Add proxy for accessing the low 32 bits of a 64-bit CSR

Use this for mstatus on RV32 so that `csrw mstatus` does not modify
the bits in `mstatush`. Fixes #1044.

1 files changed, 15 insertions, 0 deletions

diff --git a/riscv/csrs.h b/riscv/csrs.h
index 3998d79..500bde7 100644
--- a/riscv/csrs.h
+++ b/riscv/csrs.h
@@ -54,7 +54,9 @@ class csr_t {
   const unsigned csr_priv;
   const bool csr_read_only;
 
+  // For access to written_value() and unlogged_write():
   friend class rv32_high_csr_t;
+  friend class rv32_low_csr_t;
 };
 
 typedef std::shared_ptr<csr_t> csr_t_p;
@@ -249,6 +251,19 @@ class mstatus_csr_t final: public base_status_csr_t {
 
 typedef std::shared_ptr<mstatus_csr_t> mstatus_csr_t_p;
 
+// For RV32 CSRs that are split into two, e.g. mstatus/mstatush
+// CSRW should only modify the lower half
+class rv32_low_csr_t: public csr_t {
+ public:
+  rv32_low_csr_t(processor_t* const proc, const reg_t addr, csr_t_p orig);
+  virtual reg_t read() const noexcept override;
+  virtual void verify_permissions(insn_t insn, bool write) const override;
+ protected:
+  virtual bool unlogged_write(const reg_t val) noexcept override;
+ private:
+  csr_t_p orig;
+};
+
 class rv32_high_csr_t: public csr_t {
  public:
   rv32_high_csr_t(processor_t* const proc, const reg_t addr, csr_t_p orig);