Logical and Branch Instructions; Conditional Execution

Tom Kelliher, CS 220

Sept. 23, 2011

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Assignment

From Last Time

Operands and instruction formats.

Outline

1. Logical instructions.

2. Branch and jump instructions.

3. Compiling conditional statements.

Coming Up

Support for procedures.

Logical Operations

The basics:

1. NOT: complement the bits of the operand, bit by bit. (~)

2. AND: AND the bits of two operands, bit by bit. (&, not &&).

3. OR: OR the bits of two operands, bit by bit. (|, not ||).

4. Shift: Move the bits of the operand to the left or right a given ``distance'' (<< and >>). Complication: logical and arithmetic shifts.

Details:

1. MIPS has no NOT operation, but it does have NOR: ~(a | b).

   How do you use NOR to get NOT?

   ~1101 = 0010

2. 1101 & 1001 = 1001

   and $s2, $t0, $t1

3. 1001 | 0100 = 1101

4. Shifts are ``similar'' to multiplication and division.

   11001101 << 3 = 01101000

Usage example: shift and mask operations in finding a character in a word. In C:

int charinword(unsigned char c, unsigned int w)
{
   int i;

   for (i = 0; i < 4; i++)
   {
      if (c == (0xff & w))
         break;

      w = w >> 8;
   }

   return 0;
}

charinword(0xaa, 0xccddaabb);
charinword(0xaa, 0xbbccdaad);

Branch and Jump Instructions

1. I-format instructions.

2. The idea behind a branch or jump:
   

3. Branch forward or backward $2^{15}$ words.

All branch instructions are synthesized from beq, bne, and slt.

Branch instructions use a signed 16-bit offset field; hence they can jump $2^{15}-1$ instructions (not bytes) forward or $2^{15}$ instructions backwards. The jump instruction contains a 26 bit address field (the third instruction format).

Summary of branch instructions:

1. Unconditional branch: b label

   Example:

              b foo
              ...
      foo:    add $1, $1, $1
   

2. One operand branches:
   1. The list: beqz, bnez, bgez, bgtz, blez, bltz.

   2. Example: bnez $s0, label

3. Two operand branches:
   1. The list: beq, bne, bge, bgt, ble, blt.

      Unsigned versions.

   2. Second comparison operand may be a register or a constant:

         bge $sp, $ra, foo
         blt $s0, 5, bar
      

Compiling HLL Control Structures

Write MIPS code fragments corresponding to the following:

1. Compiling an if:
   

   if (i < 12)
      ++i;
   else
      --j;
   

2. Compiling a loop:
   

   i = 1;
   j = 0;
   while (i < 200)
   {
      j += i;
      i *= i;
   }
   

Class Assignment

Write MIPS code corresponding/solving each of the following:

1. j = 0;
   for (i = 0; i < 10; ++i)
      j += i;
   

2. j = 0;
   for (i = 0; i < 10; ++i)
      if (i > 5)
         j += i;
   

3. while (i > 0 && i < 10)
      ++i;
   

4. if (i < 12 && j > 3 || k != 0)
      ++i;
   else if (i == 33)
      --j;
   else
      k += 2;
   

5. int i;
   
   for (i = 0; i < 4; i++)
   {
      if (c == (0xff & w))
         return 1;
   
      w = w >> 8;
   }
   

6. (3.9 from the 3rd edition of the text) The naive way of compiling

   while (save[i] == k)
      i += k;
   

   requires execution of both a conditional branch and an unconditional jump each time through the loop. Produce the naive code.

   Optimize the naive code so that only a conditional branch is executed each time through the loop.

7. (3.24 from the 3rd edition of the text, a variation) Write a code segment which takes two ``parameters:''
   1. An ASCII character in $a0.

   2. A pointer to a NULL-terminated string in $a1.

   and ``returns'' a count of the number of occurrences of the character in the string in $v0.

Tom Kelliher