This module is a building block for application-specific shifts and rotates. It synthesizes to LUT logic and can be quite large if not specialized to a particular situation.

While a left shift by N is always equivalent to a multiplication by
2^{N} for both signed and unsigned binary integers, an arithmetic
shift right by N is only a truncating division by 2^{N} for
*positive* binary integers. For negative integers, the result is so-called
modulus division, and the quotient ends up off by one in magnitude, and
must be corrected by adding +1, *but only if an odd number results as part
of the intermediate division steps*. That is, if a non-zero bit was shifted
out to the right.

We can treat the `shift_amount`

and the `shift_direction`

together as
a signed magnitude number: the amount is an absolute value, and the
direction is the sign of the value. Here, a `shift_direction`

of `1`

,
meaning a negative number, shifts to the right. Choosing this convention
for the sign matches the behaviour of a shift when we think about it as
a multiplication or division by a power of 2:

- Multipliying by 8 is equivalent to 2
^{3}N, which is a shift-left by 3 steps. - Dividing N by 4 is equivalent to 2
^{-2}N, which is a shift-right by 2 steps.

Adding together these multiples and fractions generated by the shifts enables the creation of small, cheap scaling by constant ratios:

- 3N = N + 2
^{1}N - 10N = 8N + 2N = 2
^{3}N + 2^{1}N - 5N/4 = N + N/4 = N + 2
^{-2}N - etc...

When the shift values are constant, the shifter reduces to simple rewiring, which in turn reduces the above examples to an adder or two each.

The shifts are internally unsigned and `word_in`

and `word_out`

are
extended to the left and right so new bits can be shifted in and current
bits shifted out without loss, regardless of shift amount or direction,
which enables the creation of more complex shifts or rotates:

- Wire the most-significant bit (MSB) of
`word_in`

to all`word_in_left`

inputs and zero to all`word_in_right`

inputs to create a signed arithmetic shift. - Wire the
`word_in`

MSB to`word_in_right`

MSB (or vice-versa) to create a rotate function. - Feed
`word_out_left`

and`word_out`

to a double-word adder and set the shift to +1 (left by 1) as part of the construction of a conditional-add multiplier, which multiplies two N-bit words in N cycles, giving a 2N-bit result.

`default_nettype none module Bit_Shifter #( parameter WORD_WIDTH = 0 ) ( input wire [WORD_WIDTH-1:0] word_in_left, input wire [WORD_WIDTH-1:0] word_in, input wire [WORD_WIDTH-1:0] word_in_right, input wire [WORD_WIDTH-1:0] shift_amount, input wire shift_direction, // 0/1 -> left/right output reg [WORD_WIDTH-1:0] word_out_left, output reg [WORD_WIDTH-1:0] word_out, output reg [WORD_WIDTH-1:0] word_out_right );

Let's document the shift direction convention again here, and define our initial values for the outputs and the intermediate result.

localparam LEFT_SHIFT = 1'b0; localparam RIGHT_SHIFT = 1'b1; localparam TOTAL_WIDTH = WORD_WIDTH * 3; localparam TOTAL_ZERO = {TOTAL_WIDTH{1'b0}}; localparam WORD_ZERO = {WORD_WIDTH{1'b0}}; initial begin word_out_left = WORD_ZERO; word_out = WORD_ZERO; word_out_right = WORD_ZERO; end reg [TOTAL_WIDTH-1:0] word_in_total = TOTAL_ZERO;

Rather than do arithmetic and calculate slices of vectors to figure out where the shifted bits end up, let's concatenate the input words into one triple-wide word, shift it as an unsigned number, then deconcatenate the result into each output word. All we have to do is keep the same convention on bit significance: here LSB is on the right.

always @(*) begin word_in_total = {word_in_left, word_in, word_in_right}; {word_out_left, word_out, word_out_right} = (shift_direction == LEFT_SHIFT) ? word_in_total << shift_amount : word_in_total >> shift_amount; end endmodule