diff --git a/exe/uxnemu.ml b/exe/uxnemu.ml
index 2ea6bbd..3e14f16 100644
--- a/exe/uxnemu.ml
+++ b/exe/uxnemu.ml
@@ -2,6 +2,62 @@ open Uxn
 open Effect.Deep
 
 let debug = Option.is_some (Sys.getenv_opt "DBG")
+let banks = Array.init 15 (fun _ -> Bytes.create 65536)
+
+let get_bank_memory mach bank =
+  if bank = 0 then Machine.ram mach
+  else if bank > 0 && bank < 16 then banks.(bank - 1)
+  else Bytes.create 0
+
+let system_expansion mach cmd_addr =
+  let ram = Machine.ram mach in
+  let cmd = Bytes.get_uint8 ram cmd_addr in
+  match cmd with
+  | 0x00 ->
+      let length = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 1) in
+      let bank = Bytes.get_uint16_be ram (cmd_addr + 3) in
+      let addr = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 5) in
+      let value = Bytes.get_uint8 ram (cmd_addr + 7) in
+      if bank < 16 then begin
+        let mem = get_bank_memory mach bank in
+        for i = 0 to length - 1 do
+          let pos = (addr + i) land 0xffff in
+          Bytes.set_uint8 mem pos value
+        done
+      end
+  | 0x01 ->
+      let length = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 1) in
+      let src_bank = Bytes.get_uint16_be ram (cmd_addr + 3) in
+      let src_addr = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 5) in
+      let dst_bank = Bytes.get_uint16_be ram (cmd_addr + 7) in
+      let dst_addr = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 9) in
+      if src_bank < 16 && dst_bank < 16 then begin
+        let src_mem = get_bank_memory mach src_bank in
+        let dst_mem = get_bank_memory mach dst_bank in
+        for i = 0 to length - 1 do
+          let src_pos = (src_addr + i) land 0xffff in
+          let dst_pos = (dst_addr + i) land 0xffff in
+          let v = Bytes.get_uint8 src_mem src_pos in
+          Bytes.set_uint8 dst_mem dst_pos v
+        done
+      end
+  | 0x02 ->
+      let length = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 1) in
+      let src_bank = Bytes.get_uint16_be ram (cmd_addr + 3) in
+      let src_addr = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 5) in
+      let dst_bank = Bytes.get_uint16_be ram (cmd_addr + 7) in
+      let dst_addr = Util.get_uint16_wrap ~wrap:0xffff ram (cmd_addr + 9) in
+      if src_bank < 16 && dst_bank < 16 then begin
+        let src_mem = get_bank_memory mach src_bank in
+        let dst_mem = get_bank_memory mach dst_bank in
+        for i = length - 1 downto 0 do
+          let src_pos = (src_addr + i) land 0xffff in
+          let dst_pos = (dst_addr + i) land 0xffff in
+          let v = Bytes.get_uint8 src_mem src_pos in
+          Bytes.set_uint8 dst_mem dst_pos v
+        done
+      end
+  | _ -> Fmt.epr "System/expansion: unknown command #%02x" cmd
 
 let print_stack ~name (Machine.Stack { data; sp }) =
   Fmt.epr "%s: @[%a@]@." name
@@ -28,19 +84,41 @@ let rec run m pc =
       try
         while Bytes.get_uint8 dev 0x0f = 0 do
           match In_channel.input_byte stdin with
-          | None ->
-              if debug then Fmt.epr "EOF\n";
-              console_input 0 4;
-              raise Exit
+          | None -> raise Exit
           | Some c -> console_input c 1
         done
-      with Exit -> ())
+      with Exit -> console_input 0 4)
   | effect Machine.BRK, _ -> ()
-  | effect Machine.DEI (`Byte, port), k -> continue k (Bytes.get_uint8 dev port)
+  | effect Machine.DEI (`Byte, port), k ->
+      let value =
+        match port with
+        | 0x04 ->
+            let (Machine.Stack { sp; _ }) = Machine.wst m in
+            sp
+        | 0x05 ->
+            let (Machine.Stack { sp; _ }) = Machine.rst m in
+            sp
+        | _ -> Bytes.get_uint8 dev port
+      in
+      continue k value
   | effect Machine.DEI (`Short, port), k ->
-      continue k (Util.get_uint16_wrap dev port)
+      continue k (Util.get_uint16_wrap ~wrap:0xffff dev port)
   | effect Machine.DEO (port, value), k ->
       (match port with
+      | 0x02 -> system_expansion m value
+      | 0x04 ->
+          let (Machine.Stack s) = Machine.wst m in
+          s.sp <- value land 0xff
+      | 0x05 ->
+          let (Machine.Stack s) = Machine.rst m in
+          s.sp <- value land 0xff
+      | 0x0e ->
+          if value <> 0 then begin
+            print_stack ~name:"WST" (Machine.wst m);
+            print_stack ~name:"RST" (Machine.rst m);
+            Out_channel.flush stderr
+          end
+      | 0x0f -> Bytes.set_uint8 dev 0x0f value
       | 0x10 -> console_vector := value
       | 0x18 -> print_char (Char.chr value)
       | 0x19 -> prerr_char (Char.chr value)
diff --git a/lib/Varvara.ml b/lib/Varvara.ml
deleted file mode 100644
index e69de29..0000000
diff --git a/lib/dune b/lib/dune
index 248d9cc..ba1ed70 100644
--- a/lib/dune
+++ b/lib/dune
@@ -1,2 +1,5 @@
+(include_subdirs qualified)
+
 (library
- (name uxn))
+ (name uxn)
+ (libraries unix))
diff --git a/shell.nix b/shell.nix
new file mode 100644
index 0000000..786f1c7
--- /dev/null
+++ b/shell.nix
@@ -0,0 +1,14 @@
+{ pkgs ? import <nixpkgs> {} }:
+pkgs.mkShell {
+  buildInputs = with pkgs; [
+    xxd
+    ocamlPackages.ocaml
+    ocamlPackages.dune_3
+    ocamlPackages.findlib
+    ocamlPackages.odoc
+    ocamlPackages.ocamlformat
+    ocamlPackages.merlin
+    ocamlPackages.ocaml-lsp
+    ocamlPackages.fmt
+  ];
+}
diff --git a/uxn.opam b/uxn.opam
index 35bd3c7..9fa8c71 100644
--- a/uxn.opam
+++ b/uxn.opam
@@ -9,7 +9,7 @@ homepage: "https://codeberg.org/lobo/uxn"
 bug-reports: "https://codeberg.org/lobo/uxn/issues"
 depends: [
   "dune" {>= "3.20"}
-  "ocaml"
+  "ocaml" {>= "5.3"}
   "odoc" {with-doc}
 ]
 build: [