HDLbits——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（算数） 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.

**注意**：算数右移和逻辑右移的区别
>>逻辑右移不考虑符号位，空缺的位置补领操作即可
>>算数右移动需要考虑符号位，右移一位，如果符号位为1，则在符号位补1，如果符号位为0，则在符号位补0.即算数右移后，空缺的位置补符号位的数字即可
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)begin
        if(load)begin
            q <= data;
        end
        else if(ena) begin
            case(amount)
                2'b00:   q <= {q[62:0],1'b0};
                2'b01:   q <= {q[55:0], 8'b0};
                2'b10:   q <= {q[63],q[63:1]};//有符号数右移动  如果符号位为1，则右移后是补1；如果符号位为0，则右移后是补0
                2'b11:   q <= { { 8{q[63]} },q[63:8]};
            endcase
        end
        else q <= q;
    end

endmodule

仿真图：


原理图：