Ense 352 – Fall 2018 – Assign 3
Handed Out: 2018-10-09 Due: 2018-10-17 by 23h55
1. (10 marks) Write a function in assembly language named int_to_ascii which will extract the signed integer passed in R9 and convert it to a null-terminated ASCII string representing the value of the integer in decimal. For example, passing 0xc0010010 should create the ASCII string “-1073676272”. Note the following:
- the input value is stored in binary, in a register. I’m only showing it in hexadecimal for convenience, which is the only reason anyone ever uses hexadecimal.
- the output value does not include the double quotes.
- the output value must be terminated with a null byte so it actually will
occupy 12 bytes.
Here are some more examples of correct conversions
input output
34
-34
0x34
0x11111111 20 32 38 36 33 33 31 31 35 33 00
You should insert a minus sign (-) if the input is negative, and leave a single space otherwise.
You are required to allocate a sufficiently large RAM buffer to store the string and its terminating null byte. The buffer must start at location 0x2000 0030.
Your function should return the address of the converted string in register R10.
2. (10 marks) Write a function in assembly language named encrypt which takes a pointer to a buffer containing some cleartext, and modifies the buffer to con- tain the encrypted version of that text.
The encryption scheme is to add a (wrap-around) displacement n to each al- phabetic char where the letters of the alphabet are numbered A=1, B=2, etc.
actual hex bytes
|
” 34″ |
|
“-34” |
|
” 52″ |
|
” 286331153″ |
20 33 34 00 2d 33 34 00 20 35 32 00
1
The same encoding is used for lower-case letters: a=1, b=2, etc. For example given the clear text
AaBbCcDdEeFfGgHhIiJjKkLlMmNnOoPpQqRrSsTtUuVvWwXxYyZz
0123456789
Hello, my boots are layered like an onion.
!@#$%^&*()_-+={}[]\|
<>,.?/~‘;:"’
and assuming for example n = 13, the result should be
NnOoPpQqRrSsTtUuVvWwXxYyZzAaBbCcDdEeFfGgHhIiJjKkLlMm
0123456789
Uryyb, zl obbgf ner ynlrerq yvxr na bavba.
!@#$%^&*()_-+={}[]\|
<>,.?/~‘;:"’
Note that only the alphabetic characters have changed, case has been preserved, and each alphabetic character has been displaced by 13, for instance a Z be- ing character 26 becomes M (character 13), and a b (character 2) becomes o (character 15).
Your code should accept any n from 0 to 25. Of course 0 means no change. If a value larger than 25 is provided, your program should replace the contents of the buffer with the string: XXXXX…XXX.
The buffer pointer must be stored in R4 while the buffer size and rotate amount must be encoded in R3 as follows: size of the buffer must be stored in the bits 14:0 of R3, while the value of n must be stored in bits 19:15 of R3.
3. (5 marks) Write the complementary function decrypt which takes exactly the same parameters as encrypt in the same format, and undoes what encrypt did. Your decrypt should expect the same value of n to be passed as was used originally in encrypt. So if you called encrypt with n = 6 you must call decrypt also with n = 6 to decrypt that message. You should call encrypt as part of this process.
Rules to follow
It’s important that you follow all the requirements stated for all the questions; the marker will be testing the functions in q1, q2, and q2 by calling them from a C/assembly testing framework. If your code does not adhere to the spec that will cost significant marks.
Submit your solutions via urcourses in a single zipfile. Your submission will be re- jected if you don’t follow this. Place the solution to the questions in subfolders named
2
q1 and q23 (question 2 and 3 should be in the same project). You should only submit assembly language source files, and an explanatory text file name 00readme.txt in plain ASCII, which explains how your program works and any deficiencies in your implementation. Your code should be written in standard cortex-m3 assembly lan- guage.
3