diff --git a/.gitignore b/.gitignore
index b641b3e..538a99b 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1,4 +1,8 @@
 # SPDX-License-Identifier: MIT
 
 *~
-Development/build/
+*/build/
+*.d
+*.lib
+*.efi
+*.o
diff --git a/Development/rootfs/kaem.amd64 b/Development/rootfs/kaem.amd64
new file mode 100644
index 0000000..a73b3fd
--- /dev/null
+++ b/Development/rootfs/kaem.amd64
@@ -0,0 +1,5 @@
+# kaem
+
+# Rebuild hex0
+EFI\BOOT\hex0.efi hex0_AMD64_POSIX.hex0 hex0 # hex0
+
diff --git a/amd64/Development/Makefile b/amd64/Development/Makefile
new file mode 100644
index 0000000..ca1fbba
--- /dev/null
+++ b/amd64/Development/Makefile
@@ -0,0 +1,23 @@
+# SPDX-FileCopyrightText: 2022 Andrius Štikonas <andrius@stikonas.eu>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+cc = clang
+cflags = -ffreestanding -MMD -mno-red-zone -std=c11 -target x86_64-unknown-windows
+ld = lld-link
+lflags = -subsystem:efi_application -nodefaultlib -dll
+
+build = build
+
+all: $(build)/hex0.efi
+
+$(build)/%.o : %.S
+	mkdir -p $(build)
+	$(cc) $(cflags) -c $< -o $@
+
+$(build)/%.efi : $(build)/%.o
+	$(ld) $(lflags) -entry:efi_main $< -out:$@
+
+. PHONY : clean all
+clean:
+	rm -f $(build)
diff --git a/amd64/Development/hex0.S b/amd64/Development/hex0.S
new file mode 100644
index 0000000..ba23241
--- /dev/null
+++ b/amd64/Development/hex0.S
@@ -0,0 +1,14 @@
+# SPDX-FileCopyrightText: 2022 Andrius Štikonas <andrius@stikonas.eu>
+#
+# SPDX-License-Identifier: GPL-3.0-or-later
+
+	.text
+        .globl  efi_main
+
+efi_main:
+        sub    $16, %rsp
+        mov    %rdx, 8(%rsp)
+        mov    %rcx, (%rsp)
+        mov    $42, %eax
+        add    $16, %rsp
+        ret