VirtualDevice

The Bluespec AXI VirtualDevice is a component intended to act as a virtual device, presenting both a "device" interface and a "management" interface. The "device" interface simply blocks on any request, waiting for the "management" interface to specify and trigger a response. The assumption is that you have both a CPU that is master to this virtual device, and an independent CPU providing the virtualisation of the device which is exercising the management interface.

The VirtualDevice enables emulating I/O registers from a general-purpose CPU. The "management" interface allows a general-purpose processor to atomically observe all reads and writes to the "device" interface through the register interface defined in the following table:

offset	width	use	name
0x0000	8 bytes	ro	read_address
0x0008	4 bytes	ro	read_flit_size
0x000C	4 bytes	ro	read_burst_count
0x0040	64 bytes	rw	read_response_data
0x1000	8 bytes	ro	write_address
0x1008	4 bytes	ro	write_byte_enable
0x1040	32 bytes	ro	write_data
0x2000	4 bytes	ro	time_stamp
0x2004	2 bytes	ro	request_id
0x2006	1 byte	ro	request_is_write
0x2007	1 byte	rw	request_level/send_response
0x2008	1 byte	rw	enable_device_emulation

To emulate a read:

1. Read request_level/send_response to determine that a valid request is present, that is, if the level is non-zero.

2. Read request_is_write as 0x0 to determine that a read request is now active.

3. Read the address from the read_address register.

4. Read the read_burst_count register to determine the number of flits expected.

5. Read the read_flit_size to determine the bytes expected to be populated in each flit. This is expected to be 32 if read_burst_count is greater than 1.

6. Populate read_response_data with the data for all flits (should be either 1 or 4 flits in a standard BERI system). For a single-word access, the read_flit_size will give the number of bytes. These bytes should be inserted into memory for the first 32-byte flit at the alignment implied by read_address.

7. Write any value to request_level/send_response to send the read response and to deq this request.

To emulate a write:

1. Read request_level/send_response to determine that a valid request is present, that is, if the level is non-zero.

2. Read request_is_write as 0x1 to determine that a write request is now active.

3. Read the address from the write_address register.

4. Read the write_byte_enable register to determine the bytes expected to be updated.

5. Read the data from write_data, applying the mask from write_byte_enable, and update the relevant structure implied by the write_address.

6. Write any value to request_level/send_response to deq this request and send the write response.

Notes

The enable_device_emulation is off (with a value of 0x0) upon reset and causes this device to simply return 0s and ignore writes to the emulated_device_window to prevent system instability. The emulating processor is expected to write a 0x1 to enable_device_emulation when it begins servicing reads and writes to the emulated_device_window.