String Value Interpretation - 2021.2 English

The interpretation of the value string is determined by the declared type of the HDL object and the -radix command line option. The -radix always overrides the default radix determined by the HDL object type.

• For HDL objects of type logic, the value is a one-dimensional array of the logic type or the value is a string of digits of the specified radix.
  • If the string specifies less bits than the type expects, the string is implicitly zero-extended (not sign-extended) to match the length of the type.
  • If the string specifies more bits than the type expects, the extra bits on the MSB side must be zero; otherwise the command generates a size mismatch error.

  For example: The value 3F specifies 8 bits (4 per hex digit) with radix hex and a 6 bit logic array, equivalent to binary 0011 1111. But, because the upper two bits of 3 are zero, the value can be assigned to the HDL object. In contrast, the value 7F would generate an error, because the upper two bits are not zero.

  • A scalar (not array or record) logic HDL object has an implicit length of 1 bit.
  • For a logic array declared as a [left:right] (Verilog) or a(left TO/DOWNTO right), the left-most value bit (after extension/truncation) is assigned to a[left] and the right-most value bit is assigned to a[right].