题目:
Suppose you’re building a circuit to process scancodes from a PS/2 keyboard for a game. Given the last two bytes of scancodes received, you need to indicate whether one of the arrow keys on the keyboard have been pressed. This involves a fairly simple mapping, which can be implemented as a case statement (or if-elseif) with four cases.

解题：

module top_module (
    input [15:0] scancode,
    output reg left,
    output reg down,
    output reg right,
    output reg up  ); 

    always@(*)begin
        up=1'b0;down=1'b0;left=1'b0;right=1'b0;
        case(scancode)
            16'HE06B:left=1'b1;
            16'HE072:down=1'b1;
            16'HE074:right=1'b1;
            16'HE075:up=1'b1;
        endcase
    end
        
endmodule

结果正确：

注意点：
为避免产生锁存，必须在所有可能条件下为所有输出分配一个值（另请参阅always_if2).仅仅有一个默认案例是不够的。在所有四种情况和默认情况下，必须为所有四个输出分配一个值。这可能涉及大量不必要的键入。解决此问题的一种简单方法是在 case 语句之前为输出分配一个“默认值”：

always @(*) begin
    up = 1'b0; down = 1'b0; left = 1'b0; right = 1'b0;
    case (scancode)
        ... // Set to 1 as necessary.
    endcase
end

这种代码样式可确保在所有可能的情况下都为输出分配一个值（0），除非 case 语句覆盖赋值。这也意味着 default： case 项变得不必要。