verilog中的有符号数运算

• 有號數的計算：若有需要關於有號數的計算，應當利用Verilog 2001所提供的signed及$signed()機制。

Ex:

input  signed [7:0] a, b;
output  signed [15:0] o;
assign o = a * b;

or

input   [7:0] a, b;
output   [15:0] o;
wire signed [15:0] o_sgn;
assugb o_sgn = $signed(a) * $signed(b);
assign o = $unsigned(o_sgn);

• 正負號的擴展：應多加利用Verilog的implicity signed extension，避免手動進行轉換。

Ex:

input signed [7:0] a, b;
input signed [8:0] o;
assign o = a + b; // Verilog會自動進行符號的擴展。

• 有號數與無號數的混合計算：不要在同一個verilog敘述中進行有號數與無號數的計算。應該要分成個別獨立的敘述。在一個verilog敘述中只要有一個無號數的運算元，整個算式將被當成無號數進行計算。

input   [7:0] a;
input  signed [7:0] b;
output signed [15:0] o;

// Don't do this: assign o = a * b;
// The $signed({1'b0, a}) can convert the unsigned number to signed
number.
assign o = $signed({1'b0, a}) * b;

input signed [7:0] a;
output signed [15:0] o;

// Don't do this: assign o = a * 8'b10111111;
// Use $signed() system task
assign o = a * $signed(8'b10111111);
// or sb keyword.
assign o = a * 8'sb10111111;

• part-select運算過後的運算元是無號數。就算是選擇的範圍包含整個register或wire。

input signed [7:0] a;
input signed [7:0] b;
output signed [15:0] o1, o2;

// Don't do this: assign o1 = a[7:0];
assign o1 = a;
// Don't do this: assign o2 = a[6:0] * b;
assign o2 = $signed(a[6:0]) + b