diff --git a/source/app.d b/source/app.d
index b864b19..12fe29d 100644
--- a/source/app.d
+++ b/source/app.d
@@ -47,8 +47,8 @@ Flags:
 
 Backends and their operating systems:
   rm86   - Real mode x86, for bare-metal, DOS
-  x86_64 - 64-bit x86, for bare-metal, Linux
-  arm64  - 64-bit ARM, for Linux
+  x86_64 - 64-bit x86, for bare-metal, linux, osx
+  arm64  - 64-bit ARM, for linux
   uxn    - Varvara/Uxn
   lua    - Lua, uses subset CallistoScript
 
diff --git a/source/backends/arm64.d b/source/backends/arm64.d
index add8ab0..3a7bbc2 100644
--- a/source/backends/arm64.d
+++ b/source/backends/arm64.d
@@ -114,6 +114,7 @@ class BackendARM64 : CompilerBackend {
 		types ~= Type("size",  8);
 		types ~= Type("usize", 8);
 		types ~= Type("cell",  8);
+		types ~= Type("bool",  8);
 
 		// built in structs
 		types ~= Type("Array", 24, true, [
@@ -126,8 +127,16 @@ class BackendARM64 : CompilerBackend {
 		NewConst("Array.elements",   16);
 		NewConst("Array.sizeof",     8 * 3);
 
-		foreach (name, ref type ; types) {
-			NewConst(format("%s.sizeof", name), cast(long) type.size);
+		types ~= Type("Exception", 24 + 8, true, [
+			StructEntry(GetType("bool"),  "error"),
+			StructEntry(GetType("Array"), "msg")
+		]);
+		NewConst("Exception.bool",   0);
+		NewConst("Exception.msg",    8);
+		NewConst("Exception.sizeof", 24 + 8);
+
+		foreach (ref type ; types) {
+			NewConst(format("%s.sizeof", type.name), cast(long) type.size);
 		}
 	}
 
diff --git a/source/backends/lua.d b/source/backends/lua.d
index f18f726..84af844 100644
--- a/source/backends/lua.d
+++ b/source/backends/lua.d
@@ -96,6 +96,7 @@ class BackendLua : CompilerBackend {
 		types ~= Type("size",  1);
 		types ~= Type("usize", 1);
 		types ~= Type("cell",  1);
+		types ~= Type("bool",  1);
 
 		// built in structs
 		types ~= Type("Array", 3, true, [
@@ -108,8 +109,16 @@ class BackendLua : CompilerBackend {
 		NewConst("Array.elements",   2);
 		NewConst("Array.sizeof",     3);
 
-		foreach (name, ref type ; types) {
-			NewConst(format("%s.sizeof", name), cast(long) type.size);
+		types ~= Type("Exception", 3 + 1, true, [
+			StructEntry(GetType("bool"),  "error"),
+			StructEntry(GetType("Array"), "msg")
+		]);
+		NewConst("Exception.bool",   0);
+		NewConst("Exception.msg",    1);
+		NewConst("Exception.sizeof", 3 + 1);
+
+		foreach (ref type ; types) {
+			NewConst(format("%s.sizeof", type.name), cast(long) type.size);
 		}
 	}
 
diff --git a/source/backends/rm86.d b/source/backends/rm86.d
index 4cfac30..f14b35d 100644
--- a/source/backends/rm86.d
+++ b/source/backends/rm86.d
@@ -84,14 +84,15 @@ class BackendRM86 : CompilerBackend {
 	this() {
 		defaultOS = "dos";
 
-		types ~= Type("u8", 1);
-		types ~= Type("i8", 1);
-		types ~= Type("u16", 2);
-		types ~= Type("i16", 2);
-		types ~= Type("addr", 2);
-		types ~= Type("size", 2);
+		types ~= Type("u8",    1);
+		types ~= Type("i8",    1);
+		types ~= Type("u16",   2);
+		types ~= Type("i16",   2);
+		types ~= Type("addr",  2);
+		types ~= Type("size",  2);
 		types ~= Type("usize", 2);
-		types ~= Type("cell", 2);
+		types ~= Type("cell",  2);
+		types ~= Type("bool",  2);
 
 		// built in structs
 		types ~= Type("Array", 6, true, [
@@ -104,11 +105,19 @@ class BackendRM86 : CompilerBackend {
 		NewConst("Array.elements",   4);
 		NewConst("Array.sizeof",     2 * 3);
 
+		types ~= Type("Exception", 6 + 2, true, [
+			StructEntry(GetType("bool"),  "error"),
+			StructEntry(GetType("Array"), "msg")
+		]);
+		NewConst("Exception.bool",   0);
+		NewConst("Exception.msg",    2);
+		NewConst("Exception.sizeof", 6 + 2);
+
 		foreach (ref type ; types) {
 			NewConst(format("%s.sizeof", type.name), cast(long) type.size);
 		}
 
-		if (!opts.noDos) {
+		if (!opts.noDos) { // TODO: remove this
 			globals["__rm86_argv"] = Global(GetType("addr"), false, 0);
 			globals["__rm86_arglen"] = Global(GetType("cell"), false, 0);
 		}
diff --git a/source/backends/uxn.d b/source/backends/uxn.d
index 10c04dc..2d172d1 100644
--- a/source/backends/uxn.d
+++ b/source/backends/uxn.d
@@ -97,8 +97,16 @@ class BackendUXN : CompilerBackend {
 		NewConst("Array.elements",   4);
 		NewConst("Array.sizeof",     2 * 3);
 
-		foreach (name, ref type ; types) {
-			NewConst(format("%s.sizeof", name), cast(long) type.size);
+		types ~= Type("Exception", 6 + 2, true, [
+			StructEntry(GetType("bool"),  "error"),
+			StructEntry(GetType("Array"), "msg")
+		]);
+		NewConst("Exception.bool",   0);
+		NewConst("Exception.msg",    2);
+		NewConst("Exception.sizeof", 6 + 2);
+
+		foreach (ref type ; types) {
+			NewConst(format("%s.sizeof", type.name), cast(long) type.size);
 		}
 	}
 
diff --git a/source/backends/x86_64.d b/source/backends/x86_64.d
index 573f11c..9cfc0bb 100644
--- a/source/backends/x86_64.d
+++ b/source/backends/x86_64.d
@@ -116,6 +116,7 @@ class BackendX86_64 : CompilerBackend {
 		types ~= Type("size",  8);
 		types ~= Type("usize", 8);
 		types ~= Type("cell",  8);
+		types ~= Type("bool",  8);
 
 		// built in structs
 		types ~= Type("Array", 24, true, [
@@ -128,8 +129,16 @@ class BackendX86_64 : CompilerBackend {
 		NewConst("Array.elements",   16);
 		NewConst("Array.sizeof",     8 * 3);
 
-		foreach (name, ref type ; types) {
-			NewConst(format("%s.sizeof", name), cast(long) type.size);
+		types ~= Type("Exception", 24 + 8, true, [
+			StructEntry(GetType("bool"),  "error"),
+			StructEntry(GetType("Array"), "msg")
+		]);
+		NewConst("Exception.bool",   0);
+		NewConst("Exception.msg",    8);
+		NewConst("Exception.sizeof", 24 + 8);
+
+		foreach (ref type ; types) {
+			NewConst(format("%s.sizeof", type.name), cast(long) type.size);
 		}
 	}