Shift18

Build a 64-bit arithmetic shift register, with synchronous load. The shifter can shift both left and right, and by 1 or 8 bit positions, selected by amount.

An arithmetic right shift shifts in the sign bit of the number in the shift register (q[63] in this case) instead of zero as done by a logical right shift. Another way of thinking about an arithmetic right shift is that it assumes the number being shifted is signed and preserves the sign, so that arithmetic right shift divides a signed number by a power of two.

There is no difference between logical and arithmetic left shifts.

• load: Loads shift register with data[63:0] instead of shifting.
• ena: Chooses whether to shift.
• amount: Chooses which direction and how much to shift.
  • 2'b00: shift left by 1 bit.
  • 2'b01: shift left by 8 bits.
  • 2'b10: shift right by 1 bit.
  • 2'b11: shift right by 8 bits.
• q: The contents of the shifter.

算术右移：整体右移，高位补符号位，低位舍去。 正数0110右移一位为0011，负数1110右移一位为1111.

module top_module(
    input clk,
    input load,
    input ena,
    input [1:0] amount,
    input [63:0] data,
    output reg [63:0] q); 
	
    always@(posedge clk)
        if(load)
            q <= data;
    else if (ena) begin
        case (amount)
        	2'b00: q <= q << 1;
                2'b01: q <= q << 8;
            2'b10: q <= {q[63], q[63:1]};
            2'b11: q <= {{8{q[63]}}, q[63:8]};
                default:q <= q;
        endcase
    end
endmodule