diff --git a/config/config.json.in b/config/config.json.in
index d10ad962f5..6b06c25e8c 100644
--- a/config/config.json.in
+++ b/config/config.json.in
@@ -32,6 +32,14 @@
       // "AMOCAS_Fatal"  – raise a Sail exception, stopping execution.
       // "AMOCAS_Illegal" – treat it as an illegal instruction.
       "amocas_odd_register": "AMOCAS_Illegal",
+      // The configuration option determines how to handle execution of a floating-point
+      // instruction with DYN (dynamic) rounding mode when `fcsr.FRM` contains a
+      // reserved value (0b101, 0b110, 0b111).
+      // This does not affect the execution of floating-point instructions that have a
+      // reserved rounding mode statically encoded in the instruction.
+      // "Fcsr_RM_Fatal"  – raise a Sail exception, stopping execution.
+      // "Fcsr_RM_Illegal" – treat it as an illegal instruction.
+      "fcsr_rm": "Fcsr_RM_Illegal",
       // The configuration option determines how to handle the reserved behavior `pmpcfg_with_R0W1`.
       // "PMP_Fatal"  – raise a Sail exception, stopping execution.
       // "PMP_ClearPermissions" – convert a PMP entry with R=0, W=1 to R=0, W=0, X=0.
diff --git a/model/core/platform_config.sail b/model/core/platform_config.sail
index dea881560d..8a5a23a896 100644
--- a/model/core/platform_config.sail
+++ b/model/core/platform_config.sail
@@ -71,6 +71,17 @@ mapping AmocasOddRegisterReservedBehavior_str : AmocasOddRegisterReservedBehavio
 }
 overload to_str = {AmocasOddRegisterReservedBehavior_str}
 
+enum FcsrRmReservedBehavior = {
+  Fcsr_RM_Fatal,
+  Fcsr_RM_Illegal,
+}
+
+mapping FcsrRmReservedBehavior_str : FcsrRmReservedBehavior <-> string = {
+  Fcsr_RM_Fatal   <-> "Fcsr_RM_Fatal",
+  Fcsr_RM_Illegal <-> "Fcsr_RM_Illegal",
+}
+overload to_str = {FcsrRmReservedBehavior_str}
+
 enum PmpWriteOnlyReservedBehavior = {
   PMP_Fatal,
   PMP_ClearPermissions,
@@ -105,6 +116,7 @@ mapping RV32ZdinxOddRegisterReservedBehavior_str : RV32ZdinxOddRegisterReservedB
 overload to_str = {RV32ZdinxOddRegisterReservedBehavior_str}
 
 let amocas_odd_register_reserved_behavior : AmocasOddRegisterReservedBehavior = config base.reserved_behavior.amocas_odd_register
+let fcsr_rm_reserved_behavior : FcsrRmReservedBehavior = config base.reserved_behavior.fcsr_rm
 let pmp_write_only_reserved_behavior : PmpWriteOnlyReservedBehavior = config base.reserved_behavior.pmpcfg_write_only
 let xenvcfg_cbie_reserved_behavior : XenvcfgCbieReservedBehavior = config base.reserved_behavior.xenvcfg_cbie
 let rv32zdinx_odd_register_reserved_behavior : RV32ZdinxOddRegisterReservedBehavior = config base.reserved_behavior.rv32zdinx_odd_register
diff --git a/model/extensions/FD/fext_insts.sail b/model/extensions/FD/fext_insts.sail
index 4ff7b677db..25bcfb1430 100644
--- a/model/extensions/FD/fext_insts.sail
+++ b/model/extensions/FD/fext_insts.sail
@@ -53,7 +53,12 @@ function select_instr_or_fcsr_rm(instr_rm : rounding_mode) -> option(rounding_mo
   then {
     let fcsr_rm = fcsr[FRM];
     if (valid_rounding_mode(fcsr_rm) & fcsr_rm != encdec_rounding_mode(RM_DYN))
-      then Some(encdec_rounding_mode(fcsr_rm)) else None()
+    then Some(encdec_rounding_mode(fcsr_rm))
+    else match fcsr_rm_reserved_behavior {
+      Fcsr_RM_Fatal  => reserved_behavior("Dynamic floating-point rounding mode reserved behavior: fcsr.FRM contains reserved value " ^ bits_str(fcsr_rm)),
+      // Note: None() will be caught and converted into Illegal_Instruction() when this function is called.
+      Fcsr_RM_Illegal => None(),
+    }
   }
   else Some(instr_rm)