SCAIE-V public development has moved to SCAIE-V-2.0, this repository is no longer being maintained.

Welcome to SCAIE-V: An Open-Source SCAlable Interface for ISA Extensions for RISC-V Processors!

NEWS: SCAIE-V gets an upgrade!

In the following weeks we will release an improved version of SCAIE-V! It comes with:

• better abstraction
  • less work for ISAX module (no result multiplexing or storing destination address for decoupled execution etc.)
  • less logic within the core (more reliable tool, better verification, ease support for new cores)
• fewer stalls in case of data hazards (if the microarchitecture of the core allows, results may be returned in earlier stages than the WB stage)
• support for 5-stage VexRiscv
• improved tool structure
• some bug fixes
• more detailed use of the tool

Stay tuned!

What is SCAIE-V?

SCAIE-V is a:

• Portable = supports different microarchitectures
• Scalable = hardware cost scales with the ISAX requirements
• Flexible = supports simple and advanced features (custom control flow, decoupled, multi-cycle instructions, memory instructions)

interface for custom instructions for RISC-V processors.

Which cores do you support?

We currently support

• VexRiscv (https://github.com/SpinalHDL/VexRiscv)
• ORCA (https://github.com/cahz/orca)
• Piccolo (https://github.com/bluespec/Piccolo)
• PicoRV32 (https://github.com/YosysHQ/picorv32)

These cores provide different configurations. While testing & evaluating our tool we used the following setup:

Core	Nr. of pipeline stages	Interface
ORCA	5	AXI
VexRiscv	4	AHB
Piccolo	3	AXI
PicoRV32	non-pipelined	Native

Which operations are supported in SCAIE-V?

Operation	Meaning	Bitwidth
RdRS1/2	Reads operands	32
RdPC	Reads program counter *	32
RdInstr	Reads Instruction	32
RdIValid	Status bit informing the custom logic that a certain pipeline stage currently contains an instruction of type X	1
WrRD	Write register file *	32
Wr/RdMem	Load/Store operations *	32

* optionally, the user may request addr. and valid bit for this interface. For WrPC, the user can not request an addr. signal.

How can I use it for my custom instructions?

Let us consider the following example. The user wants to implement a module which conducts load/stores using custom addresses. He wishes to have a base value for the custom address and then increment/decrement it after/before each load/store. Therefore, he will use an internal register for storing the custom address: custom_addr. In order to load the base address into the internal register, he wants to implement a SETADDR custom instruction. For this instruction, he needs to know in which clock cycle he may read the operands from the register file. He also needs to know when there is a SETADDR instruction in the pipeline, to update the custom_addr register.

Step 1:

First, the user has to read the metadata of the core (=when is it allowed to read/update the core's state). He may do this with the following commands:

SCAIEV shim = new SCAIEV();
shim.PrintCoreNames(); // prints the names of the supported cores
shim.PrintCoreInfo("VexRiscv"); // prints metadata for a specific core (using the name from previous command)

From the metadata output of one node, only two values are relevant:

• the first value = the earliest clock cycle in which the user may read/update this data
• the third value = the latest clock cycle in which the user may read/update this data

The rest of the values are currently used in internal research projects.

Step 2:

Using the metadata in Step 1, the user decides in which clock cycles to read/update core's state. The custom ISAX module has to be designed based on this information. In our simple example, it would be something like:

module SETADDR (
    input        clk_i,
    input        rdIValid_SETADDR_2_i, 
    input [31:0] rdRS1_2_i, 
   //..value of custom regs as outputs used by other ISAXes. Inputs from other ISAXes to update custom_addr after/before a load/store
   ); 
    reg [31:0] custom_addr; 
    always @(posedge clk_i) begin 
        if(rdIValid_SETADDR_2_i)
            custom_addr <= rdRS1_2_i ;
       // rest of the logic for updating custom_addr in case of load/store ISAXes
    end 
endmodule 

Step 3:

The third step implies generating the custom instructions interface using the SCAIE-V tool. Let us consider that the user decided to read operands in the third cycle (numbering starts at 0). He does not have to modify anything in the core, but just let SCAIE-V do the work:

SCAIEV shim = new SCAIEV();
SCAIEVInstr setaddr  = shim.addInstr("SETADDRGEN","-------", "000", "0001011", "I");  
setaddr.PutSchedNode(FNode.RdRS1, 2);  
setaddr.PutSchedNode(FNode.RdIValid, 2); // valid bit for updating the custom_addr register
shim.Generate("VexRiscv"); // generates all the code

The files of the VexRiscv core will be modified so that it supports the new interface.

What is the current status of the project?

The project is quite new and we are constantly working on improving it & testing it with different configurations.

This ReadMe does not help me much. Where can I find more information?

This is the first version of the ReadMe file. In the following weeks we will upload a more detailed version.

Is there a paper to this work?

Mihaela Damian, Julian Oppermann, Christoph Spang, Andreas Koch, "SCAIE-V: An Open-Source SCAlable Interface for ISA Extensions for RISC-V Processors"

Do you have further questions?

For any questions, remarks or complaints, you can reach me at [email protected]. :)