`[Feature] Implement CSR Support and System Instructions`

[Sukuna0007Abhi]

Problem

NucleusRV lacks support for RISC-V Control Status Registers (CSR) and system instructions (ECALL, EBREAK, MRET). This prevents:

• Exception handling and system calls
• Performance monitoring
• Operating system development
• Full RISC-V compliance

🎯 Proposed Solution

Implement complete CSR system including:

Instructions

• CSR Instructions: CSRRW, CSRRS, CSRRC + immediate variants
• System Instructions: ECALL, EBREAK, MRET

CSR Registers

• Machine Info: MHARTID, MISA, MVENDORID, MARCHID, MIMPID
• Trap Setup: MSTATUS, MIE, MTVEC
• Trap Handling: MEPC, MCAUSE, MSCRATCH, MTVAL, MIP
• Performance: MCYCLE, MINSTRET (64-bit counters)

Exception Handling

• Trap entry/exit mechanism
• Privilege mode management (Machine mode)
• Exception cause detection

Implementation

Files to modify:

• Add CSR.scala and CSRUnit.scala
• Extend Control.scala for system instruction decode
• Update Core.scala for pipeline integration
• Add CSR constants to Constants.scala

Testing:

• Unit tests for CSR operations
• Assembly test programs
• Exception handling validation

Benefits

• ✅ Major step toward RISC-V compliance
• ✅ Enables system software development
• ✅ Adds performance monitoring capabilities
• ✅ Foundation for future OS/interrupt support

🤝 Contribution

I can implement this...

[Sukuna0007Abhi]

Check this @5hayanB @shahzaibk23 I am working on it