diff --git a/rtl/x86_jit.pas b/rtl/x86_jit.pas
index cb61f93d..8313762e 100644
--- a/rtl/x86_jit.pas
+++ b/rtl/x86_jit.pas
@@ -345,9 +345,13 @@ type
    APltCount       :Integer;
    //
    Allocator:t_jit_builder_allocator;
+   //
+   on_error:procedure(const Msg:RawByteString;userdata:Pointer); register;
+   on_udata:Pointer;
   //
   Function  Alloc(Size:ptruint):Pointer;
   Procedure Free;
+  procedure Assert(value:Boolean;const Msg:RawByteString=''); inline;
   Function  _new_chunk(start:QWORD):p_jit_code_chunk;
   procedure _end_chunk(__end:QWORD);
   procedure _add(const ji:t_jit_instruction;min_isize:Byte=0);
@@ -1430,6 +1434,18 @@ begin
  Allocator.Free;
 end;
 
+procedure t_jit_builder.Assert(value:Boolean;const Msg:RawByteString=''); inline;
+begin
+ if (not value) then
+ begin
+  if (on_error<>nil) then
+  begin
+   on_error(Msg,on_udata);
+  end;
+  System.Assert(value,Msg);
+ end;
+end;
+
 //
 
 Function t_jit_builder._new_chunk(start:QWORD):p_jit_code_chunk;
@@ -1470,7 +1486,7 @@ begin
  if (ACodeChunkCurr=nil) then
  begin
   _new_chunk(0);
-  Assert(ACodeChunkCurr<>nil)
+  Assert(ACodeChunkCurr<>nil);
  end;
 
  i_size:=ji.AInstructionSize;
diff --git a/sys/jit/kern_jit.pas b/sys/jit/kern_jit.pas
index 7129456a..63db5781 100644
--- a/sys/jit/kern_jit.pas
+++ b/sys/jit/kern_jit.pas
@@ -2134,6 +2134,34 @@ begin
  end;
 end;
 
+procedure on_builder_error(const Msg:RawByteString;userdata:Pointer); register;
+var
+ ctx:p_jit_context2;
+begin
+ ctx:=userdata;
+
+ with ctx^ do
+ begin
+  Writeln('original------------------------':32,' ','0x',HexStr(ptr_curr));
+  print_disassemble(code,dis.CodeIdx);
+  Writeln('original------------------------':32,' ','0x',HexStr(ptr_next));
+
+  Writeln('builder error:',
+          din.OpCode.Prefix,',',
+          din.OpCode.Opcode,',',
+          din.OpCode.Suffix,' ',
+          din.Operand[1].Size,' ',
+          din.Operand[2].Size);
+
+  Writeln('opcode=$',HexStr(dis.opcode,8),' ',
+          'MIndex=',dis.ModRM.Index,' ',
+          'SimdOp=',dis.SimdOpcode,':',SCODES[dis.SimdOpcode],' ',
+          'mm=',MCODES[dis.mm and 3],':',dis.mm);
+
+ end;
+
+end;
+
 var
  _print_stat:Integer=0;
 
@@ -2168,6 +2196,13 @@ var
 begin
  Result:=nil;
 
+ //init error cbs
+ with ctx.builder do
+ begin
+  on_error:=@on_builder_error;
+  on_udata:=@ctx;
+ end;
+
  if (cmDontScanRipRel in ctx.modes) then
  begin
   //dont scan rip relative
@@ -2374,20 +2409,24 @@ begin
   begin
    print_error_td('Unhandled jit opcode!');
 
-   Writeln('original------------------------':32,' ','0x',HexStr(ctx.ptr_curr));
-   print_disassemble(ctx.code,dis.CodeIdx);
-   Writeln('original------------------------':32,' ','0x',HexStr(ctx.ptr_next));
+   with ctx do
+   begin
+    Writeln('original------------------------':32,' ','0x',HexStr(ptr_curr));
+    print_disassemble(code,dis.CodeIdx);
+    Writeln('original------------------------':32,' ','0x',HexStr(ptr_next));
 
-   Writeln('Unhandled jit:',
-           ctx.din.OpCode.Prefix,',',
-           ctx.din.OpCode.Opcode,',',
-           ctx.din.OpCode.Suffix,' ',
-           ctx.din.Operand[1].Size,' ',
-           ctx.din.Operand[2].Size);
-   Writeln('opcode=$',HexStr(ctx.dis.opcode,8),' ',
-           'MIndex=',ctx.dis.ModRM.Index,' ',
-           'SimdOp=',ctx.dis.SimdOpcode,':',SCODES[ctx.dis.SimdOpcode],' ',
-           'mm=',ctx.dis.mm,':',MCODES[ctx.dis.mm and 3]);
+    Writeln('Unhandled jit:',
+            din.OpCode.Prefix,',',
+            din.OpCode.Opcode,',',
+            din.OpCode.Suffix,' ',
+            din.Operand[1].Size,' ',
+            din.Operand[2].Size);
+
+    Writeln('opcode=$',HexStr(dis.opcode,8),' ',
+            'MIndex=',dis.ModRM.Index,' ',
+            'SimdOp=',dis.SimdOpcode,':',SCODES[dis.SimdOpcode],' ',
+            'mm=',MCODES[dis.mm and 3],':',dis.mm);
+   end;
 
    Assert(false);
   end;
diff --git a/sys/jit/kern_jit_ctx.pas b/sys/jit/kern_jit_ctx.pas
index 0785d80f..de72b82c 100644
--- a/sys/jit/kern_jit_ctx.pas
+++ b/sys/jit/kern_jit_ctx.pas
@@ -181,7 +181,7 @@ const
  OPERAND_BYTES:array[TOperandSize] of Word=(0,1,2,4,8,6,10,16,32,64,512);
 
  SCODES:array[TSimdOpcode] of Byte=(0,0,1,3,2);
- MCODES:array[0..3] of PChar=('','0F','0F38','0F3A');
+ MCODES:array[0..3] of PChar=('_','0x0F','0x0F38','0x0F3A');
 
 function GetFrameOffset(const RegValue:TRegValue):Integer;
 function GetFrameOffset(const r:TOperand):Integer;