lab1

lab1/lab1.md

@@ -0,0 +1,81 @@
+# Find GCD (Greatest Common Divisor)
+
+**Deadline:** Friday, 1/26/2023.
+
+We will keep the submission open until 2/2/2023. 
+
+Write concise comments in code.
+
+## Learning Objectives
+
+* Write and run RISC-V programs in RARS. See 
+  [this page](https://github.com/zhijieshi/cse3666/blob/master/misc/rars.md) on 
+  how to install RARS and run RISC-V programs in RARS. 
+
+* Program with RISC-V arithmetic and control flow instructions. 
+
+* Read RISC-V assembly code and learn basic syntax and assembler directives
+  like .text and .globl.
+
+* Read and learn from help files and documentations.
+
+## Description
+
+In this lab, we write a program in RISC-V assembly language that finds
+the GCD of two positive integers.
+
+The program reads two positive integers, and then computes and prints their
+GCD.  The algorithm we use is Euclidean algorithm. Although it is common to use
+division in the algorithm, we will use subtraction only, as originally
+described by Euclid. The pseudocode is provided below.
+
+```
+# Given two positive integers a, b, repeatedly subtract the smaller one from the larger one
+# until the two integers are the same. 
+
+WHILE (a != b) 
+    IF (a > b) 
+        a = a - b
+    ELSE 
+        b = b - a
+```
+
+Skeleton code in `lab1.s` already has the instructions to read two integer and
+save them in `s1` and `s2`. Implement the following steps with RISC-V
+instructions to complete the program. 
+
+*   Compute the GCD of the two numbers with Euclidean algorithm by subtraction.
+    We recommand the following two steps.
+
+    1.  Write the while loop.
+
+    2.  Write the if-else statement inside the loop.
+
+*   Print the GCD in decimal with a system call.
+
+    The list of system calls is in RARS's help file. Select menu "Help/Help"
+    and then select "Syscalls" tab.  Scroll up to the beginning of the page if
+    necessary. **There is an example before the table.**
+
+    The skeleton code also has examples of system calls, two for reading
+    integers and one for terminating the program. Check wether you can write
+    the instructions from the information in the system call table. With
+    the system call for printing the result, the submitted code has four
+    system calls.
+
+### Testing
+
+The skeleton code includes some GCD examples. Please test your code with more
+positive numbers. In test cases 1 - 3 in Gradespce, two integers are positive
+and less than 100. In test cases 4 and 5, the integers are less than 10,000.
+
+## Deliverables
+
+Please submit `lab1.s` and take lab1-test in HuskyCT by the deadline. Please
+check the filename if the system does not grade the submission.
+
+## Tips
+
+* It can be faster to use keyboard shortcuts to assemble/run/reset the program.
+
+* Use breakpoints and backsteps to debug.

lab1/lab1.s

@@ -0,0 +1,33 @@
+#       CSE 3666 Lab 1
+
+        .globl  main
+
+        .text
+main:   
+
+        # GCD examples:
+        #     gcd(11, 121) = 11
+        #     gcd(24, 60) = 12
+        #     gcd(192, 270) = 6
+        #     gcd(14, 97) = 1
+
+        # read two positive integers from the console and 
+        # save them in s1 and s2 
+
+        # use system call 5 to read integers
+        addi    a7, x0, 5
+        ecall
+        addi    s1, a0, 0       # a in s1
+
+        addi    a7, x0, 5
+        ecall
+        addi    s2, a0, 0       # b in s2
+
+        # TODO
+        # Add you code here
+        # compute GCD(a, b) and print it
+
+
+        # sys call to exit
+exit:   addi    a7, x0, 10      
+        ecall