;convert to ASCII
STR r0,[r1]
SWI SWI_WriteC ;ouput it
SWI SWI_Exit ;all done
AREA Data1, DATA
Digit DCD &0C
AREA Data2, DATA
Result DCD 0
END