ALU: 1-bit

This ALU can perform most of the data operations in the MIPS instruction set

Another operation, useful for branching: set on less than (slt)

Set the lsb to 1 if rs < rt, and 0 otherwise

If (a-b) is negative, then a < b:

(a - b) < 0

(a - b) + b < 0 + b

a < b

Result is same as sign bit from subtraction: Connect sign bit from adder to lsb of output

Unfortunately, we can only do 1 ALU operation at a time (add or slt)

Need a new 1-bit ALU for the msb

with an extra output from adder

Extra output: Set

Additional MUX data input: Less

0 for all except lsb

Set value for lsb

Also add overflow detection

1-bit ALU with set on less than

(Fig. 4.17b)