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Basics: Number Specification

Sized Number Specification

Representation: [size]'[base][number]
  • [size] is written only in decimal and specifies the number of bits.
  • [base] could be 'd' or 'D' for decimal, 'h' or 'H' for hexadecimal, 'b' or 'B' for binary, and 'o' or 'O' for octal.
  • [number] The number is specified as consecutive digits. Uppercase letters are legal for number specification (in case of hexadecimal numbers).
Ex: 4'b1111 : 4-bit binary number
16'h1A2F : 16-bit hexadecimal number
32'd1 : 32-bit decimal number
8'o3 : 8-bit octal number

Unsized Number Specification

By default numbers that are specified without a [base] specification are decimal numbers. Numbers that are written without a [size] specification have a default number of bits that is simulator and/or machine specific (generally 32).

Ex: 123 : This is a decimal number
'hc3 : This is a hexadecimal number
Number of bits depends on simulator/machine, generally 32.

x or z values

x - Unknown value.
z - High impedance value
An x or z sets four bits for a number in the hexadecimal base, three bits for a number in the octal base, and one bit for a number in the binary base.

Note: If the most significant bit of a number is 0, x, or z, the number is automatically extended to fill the most significant bits, respectively, with 0, x, or z. This makes it easy to assign x or z to whole vector. If the most significant digit is 1, then it is also zero extended.

Negative Numbers

Representation: -[size]'[base][number]

Ex: -8'd9 : 8-bit negative number stored as 2's complement of 8
-8'sd3 : Used for performing signed integer math
4'd-2 : Illegal

Underscore(_) and question(?) mark

An underscore, "_" is allowed to use anywhere in a number except in the beginning. It is used only to improve readability of numbers and are ignored by Verilog. A question mark "?" is the alternative for z w.r.t. numbers
Ex: 8'b1100_1101 : Underscore improves readability
4'b1??1 : same as 4'b1zz1

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