diff --git a/05/build.zig b/05/build.zig
new file mode 100644
index 0000000..c228b31
--- /dev/null
+++ b/05/build.zig
@@ -0,0 +1,15 @@
+const Builder = @import("std").build.Builder;
+
+pub fn build(b: *Builder) void {
+    const mode = b.standardReleaseOptions();
+    const exe = b.addExecutable("05", "src/main.zig");
+    exe.addPackagePath("intcode", "../lib/intcode/intcode.zig");
+    exe.setBuildMode(mode);
+    exe.install();
+
+    const run_cmd = exe.run();
+    run_cmd.step.dependOn(b.getInstallStep());
+
+    const run_step = b.step("run", "Run the app");
+    run_step.dependOn(&run_cmd.step);
+}
diff --git a/05/input b/05/input
new file mode 100644
index 0000000..2109937
--- /dev/null
+++ b/05/input
@@ -0,0 +1 @@
+3,225,1,225,6,6,1100,1,238,225,104,0,1102,40,93,224,1001,224,-3720,224,4,224,102,8,223,223,101,3,224,224,1,224,223,223,1101,56,23,225,1102,64,78,225,1102,14,11,225,1101,84,27,225,1101,7,82,224,1001,224,-89,224,4,224,1002,223,8,223,1001,224,1,224,1,224,223,223,1,35,47,224,1001,224,-140,224,4,224,1002,223,8,223,101,5,224,224,1,224,223,223,1101,75,90,225,101,9,122,224,101,-72,224,224,4,224,1002,223,8,223,101,6,224,224,1,224,223,223,1102,36,63,225,1002,192,29,224,1001,224,-1218,224,4,224,1002,223,8,223,1001,224,7,224,1,223,224,223,102,31,218,224,101,-2046,224,224,4,224,102,8,223,223,101,4,224,224,1,224,223,223,1001,43,38,224,101,-52,224,224,4,224,1002,223,8,223,101,5,224,224,1,223,224,223,1102,33,42,225,2,95,40,224,101,-5850,224,224,4,224,1002,223,8,223,1001,224,7,224,1,224,223,223,1102,37,66,225,4,223,99,0,0,0,677,0,0,0,0,0,0,0,0,0,0,0,1105,0,99999,1105,227,247,1105,1,99999,1005,227,99999,1005,0,256,1105,1,99999,1106,227,99999,1106,0,265,1105,1,99999,1006,0,99999,1006,227,274,1105,1,99999,1105,1,280,1105,1,99999,1,225,225,225,1101,294,0,0,105,1,0,1105,1,99999,1106,0,300,1105,1,99999,1,225,225,225,1101,314,0,0,106,0,0,1105,1,99999,1007,226,677,224,1002,223,2,223,1005,224,329,1001,223,1,223,1007,226,226,224,1002,223,2,223,1006,224,344,101,1,223,223,1107,677,226,224,102,2,223,223,1006,224,359,1001,223,1,223,108,677,677,224,1002,223,2,223,1006,224,374,1001,223,1,223,107,677,677,224,1002,223,2,223,1005,224,389,101,1,223,223,8,677,677,224,1002,223,2,223,1005,224,404,1001,223,1,223,108,226,226,224,1002,223,2,223,1005,224,419,101,1,223,223,1008,677,677,224,1002,223,2,223,1005,224,434,101,1,223,223,1008,226,226,224,1002,223,2,223,1005,224,449,101,1,223,223,7,677,226,224,1002,223,2,223,1006,224,464,1001,223,1,223,7,226,226,224,1002,223,2,223,1005,224,479,1001,223,1,223,1007,677,677,224,102,2,223,223,1005,224,494,101,1,223,223,1108,677,226,224,102,2,223,223,1006,224,509,1001,223,1,223,8,677,226,224,102,2,223,223,1005,224,524,1001,223,1,223,1107,226,226,224,102,2,223,223,1006,224,539,1001,223,1,223,1008,226,677,224,1002,223,2,223,1006,224,554,1001,223,1,223,1107,226,677,224,1002,223,2,223,1006,224,569,1001,223,1,223,1108,677,677,224,102,2,223,223,1005,224,584,101,1,223,223,7,226,677,224,102,2,223,223,1006,224,599,1001,223,1,223,1108,226,677,224,102,2,223,223,1006,224,614,101,1,223,223,107,226,677,224,1002,223,2,223,1005,224,629,101,1,223,223,108,226,677,224,1002,223,2,223,1005,224,644,101,1,223,223,8,226,677,224,1002,223,2,223,1005,224,659,1001,223,1,223,107,226,226,224,1002,223,2,223,1006,224,674,101,1,223,223,4,223,99,226
diff --git a/05/src/main.zig b/05/src/main.zig
new file mode 100644
index 0000000..ae55384
--- /dev/null
+++ b/05/src/main.zig
@@ -0,0 +1,20 @@
+const std = @import("std");
+const intcode = @import("intcode");
+
+pub fn main() anyerror!void {
+    var arena = std.heap.ArenaAllocator.init(std.heap.page_allocator);
+    defer arena.deinit();
+
+    const alloc = &arena.allocator;
+
+    var program = try intcode.loadFromStdIn(alloc);
+    var machine = try intcode.Machine.init(alloc, program);
+
+    try machine.input.append(1); // Air conditioner unit
+    try machine.run();
+
+    std.debug.warn("Part 1\n");
+    for (machine.output.toSlice()) |value, idx| {
+        std.debug.warn("Test {}: {}\n", idx, value);
+    }
+}
diff --git a/lib/intcode/intcode.zig b/lib/intcode/intcode.zig
index fe9b82b..50460ca 100644
--- a/lib/intcode/intcode.zig
+++ b/lib/intcode/intcode.zig
@@ -30,6 +30,58 @@ pub fn loadFromStdIn(alloc: *std.mem.Allocator) anyerror![]i32 {
     return loadFromStream(alloc, stream);
 }
 
+const Opcode = enum(u8) {
+    ADD = 1,
+    MULT = 2,
+    IN = 3,
+    OUT = 4,
+    END = 99,
+};
+
+const Mode = enum(u8) {
+    Position = 0,
+    Immediate = 1,
+};
+
+const Instruction = struct {
+    op: Opcode,
+
+    // Modes for each parameter of the opcode. Parameter 0 is in position 0, etc
+    modes: [3]Mode,
+
+    pub fn decode(value: i32) !Instruction {
+        assert(value > 0 and value < 99999);
+
+        var buf: [6]u8 = undefined;
+        const str = try std.fmt.bufPrint(&buf, "{d:0<6}", value);
+        const op = ((str[4] - '0') * 10) + (str[5] - '0');
+
+        return Instruction{
+            .op = @intToEnum(Opcode, op),
+            .modes = [3]Mode{
+                @intToEnum(Mode, buf[3] - '0'),
+                @intToEnum(Mode, buf[2] - '0'),
+                @intToEnum(Mode, buf[1] - '0'),
+            },
+        };
+    }
+
+    /// Number of bytes taken up by a particular instruction
+    pub fn size(self: Instruction) usize {
+        return switch (self.op) {
+            .ADD => 4,
+            .MULT => 4,
+            .IN => 2,
+            .OUT => 2,
+            .END => 1,
+        };
+    }
+
+    pub fn halt(self: Instruction) bool {
+        return self.op == .END;
+    }
+};
+
 pub const Machine = struct {
     memory: []i32,
 
@@ -58,52 +110,52 @@ pub const Machine = struct {
         while (try self.step()) {}
     }
 
+    /// Read an immediate or position value from memory. Parameter determines
+    /// which field following the current instruction to read from
+    inline fn __read(self: *Machine, parameter: usize, mode: Mode) i32 {
+        const immediate = self.memory[self.ip + parameter + 1];
+
+        return switch (mode) {
+            .Immediate => immediate,
+            .Position => self.memory[@intCast(usize, immediate)],
+        };
+    }
+
+    inline fn __write(self: *Machine, parameter: usize, value: i32) void {
+        const dest = self.__read(parameter, .Immediate);
+        self.memory[@intCast(usize, dest)] = value;
+    }
+
     pub fn step(self: *Machine) anyerror!bool {
         var program = self.memory;
+        const insn = try Instruction.decode(program[self.ip]);
 
-        const opcode = switch (program[self.ip]) {
-            1 => {
-                var a = program[@intCast(usize, program[self.ip + 1])];
-                var b = program[@intCast(usize, program[self.ip + 2])];
-                var dest = program[self.ip + 3];
-                var result = a + b;
+        const opcode = switch (insn.op) {
+            .ADD => {
+                const a = self.__read(0, insn.modes[0]);
+                const b = self.__read(1, insn.modes[1]);
 
-                program[@intCast(usize, dest)] = result;
-                self.ip += 4;
+                self.__write(2, a + b);
             },
-            2 => {
-                var a = program[@intCast(usize, program[self.ip + 1])];
-                var b = program[@intCast(usize, program[self.ip + 2])];
-                var dest = program[self.ip + 3];
-                var result = a * b;
 
-                program[@intCast(usize, dest)] = result;
-                self.ip += 4;
-            },
-            3 => {
-                const input = self.input.orderedRemove(0);
-                const dest = program[self.ip + 1];
+            .MULT => {
+                const a = self.__read(0, insn.modes[0]);
+                const b = self.__read(1, insn.modes[1]);
 
-                program[@intCast(usize, dest)] = input;
-                self.ip += 2;
+                self.__write(2, a * b);
             },
-            4 => {
-                const output = program[@intCast(usize, program[self.ip + 1])];
 
-                try self.output.append(output);
-                self.ip += 2;
+            .IN => {
+                self.__write(0, self.input.orderedRemove(0));
             },
-            99 => {
-                self.ip += 1;
-                return false;
-            },
-            else => {
-                std.debug.warn("Unknown opcode at IP {}: {}\n", self.ip, program[self.ip]);
-                return false;
+            .OUT => {
+                try self.output.append(self.__read(0, insn.modes[0]));
             },
+            .END => {},
         };
 
-        return true;
+        self.ip += insn.size();
+        return !insn.halt();
     }
 };
 
@@ -162,3 +214,66 @@ test "day 5 example 1" {
     assert(machine.output.len == 1);
     assert(machine.output.at(0) == 666);
 }
+
+test "Instruction#decode ADD" {
+    const insn = try Instruction.decode(1);
+
+    assert(insn.op == Opcode.ADD);
+    assert(insn.modes[0] == .Position);
+    assert(insn.modes[1] == .Position);
+    assert(insn.modes[2] == .Position);
+}
+
+test "Instruction#decode MULT" {
+    const insn = try Instruction.decode(2);
+
+    assert(insn.op == Opcode.MULT);
+    assert(insn.modes[0] == .Position);
+    assert(insn.modes[1] == .Position);
+    assert(insn.modes[2] == .Position);
+}
+
+test "Instruction#decode IN" {
+    const insn = try Instruction.decode(3);
+
+    assert(insn.op == Opcode.IN);
+    assert(insn.modes[0] == .Position);
+    assert(insn.modes[1] == .Position);
+    assert(insn.modes[2] == .Position);
+}
+
+test "Instruction#decode OUT" {
+    const insn = try Instruction.decode(4);
+
+    assert(insn.op == Opcode.OUT);
+    assert(insn.modes[0] == .Position);
+    assert(insn.modes[1] == .Position);
+    assert(insn.modes[2] == .Position);
+}
+
+test "Instruction#decode END Position" {
+    const insn = try Instruction.decode(99);
+
+    assert(insn.op == Opcode.END);
+    assert(insn.modes[0] == .Position);
+    assert(insn.modes[1] == .Position);
+    assert(insn.modes[2] == .Position);
+}
+
+test "Instruction#decode END Immediate" {
+    const insn = try Instruction.decode(11199);
+
+    assert(insn.op == Opcode.END);
+    assert(insn.modes[0] == .Immediate);
+    assert(insn.modes[1] == .Immediate);
+    assert(insn.modes[2] == .Immediate);
+}
+
+test "Instruction#decode END mixed" {
+    const insn = try Instruction.decode(10099);
+
+    assert(insn.op == Opcode.END);
+    assert(insn.modes[0] == .Position);
+    assert(insn.modes[1] == .Position);
+    assert(insn.modes[2] == .Immediate);
+}