# JTAG_DMI UVM Agent

JTAG_DMI UVM Agent is extended from DV library agent classes. Contrary to what the name suggests, it actually does not come with an agent. It is called an agent because it comes with a monitor, an RAL model and a RAL frontdoor accessor class, wrapped in a package that provides some additional utility methods.

## jtag_dmi_item

This class represents a predicted DMI transaction, captured by the monitor. It consists of req_op, addr, wdata, rsp_op and rdata. The first three captures a DMI request as initiated by the host, whereas the last two capture the response type and read data respectively.

## jtag_dmi_monitor

This component converts the raw JTAG transactions captured from the jtag_monitor in the jtag_agent over its TLM analysis port into a stream of jtag_dmi_item transactions which are sent over this component’s analysis port. A JTAG DMI access is made by writing to the JTAG DTM DMI register with the request, and polling it subsequently for completion. This monitor analyzes the incoming raw JTAG transactions to see if the JTAG DMI register was accessed, by matching the IR value. If it was, then it captures the subsequent updates to its DR to extract the predicted DMI requests. It uses the JTAG DTM RAL model to parse the DTM DMI register fields from these writes and reads. Likewise, it examines the dout value of the transaction item (i.e. the read value of the DTM DMI register) which indicates the status of the previously initiated DMI request, as well as the read data. Partial requests are immediately written to the req_analysis_port and are held in a local queue. When it detects that the access completed, it pops the partial request from the queue, updates the request with the response information, and writes the completed transaction to the analysis_port.

Note that these transactions are predictions, based on reads and writes to the JTAG DTM register. The actual internal DMI interface is not probed.

Any raw JTAG transactions that were not made to the JTAG DTM DMI register are passed through the non_dmi_jtag_dtm_analysis_port.

This monitor is required to be instantiated alongside the jtag_agent in the testbench environment that seeks to consume the captured JTAG DMI transactions.

## jtag_dmi_reg_block

This class represents the JTAG DMI register space, as indicated in the RISCV debug specification 0.13.2. The registers were specified in the adjoining jtag_dmi.hjson file and converted to the model using our reggen tool. It has been hand-edited after generation to remove the comportability assumptions. See the notes in the file for details.

The package provides a convenience function jtag_dmi_agent_pkg::create_jtag_dmi_reg_block() that creates and returns a handle to this model. It also attaches the frontdoor accessor class to all registers, which replaces the traditional access of registers using a map and an adapter. Like the JTAG DMI monitor, an object of this class is also required to be manually instantiated in the parent environment.

This register model serves to be generic in nature. It does not cater to a specific implementation. The actual implementation of this register space may differ in the design. The reset values and access policies of these registers may need to be altered in the parent environment before use, based on the implementation.

## jtag_dmi_reg_frontdoor

Accesses to the DMI register space over JTAG happen indirectly via writes to the DTM register called dmi which is specified in the jtag_dtm_reg_block provided by the jtag_agent. Hence, the accesses to the DMI register space cannot be made via the traditional register map and adapter method. This sequence writes the DTM dmi register to issue a DMI read or a write request, then polls it for completion. The DTM dmi register on subsequent reads, indicates the status of the request (success, busy or fail), in addition to the read data. If the request status is busy, it keeps polling, until either a JTAG reset (trst_n) asserts or a timeout occurs.

It uses an externally created semaphore jtag_dtm_ral_sem_h to atomicize accesses to the DTM dmi register, since accesses to all DMI registers go through this shared resource. This semaphore is also created and set by the convenience function jtag_dmi_agent_pkg::create_jtag_dmi_reg_block().

## sba_access_item

This class represents the driven or predicted SBA transaction. It is used by the routines provided in sba_access_utils_pkg, as well as by the sba_access_monitor. It contains request and response related fields. It also has special control knobs to modify the behavior of the design when initiating accesses. These knobs - readonaddr, readondata, and autoincrement are written to the SBCS register, if different from the defaults.

## sba_access_monitor

This monitor retrieves the predicted JTAG DMI transactions from the jtag_dmi_monitor to further analyze accesses to the SBA subset of the DMI register space, using the handle to the jtag_dmi_reg_block instance, which is set externally. It examines the stream of reads and writes to the SBA registers to predict outgoing SBA transactions. For this prediction to work correctly, the stimulus needs to be sent correctly as well, which is facilitated by the routines provided by the sba_access_utils_pkg.

Any JTAG DMI transactions that were not made to the SBA registers are passed through the non_sba_jtag_dmi_analysis_port.

This monitor is required to be instantiated alongside the jtag_agent in the testbench environment that seeks to consume the predicted SBA transactions.

## sba_access_utils_pkg

The JTAG DMI register space has registers to initiate and manage accesses into the system through an external system bus (whatever that may be). Please see the RISCV debug specification for more details. These registers are already a part of the jtag_dmi_reg_block model.

This package provides some convenience tasks to initiate and manage accesses into the system bus access interface (SBA) using these SBA registers, including starting an access, waiting for completion and clearing the error bits. These tasks take the handle to the jtag_dmi_reg_block model as one of the arguments.