Logical Instructions and Conditional Execution

Tom Kelliher, CS 240

Sept. 7, 2005

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Assignment

Install a Linux distro on your PC.

From Last Time

Operands and instruction formats.

Outline

1. Logical instructions.

2. Branch and jump instructions.

3. Compiling HLL control structures.

4. Class teamwork assignment.

Coming Up

Intro to Linux.

Last Time

Operations, operands, and instruction formats.

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.''

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.

Branch and Jump Instructions

1. I-format instructions.

2. The idea behind a branch or jump:
   

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

The complete set, all 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).

b labelBranch instruction
Unconditionally branch to the instruction at the label.

beq Rsrc1, Src2, labelBranch on Equal
Conditionally branch to the instruction at the label if the contents of register Rsrc1 equals Src2.

beqz Rsrc, labelBranch on Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc equals 0.

bge Rsrc1, Src2, labelBranch on Greater Than Equal
bgeu Rsrc1, Src2, labelBranch on GTE Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are greater than or equal to Src2.

bgez Rsrc, labelBranch on Greater Than Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are greater than or equal to 0.

bgt Rsrc1, Src2, labelBranch on Greater Than
bgtu Rsrc1, Src2, labelBranch on Greater Than Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are greater than Src2.

bgtz Rsrc, labelBranch on Greater Than Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are greater than 0.

ble Rsrc1, Src2, labelBranch on Less Than Equal
bleu Rsrc1, Src2, labelBranch on LTE Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are less than or equal to Src2.

blez Rsrc, labelBranch on Less Than Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are less than or equal to 0.

blt Rsrc1, Src2, labelBranch on Less Than
bltu Rsrc1, Src2, labelBranch on Less Than Unsigned
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are less than Src2.

bltz Rsrc, labelBranch on Less Than Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are less than 0.

bne Rsrc1, Src2, labelBranch on Not Equal
Conditionally branch to the instruction at the label if the contents of register Rsrc1 are not equal to Src2.

bnez Rsrc, labelBranch on Not Equal Zero
Conditionally branch to the instruction at the label if the contents of Rsrc are not equal to 0.

j labelJump
Unconditionally jump to the instruction at the label.

jal labelJump and Link
jalr RsrcJump and Link Register
Unconditionally jump to the instruction at the label or whose address is in register Rsrc. Save the address of the next instruction in register 31.

jr RsrcJump Register
Unconditionally jump to the instruction whose address is in register Rsrc.

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 Teamwork Assignment

The class, working as a team, is to e-mail the solution to the following problems to me (I'll collect the solutions and e-mail them as one to the class.) Let me know who participated in the solution of what problem(s).

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. (3.9 from 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.

6. (3.24 from 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