Update ALU documentation

doc/index.rst

@@ -46,7 +46,11 @@ serv_alu
 
 .. image:: serv_alu.png
 
-serv_alu handles alu and shift operations. For shift ops, the data to be shifted resides in bufreg. It also contains the logic for comparisons used by the slt* and conditional branch instructions
+serv_alu handles alu and shift operations. The first input operand (A) comes from i_rs1 and the second operand (B) comes from i_rs2 or i_imm depending on the type of operation. The data passes through one or more of the add/sub, shift control or bool logic unit and finally ends up in o_rd to be written to the destination register. The output o_cmp is used for conditional branches to decide whether or not to take the branch.
+
+The add/sub unit can do additions A+B or subtractions A-B by converting it to A+B̅+1. Subtraction mode (i_sub = 1) is also used for the comparisions in the slt* and conditional branch instructions. Finally, it is also used to negate the B operand for left shifts by clearing the A operand (i_shift_op =1). The +1 used in subtraction mode is done by preloading the carry input with 1. Less-than comparisons are handled by converting the expression A<B to A-B<0 and checking the MSB, which will be set when the result is less than 0. This however requires sign-extending the operands to 33-bit inputs. For signed operands (when i_cmp_sig is set), the extra bit is the same as the MSB. For unsigned, the extra bit is always 0. Because the ALU is only active for 32 cycles, the 33rd bit must be calculated in parallel to the ordinary addition. The result from this operations is available in result_lt. For equality checks, result_eq checks that all bits are 0 from the subtraction.
+
+For shift operations, the data to be shifted resides in bufreg. The shift control unit in the ALU keeps track of how many steps to shift the bufreg and sign-extends/zero-pads the shifted data depending on the type (arithmetic/logic right/left) of shift operation
 
 .. image:: serv_alu_int.png