MEDELEG = 0xb109 on RISC-V: what the mask means

MEDELEG is the RISC-V Machine Exception Delegation register: a bitmask specifying which synchronous exceptions get delegated from M-mode to S-mode (the kernel) instead of being handled by M-mode firmware (OpenSBI) itself.

The common value 0xb109 decodes bit-by-bit against the RISC-V Privileged Spec exception-code table:

• Bit 0 (0x1): Instruction address misaligned
• Bit 3 (0x8): Breakpoint
• Bit 8 (0x100): Environment call from U-mode (user-ecall)
• Bit 12 (0x1000): Instruction page fault
• Bit 13 (0x2000): Load page fault
• Bit 15 (0x8000): Store/AMO page fault

Adding those: 0x1 | 0x8 | 0x100 | 0x1000 | 0x2000 | 0x8000 = 0xb109.

This is the standard shape for a Linux-capable RISC-V board. Every synchronous exception the kernel normally handles is delegated. M-mode-only causes (M-mode ecall, illegal instructions in M-mode) stay with OpenSBI.

The paired MIDELEG register does the same for interrupts; see MIDELEG = 0x222 for its typical Linux value.

Debugging: if the kernel is failing to handle a specific exception (e.g. page faults getting eaten and never reaching the kernel handler), grep the OpenSBI Boot HART report for the MEDELEG value and confirm the corresponding bit is set.