The Core-V eXtension interface, also called CV-X-IF, is an interface aimed at extending a |processor| with (custom or standardized) instructions implemented in a |coprocessor|.
It can be used to implement standard RISC-V extensions as for example B (Bit Manipulation), M (Integer Multiplication and Division), F (Single-Precision Floating Point) and D (Double-Precision Floating Point). It can also be used to implement custom extensions. Extensions implemented on the interface are unprivileged, i.e. implementing privileged extensions like H (Hypervisor) is not supported.
The goal of CV-X-IF is to enable the design and verification of instruction extensions in a |coprocessor| in a standardized manner without the need to modify the |processor| itself.
Having a common interface allows designers of RISC-V :term:`CPUs<CPU>` to reuse existing co-processor and vice versa.
Please note that the |processor| and |coprocessor| can have different license models. For example, the |coprocessor| could be closed source, connected to an open-source |processor|.
The idea of an extension interface originated from the PULP Project at ETH Zurich and University of Bologna, where it was used to decouple the floating-point unit and the CPU design.
The first version of this interface was called apu interface, and it was implemented in the CV32E40P to communicate with the CVFPU |coprocessor|.
However, this interface was tightly coupled with the |processor| pipeline, which meant that any other new |coprocessor| extension had to modify the |processor| pipeline and decoder.
Moreover, it was designed for a specific use-case. Later, the PULP team developed a more advanced interface for the CVA6 project, which could handle more complex scenarios required by the ARA vector machine. This interface was further refined in the Snitch project, where it was made more modular and independent from the pipeline, requiring only minimal changes to the decoder of the |processor|. The aim of CV-X-IF within the OpenHW Group is to take this interface to the next level and eliminate all dependencies between the |processor| and the |coprocessor|.
The interface is not only agnostic from the decoder and pipeline perspective, but also from the license and codebase standpoint, with the goal of becoming the standard interface that will enable wide reuse of RISC-V IPs.
The first CPU implementing such interface is the CV32E40X, which can be found at https://github.com/openhwgroup/cv32e40x.
The interface was also added as an option to CVA6, which can be found at https://github.com/openhwgroup/cva6.
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SPDX-License-Identifier: Apache-2.0 WITH SHL-2.1
Licensed under the Solderpad Hardware License v 2.1 (the "License"); you may not use this file except in compliance with the License, or, at your option, the Apache License version 2.0.
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Unless required by applicable law or agreed to in writing, any work distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and limitations under the License.
The CV-X-IF specification depends on the unprivileged [RISC-V-UNPRIV] and privileged [RISC-V-PRIV] RISC-V specification.
| [RISC-V-UNPRIV] | The RISC-V Instruction Set Manual, Volume I: User-Level ISA, Document Version 20191213, Editors Andrew Waterman and Krste Asanovíc, RISC-V Foundation, December 2019. |
| [RISC-V-PRIV] | The RISC-V Instruction Set Manual, Volume II: Privileged Architecture, Document Version 20211203, Editors Andrew Waterman, Krste Asanovíc, and John Hauser, RISC-V International, December 2021. |
.. glossary::
clk
Clock signal
ISA
Instruction set architecture
CPU
Central processing unit
ALU
Arithmetic logic unit
CSR
Control and status register
GPR
General purpose register
PMP
Physical memory protection
PMA
Physical memory attributes
MMU
Memory management unit
NMI
Non-maskable interrupt
UVM
Universal Verification Methodology
RTL
Register transfer language
ECS
Extension Context Status