title: Unsetting and resetting certain bits
body: <p>For a homework assignment in school, I need to use a MMIO LED display where each led is exactly 2 bits stored within a byte. For the assignment I need to "move" these LEDs up, down, left, and right. I also need to set the color (I will be using 0x40 for this). Here's my issue:</p>
  
  <p>When I click the "right" arrow to move the LED over 1 column, it remains in the current column when it should be returning to black (0x00). If I click right 4 times (moving over exactly 1 byte), I get another lit LED, leaving the original one there.</p>
  
  <p>Here is my MIPS code:</p>
  
  <pre><code>getLedPattern:
  move $t2, $s2
  andi $t1, $t2, 0x3 #remainder of x / 4 is in $t0
  sll $t0, $t2, 2 #x / 4 is in $t0
  beq $t0, 0, case0
  beq $t0, 1, case1
  beq $t0, 2, case2
  case3:
      andi $a0, 0xFFFFFFFC
      #insert $a1 into bits 0 and 1 of $a0 into $v0
      or $v0, $a0, $a1
      jr $ra
  
  case2:
      andi $a0, 0xFFFFFCFF
      #insert $a1 into bits 2 and 3 of $a0 into $v0
      #srl $a1, $a1, 2
      or $v0, $a0, $a1
      jr $ra
  
  case1:
      andi $a0, 0xFFFCFFFF
      #insert $a1 into bits 4 and 5 of $a0 into $v0
      #srl $a1, $a1, 4
      or $v0, $a0, $a1
      jr $ra
  
  case0:
      andi $a0, 0xFCFFFFFF
      #insert $a1 into bits 6 and 7 of $a0 into $v0
      #srl $a1, $a1, 6
      or $v0, $a0, $a1
      jr $ra
  
  setLED:
      addi $sp, $sp, -20
      sw $ra, 0($sp)
      sw $t0, 4($sp)
      sw $t1, 8($sp)
      sw $t2, 12($sp)
      sw $t3, 16($sp)
  
  move $t5, $a0
  sll $t6, $a1, 5     # y*32
  srl $t2, $a2, 2     # x/4
  add $t5, $t5, $t6
  add $t5, $t5, $t2
  lb $a0, 0($t5)
  move $a1, $a3
  jal getLedPattern
  sb $v0, 0($t5)
  move $s3, $t5
  
  lw $ra, 0($sp)
  lw $t0, 4($sp)
  lw $t1, 8($sp)
  lw $t2, 12($sp)
  lw $t3, 16($sp)
  addi $sp, $sp, 20
  jr $ra
  </code></pre>
  
  <p>The logic is that it starts out at at memory location 0xFFFFOOO8 (top left LED), moves down one row (+32 bytes) and over x columns (plus x*bits). However, I can't seem to unset the current LED and move it over one. Any help would be appreciated. I believe that my or in getLedPattern: is wrong, but not 100% sure.</p>
  
  <p>Hopefully, getting this correct I will be able to get this correct in a general sense (no LED display).</p>