diff --git a/addressing_mode.cc b/addressing_mode.cc
index ac53d8d..ca74bdb 100644
--- a/addressing_mode.cc
+++ b/addressing_mode.cc
@@ -10,6 +10,8 @@ static bool is_uint8(ssize_t n) { return n >= 0 && n <= 255; }
 
 static bool is_int8(ssize_t n) { return n >= -128 && n <= 127; }
 
+static bool is_imm8(ssize_t n) { return n >= -128 && n <= 255; }
+
 static bool always_valid(ssize_t _) { return true; }
 
 std::unordered_map<isa::mode, addressing_mode_t>& addressing_mode() {
@@ -20,7 +22,7 @@ std::unordered_map<isa::mode, addressing_mode_t>& addressing_mode() {
     {isa::mode::AIY,  { .len = 2, .valid = is_uint16 }},
     {isa::mode::AI,   { .len = 2, .valid = is_uint16 }},
     {isa::mode::ACC,  { .len = 0, .valid = always_valid }},
-    {isa::mode::IMM,  { .len = 1, .valid = is_uint8 }},
+    {isa::mode::IMM,  { .len = 1, .valid = is_imm8 }},
     {isa::mode::I,    { .len = 0, .valid = always_valid }},
     {isa::mode::R,    { .len = 1, .valid = is_int8 }},
     {isa::mode::S,    { .len = 0, .valid = always_valid }},
diff --git a/assembler.hh b/assembler.hh
index 212a232..a4f251f 100644
--- a/assembler.hh
+++ b/assembler.hh
@@ -36,6 +36,8 @@ namespace assembler {
       size_t start_h;
       size_t size_l;
       size_t size_h;
+      size_t end_l;
+      size_t end_h;
     } symbol;
     size_t location;
     std::shared_ptr<std::string> buf;
diff --git a/link.py b/link.py
new file mode 100644
index 0000000..2aaaf68
--- /dev/null
+++ b/link.py
@@ -0,0 +1,29 @@
+import sys
+
+assert len(sys.argv) > 2
+
+
+mem = bytearray(2 ** 15)
+
+
+with open(sys.argv[1], 'rb') as f:
+    l = f.read()
+
+
+for i, a in enumerate(l):
+    mem[i + 0x4000] = a
+
+
+# NMIB
+mem[0xfffa - 0x8000] = 0x00
+mem[0xfffb - 0x8000] = 0xc0
+# RESB
+mem[0xfffc - 0x8000] = 0x00
+mem[0xfffd - 0x8000] = 0xc0
+# IRQB
+mem[0xfffe - 0x8000] = 0x02
+mem[0xffff - 0x8000] = 0xc0
+
+
+with open(sys.argv[2], 'wb') as f:
+    f.write(mem)
diff --git a/main.cc b/main.cc
index 045c622..c824791 100644
--- a/main.cc
+++ b/main.cc
@@ -3,6 +3,7 @@
 #include <iostream>
 #include <unordered_map>
 #include <memory>
+#include <cstdio>
 
 #include "assembler.hh"
 #include "addressing_mode.hh"
@@ -13,9 +14,9 @@
 
 using namespace std::literals;
 
-void serialize_program(const assembler::program_t& program, const assembler::symbol_table_t &symbol_table, std::ostream& out)
+bool serialize_program(const assembler::program_t& program, const assembler::symbol_table_t &symbol_table, std::ostream& out)
 {
-  auto serialize_instruction = [&](assembler::instruction_t& ins) -> void {
+  auto serialize_instruction = [&](assembler::instruction_t& ins) -> bool {
     //std::cerr << "enc " << (int)ins.op << ' ' << (int)ins.mode << '\n';
     char buf[3];
 
@@ -24,15 +25,9 @@ void serialize_program(const assembler::program_t& program, const assembler::sym
       std::cerr << ins.row << ": illegal op,mode: "
                 << tostring::op().find(ins.op)->second << ' '
                 << tostring::mode().find(ins.mode)->second << '\n';
-      return;
+      return false;
     }
 
-    /*
-    std::cerr << tostring::op().find(ins.op)->second << ' '
-              << tostring::mode().find(ins.mode)->second << ' '
-              << std::hex << (int)opcode_it->second << '\n';
-    */
-
     buf[0] = opcode_it->second;
 
     auto am_it = addressing_mode().find(ins.mode);
@@ -67,7 +62,7 @@ void serialize_program(const assembler::program_t& program, const assembler::sym
     size_t value = get_value();
     if (!am_it->second.valid(value)) {
       std::cerr << "overflow at " << ins.row << " value: " << value << '\n';
-      return;
+      return false;
     }
 
     //std::cerr << "value " << std::hex << value << ' ' << am_it->second.len << '\n';
@@ -77,14 +72,28 @@ void serialize_program(const assembler::program_t& program, const assembler::sym
     if (am_it->second.len >= 1)
       buf[1] = (value >> 0) & 0xff;
 
+    std::cerr << std::hex << ins.location << ' '
+              << tostring::op().find(ins.op)->second << ' '
+              << tostring::mode().find(ins.mode)->second << ' ';
+    if (am_it->second.len == 2)
+      std::cerr << std::hex << (value & 0xffff) << '\n';
+    else if (am_it->second.len == 1)
+      std::cerr << std::hex << (value & 0xff) << '\n';
+    else
+      std::cerr << '\n';
+
     out.write(buf, 1 + am_it->second.len);
+
+    return true;
   };
 
 
   for (auto prog_it = program.cbegin(); prog_it != program.cend(); prog_it++) {
     if (std::holds_alternative<assembler::instruction_t>(*prog_it)) {
       auto ins = std::get<assembler::instruction_t>(*prog_it);
-      serialize_instruction(ins);
+      auto ok = serialize_instruction(ins);
+      if (!ok)
+        return false;
     } else if (std::holds_alternative<assembler::blob_t>(*prog_it)) {
       auto blob = std::get<assembler::blob_t>(*prog_it);
       out.write(blob.buf->data(), blob.buf->size());
@@ -92,6 +101,8 @@ void serialize_program(const assembler::program_t& program, const assembler::sym
       assert (false);
     }
   }
+
+  return true;
 }
 
 int main(int argc, char * argv[])
@@ -128,7 +139,7 @@ int main(int argc, char * argv[])
       ok = parser::parse(*buf, program, symbol_strings);
       if (!ok)
         return -1;
-    } else if (ext == ".bin"s) {
+    } else if (ext == ".bin"s or ext == ".txt"s) {
       std::string name = input_filename.substr(0, input_filename.size() - 4);
       assembler::blob_t blob {
         .symbol = {
@@ -137,6 +148,8 @@ int main(int argc, char * argv[])
           .start_h = symbol::get(symbol_strings, "_bin_"s + name + "_start_h"s),
           .size_l = symbol::get(symbol_strings, "_bin_"s + name + "_size_l"s),
           .size_h = symbol::get(symbol_strings, "_bin_"s + name + "_size_h"s),
+          .end_l = symbol::get(symbol_strings, "_bin_"s + name + "_end_l"s),
+          .end_h = symbol::get(symbol_strings, "_bin_"s + name + "_end_h"s),
         },
         .location = 0xeeee,
         .buf = std::move(buf),
@@ -159,14 +172,19 @@ int main(int argc, char * argv[])
 
   std::cerr << "output: " << argv[argc - 1] << '\n';
   std::string output_filename {argv[argc - 1]};
-  std::ofstream out {output_filename, std::ios::binary};
+  std::ofstream out {output_filename + ".part", std::ios::binary};
   if (!out.is_open()) {
     std::cerr << "failed to open " << output_filename << '\n';
     return -1;
   }
 
-  serialize_program(program, symbol_table, out);
+  ok = serialize_program(program, symbol_table, out);
+  if (!ok) {
+    return -1;
+  }
 
   out.close();
+  std::rename((output_filename + ".part").data(), output_filename.data());
+
   return 0;
 }
diff --git a/symbol.cc b/symbol.cc
index 82192ee..73122b5 100644
--- a/symbol.cc
+++ b/symbol.cc
@@ -47,7 +47,9 @@ bool resolve(size_t link_location, assembler::program_t& program, assembler::sym
       auto [_3, ok3] = symbol_table.insert({blob.symbol.start_h, (offset >> 8) & 0xff});
       auto [_4, ok4] = symbol_table.insert({blob.symbol.size_l, (blob.buf->size() >> 0) & 0xff});
       auto [_5, ok5] = symbol_table.insert({blob.symbol.size_h, (blob.buf->size() >> 8) & 0xff});
-      if (!ok1 || !ok2 || !ok3 || !ok4 || !ok5) {
+      auto [_6, ok6] = symbol_table.insert({blob.symbol.end_l, ((offset + blob.buf->size() - 1) >> 0) & 0xff});
+      auto [_7, ok7] = symbol_table.insert({blob.symbol.end_h, ((offset + blob.buf->size() - 1) >> 8) & 0xff});
+      if (!ok1 || !ok2 || !ok3 || !ok4 || !ok5 || !ok6 || !ok7) {
         std::cerr << "duplicate blob symbol\n";
         return false;
       }