# Create Program Using MIPS Language Using MARS

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### Question Description

You must write a MIPS assembly language program to generate pseudo-random numbers. They are not true random numbers because the algorithm always generates the same sequence when started with the same initial “seed” value.

Write a 32 bit pseudo-random number generator function, using a 32 bit LFSR (Linear Feedback Shift Register) – see:

https://en.wikipedia.org/wiki/Linear-feedback_shift_register

Animated graphic illustrating a 4 bit LFSR using taps 3 and 4: https://upload.wikimedia.org/wikipedia/commons/7/7f/LFSR-F4.GIF

Note that in the schematic and table at the bottom of the article, the bit numbering used for this notation is reversed from that normally used: bit 1 is the MSB and bit 4 is the lsb for the animation above.

The initial value loaded into the LFSR is called the “seed” for the random number sequence. LFSRs must not be initialized to all zeros because they would remain stuck in that state, and the pseudo-random sequence must not generate all zeros.

The LFSR must be stored as a 32 bit word in the .data memory area and the argument in \$a0 = 0 at entry to the subroutine for normal operation, and the next pseudo-random value is returned in \$a0 and stored in the LFSR .data memory location. If the value of \$a0 is not 0, then the value in \$a0 is to be used as the seed and is simply stored in the LFSR .data memory location like this:

.data
lfsr: .word 0 # a place to store the random number

# The value in \$a0 is an argument passed from the main program to the random number routine

# if argument \$a0==0 the subroutine calculates the next 32 bit pseudo-random number and

# stores it in memory location lfsr, and returns it in \$a0

# if argument \$a0!=0 then the content of \$a0 is stored in .data memory as the initial “seed” value

#

You are required to write your own unique and original commented MIPS assembly language subroutine that is *different* than the code generated by a compiler. To verify that the output is correct, you should work through the algorithm by hand for one 32 bit shift cycle, or if you wish you may write the function in C to compare the results of your assembly program. You must call your random number function using the MIPS instruction jal (jump and link) from your main program.

Note that one bit shift generates one bit and 32 NEW bits must be generated for each time the function is called.

You must write your MIPS assembly lfsr function and display the first ten 32 bit values generated when starting with a seed value of 0x F00F5AA5. The LFSR assembly function should use the following “taps” or bit numbers to XOR together: 32 30 26 and 25 (Where bit 32 is the rightmost bit, the LS Bit.) Those 4 bits are XOR’d and the result is shifted into bit 1, the MSB, and all the other bits are shifted right one position (what was in bit 1 shifts to bit 2, bit 2 shifts to 3, and so on). When XOR is performed on those 4 bits, the XOR result is 1 iff the input has an odd number of ones, and 0 otherwise.

The selection of taps for the LFSR above generates a maximum length pseudo-random sequence with all 4294967295 possible unique values not including 0.

You may use Wikipedia and other resources for reference, but you must come up with your own unique and original assembly language solution, and you may NOT submit code generated by a compiler or copy code from any other source. From now on your code will be checked for plagiarism, so be sure to cite any references and write your own code! Do not share your code with anyone else.

As always, include the template in a document of type .doc .docx or .pdf containing a description of your code, ALL the assembly source code pasted into the document, as well as a screen shot of the output of your program. You must also submit separate file(s) containing the source code so that we can reproduce your results.

Please submit the required .doc and .asm files separately (not in a zip, tar or other compressed form) using the link above. You may re-submit if necessary, but include all the required files again and make a comment in the text box explaining what you changed when you resubmit.

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