diff --git a/shared/context_struct.h b/shared/context_struct.h
index ae6ffeb..692b911 100644
--- a/shared/context_struct.h
+++ b/shared/context_struct.h
@@ -1,6 +1,15 @@
 #include <stdint.h>
 
+typedef enum {
+    UNWF_RIP=0,
+    UNWF_RSP=1,
+    UNWF_RBP=2,
+
+    UNWF_ERROR=7
+} unwind_flags_t;
+
 typedef struct {
+    uint8_t flags;
     uintptr_t rip, rsp, rbp;
 } unwind_context_t;
 
diff --git a/src/CodeGenerator.cpp b/src/CodeGenerator.cpp
index f5b165c..a55a4c0 100644
--- a/src/CodeGenerator.cpp
+++ b/src/CodeGenerator.cpp
@@ -1,6 +1,7 @@
 #include "CodeGenerator.hpp"
 #include "gen_context_struct.hpp"
 #include "settings.hpp"
+#include "../shared/context_struct.h"
 
 #include <algorithm>
 #include <limits>
@@ -28,16 +29,6 @@ CodeGenerator::CodeGenerator(
     }
 }
 
-static std::string undefined_register() {
-    static std::string val = "";
-    if(val.empty()) {
-        std::ostringstream oss;
-        oss << std::numeric_limits<uintptr_t>::max() << "ull";
-        val = oss.str();
-    }
-    return val;
-}
-
 void CodeGenerator::generate() {
     gen_of_dwarf();
 }
@@ -90,12 +81,33 @@ void CodeGenerator::gen_unwind_func_header(const std::string& name) {
        << "\tswitch(pc) {" << endl;
 }
 
+struct UnwFlags {
+    UnwFlags():
+        error(false), rip(false), rsp(false), rbp(false) {}
+
+    bool error, rip, rsp, rbp;
+
+    uint8_t to_uint8() const {
+        uint8_t out = 0;
+        if(rip)
+            out |= (1 << UNWF_RIP);
+        if(rsp)
+            out |= (1 << UNWF_RSP);
+        if(rbp)
+            out |= (1 << UNWF_RBP);
+        if(error)
+            out |= (1 << UNWF_ERROR);
+
+        return out;
+    }
+};
+
 void CodeGenerator::gen_unwind_func_footer() {
+    UnwFlags flags;
+    flags.error = true;
     os << "\t\tdefault:\n"
-       << "\t\t\tout_ctx.rsp = " << undefined_register() << ";\n"
-       << "\t\t\tout_ctx.rbp = " << undefined_register() << ";\n"
-       << "\t\t\tout_ctx.rip = " << undefined_register() << ";\n"
-       << "\t\t\treturn out_ctx;"
+       << "\t\t\tout_ctx.flags = " << (int) flags.to_uint8() << "u;\n"
+       << "\t\t\treturn out_ctx;\n"
        << "\t}\n"
        << "}" << endl;
 }
@@ -126,26 +138,51 @@ void CodeGenerator::gen_of_row(
         uintptr_t row_end)
 {
     gen_case(row.ip, row_end);
+    UnwFlags flags;
+
 
     try {
-        os << "\t\t\t" << "out_ctx.rsp = ";
-        gen_of_reg(row.cfa);
-        os << ';' << endl;
+        if(!check_reg_valid(row.ra)) {
+            // RA might be undefined (last frame), but if it is defined and we
+            // don't implement it (eg. EXPR), it is an error
+            flags.error = true;
+            goto write_flags;
+        }
 
-        os << "\t\t\t" << "out_ctx.rbp = ";
-        gen_of_reg(row.rbp);
-        os << ';' << endl;
+        if(check_reg_valid(row.cfa)) {
+            flags.rsp = true;
+            os << "\t\t\t" << "out_ctx.rsp = ";
+            gen_of_reg(row.cfa);
+            os << ';' << endl;
+        }
+        else { // rsp is required (CFA)
+            flags.error = true;
+            goto write_flags;
+        }
+
+        if(check_reg_defined(row.rbp)) {
+            flags.rbp = true;
+            os << "\t\t\t" << "out_ctx.rbp = ";
+            gen_of_reg(row.rbp);
+            os << ';' << endl;
+        }
+
+        if(check_reg_defined(row.ra)) {
+            flags.rip = true;
+            os << "\t\t\t" << "out_ctx.rip = ";
+            gen_of_reg(row.ra);
+            os << ';' << endl;
+        }
 
-        os << "\t\t\t" << "out_ctx.rip = ";
-        gen_of_reg(row.ra);
-        os << ';' << endl;
     } catch(const UnhandledRegister& exn) {
-        os << ";\n"
-           << "\t\t\tout_ctx.rip = " << undefined_register() << ";\n"
-           << "\t\t\tout_ctx.rsp = " << undefined_register() << ";\n"
-           << "\t\t\tout_ctx.rbp = " << undefined_register() << ";\n";
+        // This should not happen, since we check_reg_*, but heh.
+        flags.error = true;
+        os << ";\n";
     }
 
+write_flags:
+    os << "\t\t\tout_ctx.flags = " << (int)flags.to_uint8() << "u;" << endl;
+
     os << "\t\t\treturn " << "out_ctx" << ";" << endl;
 }
 
@@ -188,10 +225,25 @@ static const char* ctx_of_dw_name(SimpleDwarf::MachineRegister reg) {
     return "";
 }
 
+bool CodeGenerator::check_reg_defined(const SimpleDwarf::DwRegister& reg) {
+    switch(reg.type) {
+        case SimpleDwarf::DwRegister::REG_UNDEFINED:
+        case SimpleDwarf::DwRegister::REG_NOT_IMPLEMENTED:
+            return false;
+        default:
+            return true;
+    }
+}
+bool CodeGenerator::check_reg_valid(const SimpleDwarf::DwRegister& reg) {
+    return reg.type != SimpleDwarf::DwRegister::REG_NOT_IMPLEMENTED;
+}
+
 void CodeGenerator::gen_of_reg(const SimpleDwarf::DwRegister& reg) {
     switch(reg.type) {
         case SimpleDwarf::DwRegister::REG_UNDEFINED:
-            os << undefined_register();
+            // This function is not supposed to be called on an undefined
+            // register
+            throw UnhandledRegister();
             break;
         case SimpleDwarf::DwRegister::REG_REGISTER:
             os << ctx_of_dw_name(reg.reg)
diff --git a/src/CodeGenerator.hpp b/src/CodeGenerator.hpp
index 7669459..a559f9c 100644
--- a/src/CodeGenerator.hpp
+++ b/src/CodeGenerator.hpp
@@ -48,6 +48,9 @@ class CodeGenerator {
 
         void gen_lookup(const std::vector<LookupEntry>& entries);
 
+        bool check_reg_defined(const SimpleDwarf::DwRegister& reg);
+        bool check_reg_valid(const SimpleDwarf::DwRegister& reg);
+
     private:
         SimpleDwarf dwarf;
         std::ostream& os;