what is the zero connected to some MUX in this 8 bit right shifter

Question

in this picture of an 8 bit right shifter, can someone please tell me what those 0's represent ? in the first row of MUX there is one zero connect, in the second row the first two have 0 connected, and in the third row there are 4 zeros connected?

Answer 1

There are three shifting possibilities:

• shift(0) is the bit0 of the shift amount. It weights 2⁰ = 1.
  Setting that bit shift at least 1 bit.
• shift(1) is the bit1 of the shift amount. It weights 2¹ = 2.
  Setting that bit shift at least 2 bits.
• shift(2) is the bit2 of the shift amount. It weights 2² = 4.
  Setting that bit shift at least 4 bits.

When shifting the incoming bits must be set to zero for logical shifts ( 01001101 shifted left by 3 becomes 01101000) and that's the reason for those zeros.

The circuit (a barrel shifter) basically decomposes a shift amount into a sum of powers of two (e.g. 5 as 1 + 4) so that any amount in the range 0-7 (3-bit of a number) can be done using a fixed arrangement of wires in one clock.
A SISO (Serial In, Serial Out) register would require multiple clock cycles.