diff --git a/py/runtime.c b/py/runtime.c
index 81881c403..006a00d90 100644
--- a/py/runtime.c
+++ b/py/runtime.c
@@ -1027,7 +1027,8 @@ void rt_assign_byte_code(int unique_code_id, byte *code, uint len, int n_args, i
         DEBUG_printf(" %02x", code[i]);
     }
     DEBUG_printf("\n");
-    py_un_byte_code(code, len);
+    extern void py_show_byte_code(const byte *code, int len);
+    py_show_byte_code(code, len);
 
 #ifdef WRITE_CODE
     if (fp_write_code != NULL) {
diff --git a/py/showbc.c b/py/showbc.c
new file mode 100644
index 000000000..a84d98de9
--- /dev/null
+++ b/py/showbc.c
@@ -0,0 +1,371 @@
+#include <stdint.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <string.h>
+#include <assert.h>
+
+#include "misc.h"
+#include "mpyconfig.h"
+#include "runtime.h"
+#include "bc.h"
+
+#define DECODE_UINT do { unum = *ip++; if (unum > 127) { unum = ((unum & 0x3f) << 8) | (*ip++); } } while (0)
+#define DECODE_ULABEL do { unum = (ip[0] | (ip[1] << 8)); ip += 2; } while (0)
+#define DECODE_SLABEL do { unum = (ip[0] | (ip[1] << 8)) - 0x8000; ip += 2; } while (0)
+#define DECODE_QSTR do { qstr = *ip++; if (qstr > 127) { qstr = ((qstr & 0x3f) << 8) | (*ip++); } } while (0)
+
+void py_show_byte_code(const byte *ip, int len) {
+    const byte *ip_start = ip;
+    machine_uint_t unum;
+    qstr qstr;
+    while (ip - ip_start < len) {
+        printf("%02u ", (uint)(ip - ip_start));
+        int op = *ip++;
+        switch (op) {
+            case PYBC_LOAD_CONST_FALSE:
+                printf("LOAD_CONST_FALSE");
+                break;
+
+            case PYBC_LOAD_CONST_NONE:
+                printf("LOAD_CONST_NONE");
+                break;
+
+            case PYBC_LOAD_CONST_TRUE:
+                printf("LOAD_CONST_TRUE");
+                break;
+
+            case PYBC_LOAD_CONST_SMALL_INT:
+                unum = (ip[0] | (ip[1] << 8) | (ip[2] << 16)) - 0x800000;
+                ip += 3;
+                printf("LOAD_CONST_SMALL_INT %d", (int)unum);
+                break;
+
+                /*
+            case PYBC_LOAD_CONST_DEC:
+                DECODE_QSTR;
+                PUSH(rt_load_const_dec(qstr));
+                break;
+
+            case PYBC_LOAD_CONST_ID:
+                DECODE_QSTR;
+                PUSH(rt_load_const_str(qstr)); // TODO
+                break;
+
+            case PYBC_LOAD_CONST_STRING:
+                DECODE_QSTR;
+                PUSH(rt_load_const_str(qstr));
+                break;
+                */
+
+            case PYBC_LOAD_FAST_0:
+                printf("LOAD_FAST_0");
+                break;
+
+            case PYBC_LOAD_FAST_1:
+                printf("LOAD_FAST_1");
+                break;
+
+            case PYBC_LOAD_FAST_2:
+                printf("LOAD_FAST_2");
+                break;
+
+                /*
+            case PYBC_LOAD_FAST_N:
+                DECODE_UINT;
+                PUSH(fastn[unum]);
+                break;
+                */
+
+            case PYBC_LOAD_NAME:
+                DECODE_QSTR;
+                printf("LOAD_NAME %s", qstr_str(qstr));
+                break;
+
+            case PYBC_LOAD_GLOBAL:
+                DECODE_QSTR;
+                printf("LOAD_GLOBAL %s", qstr_str(qstr));
+                break;
+
+                /*
+            case PYBC_LOAD_ATTR:
+                DECODE_QSTR;
+                *sp = rt_load_attr(*sp, qstr);
+                break;
+                */
+
+            case PYBC_LOAD_METHOD:
+                DECODE_QSTR;
+                printf("LOAD_METHOD %s", qstr_str(qstr));
+                break;
+
+                /*
+            case PYBC_LOAD_BUILD_CLASS:
+                PUSH(rt_load_build_class());
+                break;
+                */
+
+            case PYBC_STORE_FAST_0:
+                printf("STORE_FAST_0");
+                break;
+
+            case PYBC_STORE_FAST_1:
+                printf("STORE_FAST_1");
+                break;
+
+            case PYBC_STORE_FAST_2:
+                printf("STORE_FAST_2");
+                break;
+
+                /*
+            case PYBC_STORE_FAST_N:
+                DECODE_UINT;
+                fastn[unum] = POP();
+                break;
+                */
+
+            case PYBC_STORE_NAME:
+                DECODE_QSTR;
+                printf("STORE_NAME %s", qstr_str(qstr));
+                break;
+
+                /*
+            case PYBC_STORE_GLOBAL:
+                DECODE_QSTR;
+                rt_store_global(qstr, POP());
+                break;
+
+            case PYBC_STORE_ATTR:
+                DECODE_QSTR;
+                rt_store_attr(sp[0], qstr, sp[1]);
+                sp += 2;
+                break;
+
+            case PYBC_STORE_SUBSCR:
+                rt_store_subscr(sp[1], sp[0], sp[2]);
+                sp += 3;
+                break;
+
+            case PYBC_DUP_TOP:
+                obj1 = *sp;
+                PUSH(obj1);
+                break;
+
+            case PYBC_DUP_TOP_TWO:
+                sp -= 2;
+                sp[0] = sp[2];
+                sp[1] = sp[3];
+                break;
+                */
+
+            case PYBC_POP_TOP:
+                printf("POP_TOP");
+                break;
+
+                /*
+            case PYBC_ROT_TWO:
+                obj1 = sp[0];
+                sp[0] = sp[1];
+                sp[1] = obj1;
+                break;
+
+            case PYBC_ROT_THREE:
+                obj1 = sp[0];
+                sp[0] = sp[1];
+                sp[1] = sp[2];
+                sp[2] = obj1;
+                break;
+                */
+
+            case PYBC_JUMP:
+                DECODE_SLABEL;
+                printf("JUMP %lu", ip + unum - ip_start);
+                break;
+
+            case PYBC_POP_JUMP_IF_TRUE:
+                DECODE_SLABEL;
+                printf("POP_JUMP_IF_TRUE %lu", ip + unum - ip_start);
+                break;
+
+                /*
+            case PYBC_POP_JUMP_IF_FALSE:
+                DECODE_SLABEL;
+                if (!rt_is_true(POP())) {
+                    ip += unum;
+                }
+                break;
+
+            case PYBC_JUMP_IF_TRUE_OR_POP:
+                DECODE_SLABEL;
+                if (rt_is_true(*sp)) {
+                    ip += unum;
+                } else {
+                    sp++;
+                }
+                break;
+
+            case PYBC_JUMP_IF_FALSE_OR_POP:
+                DECODE_SLABEL;
+                if (rt_is_true(*sp)) {
+                    sp++;
+                } else {
+                    ip += unum;
+                }
+                break;
+
+            case PYBC_SETUP_EXCEPT:
+                DECODE_ULABEL; // except labels are always forward
+                *++exc_sp = (machine_uint_t)ip + unum;
+                *++exc_sp = (machine_uint_t)sp;
+                break;
+
+            case PYBC_END_FINALLY:
+                // not implemented
+                // if TOS is an exception, reraises the exception (3 values on TOS)
+                // if TOS is an integer, does something else
+                // if TOS is None, just pops it and continues
+                // else error
+                assert(0);
+                break;
+
+            case PYBC_GET_ITER:
+                *sp = rt_getiter(*sp);
+                break;
+
+            case PYBC_FOR_ITER:
+                DECODE_ULABEL; // the jump offset if iteration finishes; for labels are always forward
+                obj1 = rt_iternext(*sp);
+                if (obj1 == py_const_stop_iteration) {
+                    ++sp; // pop the exhausted iterator
+                    ip += unum; // jump to after for-block
+                } else {
+                    PUSH(obj1); // push the next iteration value
+                }
+                break;
+
+            case PYBC_POP_BLOCK:
+                // pops block and restores the stack
+                assert(0);
+                break;
+
+            case PYBC_POP_EXCEPT:
+                // TODO need to work out how blocks work etc
+                // pops block, checks it's an exception block, and restores the stack, saving the 3 exception values to local threadstate
+                assert(exc_sp >= &exc_stack[0]);
+                //sp = (py_obj_t*)(*exc_sp--);
+                //exc_sp--; // discard ip
+                exc_sp -= 2;
+                //sp += 3; // pop 3 exception values
+                break;
+
+            case PYBC_UNARY_OP:
+                unum = *ip++;
+                *sp = rt_unary_op(unum, *sp);
+                break;
+                */
+
+            case PYBC_BINARY_OP:
+                unum = *ip++;
+                printf("BINARY_OP %lu", unum);
+                break;
+
+            case PYBC_COMPARE_OP:
+                unum = *ip++;
+                printf("COMPARE_OP %lu", unum);
+                break;
+
+                /*
+            case PYBC_BUILD_TUPLE:
+                DECODE_UINT;
+                obj1 = rt_build_tuple(unum, sp);
+                sp += unum - 1;
+                *sp = obj1;
+                break;
+
+            case PYBC_BUILD_LIST:
+                DECODE_UINT;
+                obj1 = rt_build_list(unum, sp);
+                sp += unum - 1;
+                *sp = obj1;
+                break;
+
+            case PYBC_LIST_APPEND:
+                DECODE_UINT;
+                // I think it's guaranteed by the compiler that sp[unum] is a list
+                rt_list_append(sp[unum], sp[0]);
+                sp++;
+                break;
+
+            case PYBC_BUILD_MAP:
+                DECODE_UINT;
+                PUSH(rt_build_map(unum));
+                break;
+
+            case PYBC_STORE_MAP:
+                sp += 2;
+                rt_store_map(sp[0], sp[-2], sp[-1]);
+                break;
+
+            case PYBC_MAP_ADD:
+                DECODE_UINT;
+                // I think it's guaranteed by the compiler that sp[unum + 1] is a map
+                rt_store_map(sp[unum + 1], sp[0], sp[1]);
+                sp += 2;
+                break;
+
+            case PYBC_BUILD_SET:
+                DECODE_UINT;
+                obj1 = rt_build_set(unum, sp);
+                sp += unum - 1;
+                *sp = obj1;
+                break;
+
+            case PYBC_SET_ADD:
+                DECODE_UINT;
+                // I think it's guaranteed by the compiler that sp[unum] is a set
+                rt_store_set(sp[unum], sp[0]);
+                sp++;
+                break;
+                */
+
+            case PYBC_MAKE_FUNCTION:
+                DECODE_UINT;
+                printf("MAKE_FUNCTION %lu", unum);
+                break;
+
+            case PYBC_CALL_FUNCTION:
+                DECODE_UINT;
+                assert((unum & 0xff00) == 0); // n_keyword
+                unum &= 0xff; // n_positional
+                printf("CALL_FUNCTION %lu", unum);
+                break;
+
+            case PYBC_CALL_METHOD:
+                DECODE_UINT;
+                assert((unum & 0xff00) == 0); // n_keyword
+                unum &= 0xff;
+                printf("CALL_METHOD %lu", unum);
+                break;
+
+            case PYBC_RETURN_VALUE:
+                printf("RETURN_VALUE");
+                break;
+
+                /*
+            case PYBC_YIELD_VALUE:
+                nlr_pop();
+                *ip_in_out = ip;
+                fastn[0] = fast0;
+                fastn[1] = fast1;
+                fastn[2] = fast2;
+                *sp_in_out = sp;
+                return true;
+                */
+
+            default:
+                printf("code %p, byte code 0x%02x not implemented\n", ip, op);
+                assert(0);
+                return;
+        }
+        printf("\n");
+    }
+}