In-Class Exercise
4/5/2010

Suppose the data-path is 4 bits rather than 32 bits.  
For each of the instructions below, what are the following?  You can
write your answer on the handout

   Ainvert
   Binvert
   Operation
   CarryIn to ALU0
   a0
   b0
   Output of the adder of ALU0 
   CarryOut of ALU0
   Result0
   a1
   b1
   Output of the adder of ALU1 
   CarryOut of ALU1
   Result1
   a2
   b2
   Output of the adder of ALU2 
   CarryOut of ALU2
   Result2
   a3
   b3
   Output of the adder of ALU3 
   CarryOut of ALU3
   Result3
   Set

add $t0,$t1,$t2, where $t1 = 0b0011 and $t2 = 0b0001.

sub $t0,$t1,$t2, where $t1 = 0b0011 and $t2 = 0b0100.

slt $t0,$t1,$t2, where $t1 = 0b0011 and $t2 = 0b0101.

== Question 4

When we looked at SLT in our construction of the ALU, the destination
register ($t0 in the example above) was set depending only on whether
the result of the subtraction is negative (i.e., the Set output of
ALU31).  

But this isn't, in general, sufficient.  Suppose our datapath is 4
bits, and we execute:

slt $t0,$t1,$t2

where $t1 = -7dec = 0b1001
      $t2 =  6dec = 0b0110

What value would $t0 be set to?  

Be sure to review the notes from Friday about modifications needed to
the ALU to handle SLT properly in these cases.