Question

MIPS CODE

A simple problem that makes use both the shift and logic operations is the digit-packing example. Suppose that two decimal digits represented in ASCII code are located in the memory at byte locations LOC and LOC+1. We wish to represent each of these digits in the 4- bit BCD code and store both of them in a single byte location PACKED. The result is said to be in packed-BCD format. Hence, the required task is to extract the low-order four bits in LOC and LOC+1 and concatenate them into the single byte at PACKED. The pseudo-code version of the program in generic assembly is available in the figure below Move LoadByte LShiftL Add LoadByte And Or StoreByte R2. #LOC R3. (R2) R3, R3, #4 R2, R2. #1 R4. (R2) R4. R4. #oxF R3. R3. R4 R3, PACKED R2 points to data Load first byte into R3 Shift left by 4 bit positions Increment the pointer Load second byte into R4 Clear high-order bits to zero Concatenate the BCD digits Store the result

Answer 1

// none of these allocate any storage
    #define MAX_SIZE  256
    #define IF(a)    if (a) {
    #define ENDIF    }
    typedef struct {
        unsigned char red;      // 'unsigned char' is an unsigned, 8-bit int
        unsigned char green;
        unsigned char blue;
        unsigned char alpha;
    } RGBa;

    // these allocate storage
    int     i;
    int     N = 20;
    char    prompt[] = "Enter an integer:";
    int     A[MAX_SIZE];
    int*    pBArray;
    int     BSize;
    RGBa    background = {0xff, 0xff, 0xff, 0x0};

• C code:

      i = N*N + 3*N
  

  "Unoptimized":
  (Note: There are some small disagreements in the syntax of assembler between SPIM, which is used in the book, and Cebollita, which is the tool we will be using. I have tried to follow the conventions of Cebollita here.)

      lw     $t0, 4($gp)       # fetch N
      mult   $t0, $t0, $t0     # N*N
      lw     $t1, 4($gp)       # fetch N
      ori    $t2, $zero, 3     # 3
      mult   $t1, $t1, $t2     # 3*N
      add    $t2, $t0, $t1     # N*N + 3*N
      sw     $t2, 0($gp)       # i = ...
  

  "Optimized":

      lw     $t0, 4($gp)       # fetch N
      add    $t1, $t0, $zero   # copy N to $t1
      addi   $t1, $t1, 3       # N+3
      mult   $t1, $t1, $t0     # N*(N+3)
      s