lab 4

lab4/lab4.md

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+# Binary search
+
+**Firm Deadline:** Monday, 2/19/2024
+
+**Important:** This is a firm deadline. Lab4-test is due on the same day.
+
+Write concise comments in code.
+
+## Learning Objectives
+
+* Review RISC-V ISA, e.g., support for control flow and memory access 
+
+* Implement nested functions and recursion
+
+* Save/restore registers in a function
+
+## Description
+
+In this lab, we write function `binary_search` in RISC-V. The prototype of the
+function and an implementation in C is at the end of this page. Translate the
+C code at the bottom of the page to RISC-V assembly code. 
+
+The skeleton code is in `lab4.s`. Function `binary_search` is located at the
+end of the file. It is empty in the skeleton code.
+
+There are some constraints/tips.  
+
+1.  To ensure we do not use pseudoinstructions, we turn off the feature in
+    RARs. In Settings, uncheck "Permit extended (pseudo) instructions and
+    formats".
+
+2.  Follow the C code closely. Although binary search is simple, it is very
+    easy to make mistakes if you have not written it many times.
+
+3.  We keep variable `left` in register `s1`. Other local variables do not have
+    to be in a saved register. Think about why we need to keep `left`, but not
+    other variables, in a saved register.
+
+4.  There should be only one exit (one return instruction) in the function. In
+    the C code, we use `goto f_exit` on purpose. With one exit, we do not have
+    multiple copies of clean up code, e.g., restoring saved registers.  
+
+5.  There is an IF-ELSEIF-ELSE structure. Write the outline first, without
+    instructions in each branch. Follow the order of branches in C code: IF
+    branch, then the ELSEIF branch, and finally the ELSE branch. It is easier
+    to check the code this way. 
+
+6.  There is only one LW instruction, for reading `a[half]`, between the code
+    that saves/restores registers.
+
+7.  During the execution of the program, examine how `ra` and `sp` registers
+    are changed and what have been saved on the stack. It helps to understand
+    how funciton and local storage work.
+
+### Testing
+
+The array used for testing has multiple of 10s between 0 and 990, inclusive.
+Here are some inputs/outputs of the binary search function. Note the program
+does not print anything to the terminal. We check the return value in RARS in
+the Registers tab.
+
+```
+0
+0
+
+460
+46
+
+990
+99
+
+45
+-1
+
+1000
+-1
+
+-3
+-1
+```
+
+## Deliverables
+
+Please submit lab4.s and take lab4-test in HuskyCT by the deadline.
+
+In addition to the output, the auto grader also checks some values in registers
+and in memory. For example, the value of `s1` should be preserved through the
+function all.
+
+## C code 
+
+Here is the C code of the function.
+
+``` C
+int     binary_search(int a[], int n, int v)
+{
+    int     rv;
+
+    if (n == 0) {        // nothing in the array
+        rv = -1;
+        goto  f_exit;    // return -1
+    }
+
+    int half = n / 2;   // integer division
+    int half_v = a[half];
+
+    if (half_v == v) {
+        rv = half;
+    }
+    else if (v < half_v) {
+        // search the first half, excluding a[half]
+        rv = binary_search(a, half, v); 
+    }
+    else {  // v > half_v
+        // search the second half, excluding a[half]
+        int left = half + 1;
+
+        // &a[left] means the address of a[left]
+        rv = binary_search(&a[left], n - left, v);
+
+        if (rv >= 0) {
+            rv += left;
+        }
+    }
+
+f_exit:
+    return rv;
+}
+
+```

lab4/lab4.s

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+#       CSE 3666 Lab 4
+#	TAG: 5fd71ac11b8a14746aa31ed0caf142197fb20839
+
+	.data
+	.align	2	
+word_array:     .word
+        0,   10,   20,  30,  40,  50,  60,  70,  80,  90, 
+        100, 110, 120, 130, 140, 150, 160, 170, 180, 190,
+        200, 210, 220, 230, 240, 250, 260, 270, 280, 290,
+        300, 310, 320, 330, 340, 350, 360, 370, 380, 390,
+        400, 410, 420, 430, 440, 450, 460, 470, 480, 490,
+        500, 510, 520, 530, 540, 550, 560, 570, 580, 590,
+        600, 610, 620, 630, 640, 650, 660, 670, 680, 690,
+        700, 710, 720, 730, 740, 750, 760, 770, 780, 790,
+        800, 810, 820, 830, 840, 850, 860, 870, 880, 890,
+        900, 910, 920, 930, 940, 950, 960, 970, 980, 990
+
+        # code
+        .text
+        .globl  main
+main:   
+	addi	s0, x0, -1
+	addi	s4, x0, -1
+	addi	s5, x0, -1
+	addi	s6, x0, -1
+	addi	s7, x0, -1
+
+	# help to check if any saved registers are changed during the function call
+	# could add more...
+
+        # la      s1, word_array
+        lui     s1, 0x10010      # starting addr of word_array in standard memory config
+        addi    s2, x0, 100      # 100 elements in the array
+
+        # read an integer from the console
+        addi    a7, x0, 5
+        ecall
+
+        addi    s3, a0, 0       # keep a copy of v in s3
+
+        # call binary search
+        addi	a0, s1, 0
+        addi	a1, s2, 0
+        addi	a2, s3, 0
+        jal	ra, binary_search
+
+exit:   addi    a7, x0, 10      
+        ecall
+
+#### Do not change lines above
+binary_search:
+
+        # TODO
+