JZ-HDL Reference

Global Constant Blocks (@global) ​

Purpose ​

Define named collections of sized literal constants visible to all modules and projects within the compilation unit. Useful for instruction encodings, CSR numbers, protocol constants, and other bit-patterns referenced throughout the design.

Syntax ​

text
@global <global_name>
  <const_id> = <sized_literal>;
  <const_id> = <sized_literal>;
  ...
@endglob

Where each sized literal follows the literal syntax: width'base_value

Semantics ​

  • The global name creates a namespace root.
  • Constants are referenced as global_name.CONST_ID.
  • Each constant is a sized literal with explicit, unambiguous bit-width.
  • Width must be >= intrinsic width of the value (standard overflow rules apply).
  • Multiple @global blocks are allowed; each must have a unique name.

Value semantics ​

Unlike CONFIG and CONST, global constants are values and may be used anywhere a value expression is permitted:

  • RHS of assignments (ASYNCHRONOUS or SYNCHRONOUS)
  • Operands to operators
  • Arguments to intrinsic operators
  • Part of expressions, concatenations, conditionals

Standard width rules apply: same width as target for direct assignment; use =z/=s modifiers for extension.

Errors ​

  • Duplicate global name across the compilation
  • Duplicate const_id inside a single block
  • Invalid identifier syntax
  • Forward reference inside a block
  • Non-integer or negative expressions
  • Value that is not a properly-sized literal (bare decimal, CONST reference, or CONFIG reference)
  • Literal overflow (intrinsic width exceeds declared width)
  • Attempting to assign to a global constant (read-only)

Example ​

text
@global ISA
  INST_ADD   = 17'b0110011_0000000_000;
  INST_SUB   = 17'b0110011_0100000_000;
  INST_SLL   = 17'b0110011_0000000_001;
  INST_AUIPC = 10'b0010111;
  INST_LUI   = 10'b0110111;
@endglob

Usage inside a module:

text
ASYNCHRONOUS {
  IF (opcode == ISA.INST_ADD) {
    alu_op <= 4'b0001;
  }
}