lab2

lab2/lab2.md

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+# Reverse bits
+
+**Deadline:** Friday, 2/2/2024
+
+*If you work on a lab computer, save your files to cloud storage like OneDrive.
+Otherwise, you may lose your files.*
+
+Write concise comments in code.
+
+## Learning Objectives
+
+* Logical and bit-wise operations 
+
+* Binary representation of signed integers
+
+* ASCII characters
+
+## Description
+
+In this lab, we write RISC-V code to reverse the order of the bits in a
+register. The skeleton code, in `lab2.s`, reads an integer and saves it
+in `s1`. 
+
+Implement the following steps with RISC-V instructions.
+
+*   Write a loop to reverse the bits in `s1` and save the results in `s2`. 
+
+    Think aobut the strategy/algorithm first and then implement the method
+    provided at the bottom of this page. Note that we are working on bits in
+    registers and do not interprete them as signed values.
+
+*   Use a system call to print `s1` in binary. 
+
+    Find the system call number in RARS's help.
+
+*   Use a system call to print a newline character (ASCII value 10). 
+
+    We also need to find the system call number for printing a character.
+
+    Tips: We can use `'\n'` as the immediate in instructions. 
+
+*   If `s1` is a binary palindrome (i.e., bits are the same from left to right,
+    and from right to left), print character `=`. Otherwise, use a system call
+    to print `s2` in binary. 
+
+*   Use a system call to print a newline character.   
+
+Here are some example inputs/outputs of the program. 
+
+```
+170
+00000000000000000000000010101010
+01010101000000000000000000000000
+
+20232023
+00000001001101001011011101010111
+11101010111011010010110010000000
+
+1431655765
+01010101010101010101010101010101
+10101010101010101010101010101010
+
+-3666
+11111111111111111111000110101110
+01110101100011111111111111111111
+
+-2010819982
+10001000001001010101001001110010
+01001110010010101010010000010001
+
+0
+00000000000000000000000000000000
+=
+
+-1
+11111111111111111111111111111111
+=
+
+1342545930
+01010000000001011010000000001010
+=
+
+```
+
+## Deliverables
+
+Please submit "lab2.s" and take lab2-test in HuskyCT by the deadline.
+
+## Pseudocode 
+
+Here is one of the algorithms that reverse bits. 
+
+```
+1.  t0 = s1         // copy s1 to t0
+2.  s2 = 0
+2.  FOR (i = 0; i < 32; i += 1)  // repeat 32 times
+        // copy the the LSB in t0 to s2 
+        s2 <<= 1            // s2 = s2 << 1
+        s2 |= (t0 & 1)
+        t0 >>= 1            
+```

lab2/lab2.s

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+#       CSE 3666 Lab 2
+
+        .globl  main
+
+        .text
+main:   
+        # use system call 5 to read integer
+        addi    a7, x0, 5
+        ecall
+        addi    s1, a0, 0       # copy to s1
+
+        # TODO
+        # Add you code here
+        #   reverse bits in s1 and save the results in s2
+        #   print s1 in binary, with a system call
+        #   print newline
+        #   print '=' if s1 is palindrome, otherwise print s2 in binary
+        #   print newline
+
+        # exit
+exit:   addi    a7, x0, 10      
+        ecall