# Loadable Targets

Icarus Verilog supports dynamically loading code generator modules to perform the back-end processing of the completed design. The user specifies on the command line the module to load. The compiler loads the module (once the design is compiled and elaborated) and calls it to finally handle the design.

Loadable target modules implement a set of functions that the core compiler calls to pass the design to it, and the module in turn uses a collection of functions in the core (the API) to access details of the design.

## Loading Target Modules

The target module loader is invoked with the ivl flag “-tdll”. That is, the DLL loader is a linked in target type. The name of the target module to load is then specified with the DLL flag, i.e. “-fDLL=<path>”.

## Compiling Target Modules

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## Loadable Target Module Api

The target module API is defined in the ivl_target.h header file. This declares all the type and functions that a loadable module needs to access the design.

## About Specific Expression Types

In this section find notes about the various kinds of expression nodes. The notes here are in addition to the more general documentation in the ivl_target.h header file.

- IVL_EX_CONCAT

  The concatenation operator forms an expression node that holds the repeat count and all the parameter expressions. The repeat count is an integer that is calculated by the core compiler so it fully evaluated, and not an expression.

  The parameter expressions are retrieved by the ivl_expr_parm method, with the index increasing as parameters go from left to right, from most significant to least significant. (Note that this is different from the order of bits within an expression node.)

- IVL_EX_NUMBER

  This is a constant number. The width is fully known, and the bit values are all represented by the ASCII characters 0, 1, x or z. The ivl_expr_bits method returns a pointer to the least significant bit, and the remaining bits are ordered from least significant to most significant. For example, 5’b1zzx0 is the 5 character string “0xzz1”.