cpu_instruction_decoder

Description

This interface is used to replace an existing instruction with a user defined one or add new instructions to the simulation. The interface can only be used from a cpu_instruction_decoder_cb_t callback registered by the register_instruction_decoder_cb method in the cpu_instrumentation_subscribe interface.

The interface consist of just one method and looks like this:

SIM_INTERFACE(cpu_instruction_decoder) {
        void (*register_emulation_cb)(conf_object_t *cpu,
                                      cpu_emulation_cb_t cb,
                                      decoder_handle_t *handle,
                                      lang_void *user_data,
                                      cpu_callback_free_user_data_cb_t free_cb);
};

#define CPU_INSTRUCTION_DECODER_INTERFACE "cpu_instruction_decoder"

register_emulation_cb is used to set a callback function that implements the semantics of the new or changed instruction. Every time the instructions is executed on the cpu this function will be called instead of the build-in implementation. The handle is the decoder_handle_t handle passed to the cpu_instruction_decoder_cb_t callback. The user_data argument is user data for the callback. The free_cb is a clean-up callback function that Simics calls when the instruction is overwritten or if Simics wants to flush decoding caches. This function should deallocate any the user data if present. Can be NULL if not used.

The signature of the emulation callback looks like this:

typedef cpu_emulation_t (*cpu_emulation_cb_t)(
        conf_object_t *obj, conf_object_t *cpu,
        lang_void *user_data);

obj is the connection object, the same object as passed to the cpu_instruction_decoder_cb_t callback. cpu is the processor executing the replaced instruction. user_data is user data for the emulation callback. This is a useful place for storing immediate or register values for the new instruction. In the emulation function the whole Cell Context API is available for use.

CPU_Emulation_Fall_Through should be returned from the emulation callback if replaced one is a fall through instruction. The program counter does not need to be updated. If the replaced instruction is doing any control flow then CPU_Emulation_Control_Flow should be returned and the program counter should be set to the destination address. This can be done for fall through instruction as well but is less efficient.

CPU_Emulation_Default_Semantics can also be returned to indicate that the default semantics should be used instead of the user defined. This can be useful if the instruction only should be replaced under certain circumstances.

Execution Context

Threaded Context for all methods, but must be called from a callback registered by the register_instruction_decoder_cb method in the cpu_instrumentation_subscribe interface.