- 無符號數運算，左值位寬不夠，發生截斷的現象
reg [3:0] a = 4'b1111;//15
reg [3:0] b = 4'b0010;//2
wire [3:0] c;
wire [3:0] show_c;
//17 10001
assign c = a + b;//1
assign show_c = a + b;//1
2. 兩個無符號數運算，賦值給一個有符號的數。可以看出，右側先按照無符號數進行運算，取得的運算結果按照左側的符號進行數據顯示。
reg [3:0] a = 15;//4'b1111
reg [3:0] b = 2;//4'b0010
reg signed [3:0] c;
reg signed [4:0] d;
reg signed [4:0] e;
initial begin
c = a + b;
d = a + b;
e = b - a;
//17 10001
$display("c = %d",c); //1 0001
$display("d = %d",d); //-15 10001
$display("e = %d",e); //-13 10011
end
3. 兩個無符號數運算，無符號數賦負值（補碼）。按照該補碼對應的正值進行處理。結果同上。
reg [3:0] a = -1;//4'b1111 15
reg [3:0] b = 2 ;//4'b0010
wire signed [3:0] c;
wire signed [3:0] show_c;
wire signed [4:0] d;
wire signed [4:0] show_d;
//17 10001
assign c = a + b;//0001
assign show_c = a + b;//1
assign d = a + b;//10001
assign show_d = a + b; //-15 signed
4. 有符號數和無符號數運算，賦值給有符號數。補碼進行運算，如果左值位寬不夠，進行截位，取得的結果爲：3（4’b0011）。位寬足夠，取得結果爲-13（10011）。
reg signed [4:0] a = -15;//5'b10001
reg [3:0] b = 2;//4'b0010
wire signed [3:0] c;
wire signed [3:0] show_c;
wire signed [4:0] d;
wire signed [4:0] show_d;
assign c = a + b;//0011
assign show_c = a + b;//3
assign d = a + b;//10011
assign show_d = a + b; //-13
5. 有符號數和無符號數的比較
一個無符號數和一個有符號數比較，都視爲無符號數，按照各自補碼的大小進行比較；
reg [3:0] a = 15;//4'b1111
reg [3:0] b = 14;//4'b1110
reg signed [3:0] c =-1 ;//4'b1111
reg signed [4:0] d =-2; //5'b11110
initial begin
if(a <= c) begin
$display("a <= c");//成立
end
if(b <= c) begin
$display("b <= c");//成立
end
if(c > d) begin
$display("c > d");//成立
end
end
6. 兩個無符號數已經進行移位。數據先按照左值進行截斷，然後再進行移位。算數右移時，不管左值是否有符號，都按照無符號數進行補零操作。
reg [3:0] a = 15;//4'b1111
reg [3:0] b = 4;//4'b0100
reg [3:0] c ;
reg signed [3:0] d ;
reg signed [4:0] e ;
reg signed [4:0] f;
initial begin
c = ( a + b ) >> 1;
d = ( a + b ) >> 1;
e = ( a + b ) >> 1;
f = ( a + b ) >>> 1;
$display("c = %d",c);//1 0001
$display("d = %d",d);//1 0001
$display("e = %d",e);//9 1001
$display("f = %d",f);//9 1001
7.有符號數進行移位。只有參與運算的兩個數都是有符號數，才按照有符號數進行算數右移，補最高位。不然，就補0.
reg signed [3:0] a = -7;//4'b1001
reg [3:0] b = 4;//4'b0100
reg signed [3:0] c = 4;//4'b0100
reg signed [3:0] d ;
reg signed [3:0] e ;
reg signed [3:0] f;
initial begin
d = ( a + b ) >> 1;//a + b =-3 1101
e = ( a + b ) >>> 1;
f = ( a + c ) >>> 1;
$display("d = %d",d);//6 0110
$display("e = %d",e);//6 0110
$display("f = %d",f);//-2 1101
