System Verilog LRM 学习笔记 — 字面常量
1. 整数型字面常量
1.1 整数型字面常量表示方法有两种：
- 数字直接表示：会被系统识别为32bit宽的有符号数。
- 指定size和进制，如12‘hDBA, 有可选的宽度,（’）, s/S有符号数指示，和数字组成。 默认是无符号数，只有含s/s符号数指示时被识别为有符号数， 如12‘shDBA.
1.2 负数是二进制补码的形式表示
1.3 整数型字面常量的补齐和截断，padded to left， truncated：
对于无符号数，如果向size小的数赋值，则需要将左侧多余的bit截断后赋值。如果向size大的数赋值，则将其左侧填0后赋值，但如果其左侧是x/z,则将其左侧补x/z，之后再进行赋值操作。
1.4 所有bit设置成0/1/x/z的表示方法：
‘0, ‘1, ‘X, ‘x, ‘Z, ‘z // sets all bits to specified value
//Example 1: Unsized literal constant numbers
659 // is a decimal number
'h 837FF // is a hexadecimal number
'o7460 // is an octal number
4af // is illegal (hexadecimal format requires 'h)
//Example 2: Sized literal constant numbers
4'b1001 // is a 4-bit binary number
5 'D 3 // is a 5-bit decimal number
3'b01x // is a 3-bit number with the least
// significant bit unknown
12'hx // is a 12-bit unknown number
16'hz // is a 16-bit high-impedance number
//Example 3: Using sign with literal constant numbers
8 'd -6 // this is illegal syntax
-8 'd 6 // this defines the two's-complement of 6,
// held in 8 bits—equivalent to -(8'd 6)
4 'shf // this denotes the 4-bit number '1111', to
// be interpreted as a two's-complement number,
// or '-1'. This is equivalent to -4'h 1
-4 'sd15 // this is equivalent to -(-4'd 1), or '0001'
16'sd? // the same as 16'sbz
//Example 4: Automatic left padding of literal constant numbers
logic [11:0] a, b, c, d;
logic [84:0] e, f, g;
initial begin
a = 'h x; // yields xxx
b = 'h 3x; // yields 03x
c = 'h z3; // yields zz3
d = 'h 0z3; // yields 0z3
e = 'h5; // yields {82{1'b0},3'b101}
f = 'hx; // yields {85{1'hx}}
g = 'hz; // yields {85{1'hz}}
end
//Example 5: Automatic left padding of constant literal numbers using a single-bit value
logic [15:0] a, b, c, d;
a = '0; // sets all 16 bits to 0
b = '1; // sets all 16 bits to 1
c = 'x; // sets all 16 bits to x
d = 'z; // sets all 16 bits to z
//Example 6: Underscores in literal constant numbers
27_195_000 // unsized decimal 27195000
16'b0011_0101_0001_1111 // 16-bit binary number
32 'h 12ab_f001 // 32-bit hexadecimal number
2. 实数型字面常量
在SV中，实数型字面常量代表的是双精度浮点数，有普通表示法(如12.73)，和科学表示法(如32e6）两种形式。
3. 时间常量
用整数或小数+时间单位(fs,ps,ns,us,ms,s)来表示，如2.1ns，49ps;
4. 字符串常量
本质是byte类型的可变数组，用“”来表示一个字符串。可以将字符串赋给整数变量，如
byte c1 = "A" ;
bit [7:0] d = "\n" ;
bit [8*12:1] stringvar = "Hello world\n"; //store the 12-character string "Hello world\n"
//requires a variable 8 x 12, or 96 bits wide.
bit [0:11] [7:0] stringvar = "Hello world\n" ; //Alternatively, a multidimensional packed
//array can be used, with 8-bit subfields
//A string literal can be assigned to an unpacked array of bytes. If the size differs, it is left justified.
byte c3 [0:12] = "hello world\n" ;
左侧变量应该和字符串长度一致，是8bit的整数倍，否则需要进行补0或截断的操作。
