For this assignment you will write MAL subprograms for converting a
string of hexadecimal characters to an integer.
You will first write a subprogram hexVal that
converts hexadecimal digit characters to their MIPS long value.
Then you will add the subprogram toInt that uses hexVal
to convert strings of hexadecimal digits to MIPS long values.
Test software will be provided.
In this program you will get experience using branches for more complex
control structures and using the jalr instruction for
subprogram calls and the jr instruction for subprogram
returns.
The test software is provided in the file conversion.zip. This unzips to create a folder named "conversion". It contains the following files:
hexVal subprogram.
All it does is just return.
toInt subprogram.
All it does is just return.
hexValTest subprogram for testing
hexVal.
toIntTest subprogram for testing
toInt.
hexValTest and toIntTest.
If you have configured Mars settings correctly you should be able to open any one of these files in Mars, then assemble and run the program. It won't, of course, give correct results until you have implemented the two subprograms.
hexVal Subprogram
The algorithm for hexVal is described by the following C
code:
/** hexVal(ch) returns the hexadecimal value of the character ch, or
* -1 if it cannot be a hexadecimal digit.
*/
int hexVal(char ch) {
if ((ch1 >= '0') && (ch <= '9')) {
return ch - '0';
}
if (ch >= 'a') and (ch <= 'f') {
return ch - 'a' + 10;
}
if (ch >= 'A') and (ch <= 'F') {
return ch - 'A' + 10;
}
return -1;
}
If you implement this algorithm as shown you should not need to use any
$t registers (do not use $s registers in any case).
This simplifies the coding of the toInt subprogram.
Note that in MAL you can do a subtraction that mixes characters and integers. MAL treats a character literal as if it was the integer ASCII code for the character.
hexVal Tests
The test software tests toInt with characters from the
string:
testString:
.asciiz "/059:@ACFG`acfg"
The characters in this test string should reveal any problems with your
toInt implementation.
toInt Subprogram
toInt converts a null-terminated string of hexadecimal
digits to a word size integer.
It should have an input parameter which is the address of the string to
be converted.
It should also have two output parameters.
The first is a word representing the integer value of the string,
interpreted as a hexadecimal number.
The second is a boolean success parameter.
It should be set to false (0) if any non-hexadecimal characters are
encountered in the string or if the string is empty.
Otherwise, the success parameter should be set to true (nonzero).
Your toInt subprogram should call the hexVal subprogram for
conversion of the individual hexadecimal digits (characters) to integer
values.
It should use a repeated multiplication algorithm for conversion.
Since the toInt subprogram calls another subprogram,
hexVal, you cannot use the simple subprogram call protocol
because you need to push the return address for toInt onto
the run-time stack before calling hexVal and pop it after
the call.
Both of the test subprograms illustrate how this is done.
If your hexVal subprogram uses $t registers then the
toInt subprogram will also need to save them on the
run-time stack before calling toInt and restore them after
toInt returns.
toInt Tests
The test software provides the following test strings for
toInt:
test1:
.asciiz "a"
test2:
.asciiz "ff"
test3:
.asciiz "43E"
test4:
.asciiz "C840"
test5:
.asciiz "7FfFfFfF"
test6:
.asciiz "7FfFgFfF"
test7:
.asciiz ""
For the last two test strings the success return value should be false — the strings are not valid hexadecimal digit strings.
Turn in both a copy of your program and a Mars session record showing a
session assembling and running the provided test software with your
hexVal and toInt subprograms.
Your TA may also ask you to demonstrate your program in lab.
hexVal.
toInt.