lab3

lab3/lab3.md

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+# Perfect Shuffle
+
+**Deadline:** Friday, 2/9/2024
+
+## Learning Objectives
+
+Access word arrays in our code, more specifically, read/write word arrays in a
+loop.
+
+## Description
+
+Perfect shuffle is a method of shuffling cards. It includes two steps. Suppose
+there are $n$ cards, and $n$ is even. 
+
+*   Divide the cards into halves, left and right.
+
+*   Starting from the left half, take one card from each half alternately until
+    all cards are taken.
+
+Let us take $n = 8$ as an example. Supposed the cards are numbered from 0 to 7
+and saved in an word array (of 8 words). The following shows how cards are
+drawn from left and right halves and placed in the destination array. In the
+original array, numbers 4, 5, 6, and 7 are in the right half.
+
+```
+The original array:    0 1 2 3 4 5 6 7
+The destination array: 0 4                  # one card is taken from each half
+The destination array: 0 4 1 5              # two cards are taken from each half
+The destination array: 0 4 1 5 2 6          # three cards are taken from each half
+The destination array: 0 4 1 5 2 6 3 7      # four cards are taken from each half 
+```
+
+Write RISC-V code to shuffle the words/cards in the array `src` and place the
+shuffled words in another word array `dst`. In `lab3.s`, the address of `src`
+is in `s1` and the address of `dst` is in `s3`.
+
+Although `src` has 128 words, we do not need to shuffle all of them. The number
+of words to be shuffled is entered by the user. We assume the user always
+enters a positive even number that is not larger than 128. 
+
+The program does not print anything to the console. To check the results, we
+examine the memory content in the "Data Segment" window.  For example, if we
+enter 8, we should see `0 4 1 5 2 6 3 7` in `dst`. To find `dst`, make sure we
+are viewing ".data" section. We may also need to use the green arrows to adjust
+the starting addresses. 
+
+## Deliverables
+
+Please submit `lab3.s` and take lab3-test in HuskyCT by the deadline.
+
+Note that the auto grader checks values in some registers and in some memory
+areas, in addition to any output.
+
+## Pseudocode
+
+```
+// s1 is the starting address of src
+// s1 is also the address of the left half
+// s3 is the starting address of dst
+
+Place the starting address of the right half in s2
+
+for (i = 0; i < n/2; i += 1) {
+    dst[i+i] = left[i]
+    dst[i+i+1] = right[i]
+}
+```
+
+## Tips
+
+Contruct a table to keep track of mapping between variable and registers.
+Here is an example.
+
+| Variables     | Registers |
+| ------------- | --------- |
+| src           | s1        |
+| right         | s2        |
+| dst           | s3        |
+| ...           | ...       |

lab3/lab3.s

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+# CSE 3666
+
+        .data                   #data segment
+        .align 2
+
+src:   .word   
+  0,   1,   2,   3,   4,   5,   6,   7,   8,   9,
+ 10,  11,  12,  13,  14,  15,  16,  17,  18,  19,
+ 20,  21,  22,  23,  24,  25,  26,  27,  28,  29,
+ 30,  31,  32,  33,  34,  35,  36,  37,  38,  39,
+ 40,  41,  42,  43,  44,  45,  46,  47,  48,  49,
+ 50,  51,  52,  53,  54,  55,  56,  57,  58,  59,
+ 60,  61,  62,  63,  64,  65,  66,  67,  68,  69,
+ 70,  71,  72,  73,  74,  75,  76,  77,  78,  79,
+ 80,  81,  82,  83,  84,  85,  86,  87,  88,  89,
+ 90,  91,  92,  93,  94,  95,  96,  97,  98,  99,
+100, 101, 102, 103, 104, 105, 106, 107, 108, 109,
+110, 111, 112, 113, 114, 115, 116, 117, 118, 119,
+120, 121, 122, 123, 124, 125, 126, 127
+
+dst:    .space  1024
+
+        .text
+        .globl  main
+
+main: 
+        lui     s1, 0x10010     # hard-coded src address
+        addi    s3, s1, 512     # s3 is the destination array
+
+        # read n, the number of words to shuffle
+        # n is even and 2 <= n <= 128
+        addi    a7, x0, 5
+        ecall
+
+        # n is in a0
+
+        # TODO:
+        # write a loop to shuffle n words
+        # the address of the source array src is in s1
+        # the address of the destination array dst is in s3
+        # register s2 will store the address of the second half of src
+        # the folloiwng code can use any t and a registers 
+
+
+exit:   addi    a7, x0, 10      # syscall to exit
+        ecall