Add readme

README.md

@@ -1 +1,69 @@
-WIP
+This repository host an RISC-V implementation written in SpinalHDL. There is some specs :
+
+- RV32IM instruction set
+- Interrupts and exception handling with the Machine mode from the riscv-privileged-v1.9.1 specification.
+- Pipelined on 5 stages (Fetch, Decode, Execute, Memory, WriteBack)
+- 1.17 DMIPS/Mhz with all extension
+- Optimized for FPGA
+- Optional MUL/DIV/REM extension
+- Two implementation of shift instructions, Single cycle / shiftNumber cycle
+- Each stage could have bypass or interlock hazard logic
+- FreeRTOS port https://github.com/Dolu1990/FreeRTOS-RISCV
+
+The hardware description of this CPU is done by using an very software oriented approach
+(without any overhead in the generated hardware). There is a list of software concepts used :
+
+- There is very few fixed things. Nearly everything is plugin based. The PC manager is a plugin, the register file is a plugin, the hazard controller is a plugin ...
+- There is an automatic a tool which allow plugins to insert data in the pipeline at a given stage, and allow other plugins to read it in another stages through automatic pipelining.
+- There is an service system which provide a very dynamic framework. As instance, a plugin could provide an exception service which could then be used by others plugins to emit exceptions from the pipeline.
+
+## Plugin structure
+
+```scala
+
+//Define an signal name/type which could be used in the pipeline
+object ALU_ENABLE extends Stageable(Bool)
+object ALU_OP     extends Stageable(Bits(2 bits))  // ADD, SUB, AND, OR
+object ALU_SRC1   extends Stageable(UInt(32 bits))
+object ALU_SRC2   extends Stageable(UInt(32 bits))
+object ALU_RESULT extends Stageable(UInt(32 bits))
+
+class AluPlugin() extends Plugin[VexRiscv]{
+
+
+
+  override def setup(pipeline: VexRiscv): Unit = {
+    import pipeline.config._
+    //Do some setups as for example specifying some instruction decoding by using the Decoding service
+    val decoderService = pipeline.service(classOf[DecoderService])
+
+    decoderService.addDefault(ALU_ENABLE,False)
+    decodingService.add(List(
+        //.....
+    ))
+  }
+
+  override def build(pipeline: VexRiscv): Unit = {
+    import pipeline._
+
+    execute plug new Area {
+      import execute._
+      //Add some logic in the execute stage
+      insert(ALU_RESULT) := input(ALU_OP).mux(
+        B"00" -> input(ALU_SRC1) + input(ALU_SRC2),
+        B"01" -> input(ALU_SRC1) - input(ALU_SRC2),
+        B"10" -> input(ALU_SRC1) & input(ALU_SRC2),
+        B"11" -> input(ALU_SRC1) | input(ALU_SRC2),
+      )
+    }
+
+    writeBack plug new Area {
+      import writeBack._
+      //Add some logic in the execute stage
+      when(input(ALU_ENABLE)){
+        input(REGFILE_WRITE_DATA) := input(ALU_RESULT)
+      }
+    }
+  }
+}
+```