always @(posedge KEY[1] ,negedge KEY[0])
beign
if(!KEY[0]) //复位
begin
Areg<=0; Breg<=0; Creg<=0; Dreg<=0; cout<=0; S<=0;
end
else
begin
if(SW17) //寄存器写使能
begin
if(SW16) begin //选择A,B
Areg<=SW[15:8]; Breg<=SW[7:0];
end
else //选择C,D
begin
Creg<=SW[15:8]; Dreg<=SW[7:0];
end
end
else //写使能无效,禁止写入,即保持
begin
Areg<=Areg; Breg<=Breg; Creg<=Creg; Dreg<=Dreg;
end
end