diff --git a/02/src/main.zig b/02/src/main.zig
index 94ee58d..412f1b4 100644
--- a/02/src/main.zig
+++ b/02/src/main.zig
@@ -28,7 +28,7 @@ pub fn main() anyerror!void {
     var program = try intcode.loadFromStdIn(alloc);
 
     // Part 1
-    std.debug.warn("Part 1: {}\n", try runCopy(alloc, program, 12, 2));
+    std.debug.warn("Day 2, Part 1: {}\n", try runCopy(alloc, program, 12, 2));
 
     // Part 2: 100*100 = 10,000 combinations to try.
     var memory: []i32 = try alloc.alloc(i32, program.len);
@@ -36,14 +36,14 @@ pub fn main() anyerror!void {
     var noun: i32 = 0;
     var verb: i32 = 0;
 
-    std.debug.warn("Part 2: Searching...");
+    std.debug.warn("Day 2, Part 2: ");
     while (!done) {
         std.mem.copy(i32, memory, program);
         const result = try run(alloc, memory, noun, verb);
 
         // Too high: 250800 (noun=33, verb=76)
         if (result == 19690720) {
-            std.debug.warn("OK! noun={} verb={}\n", noun, verb);
+            std.debug.warn("noun={} verb={}\n", noun, verb);
             done = true;
         }
 
@@ -54,8 +54,6 @@ pub fn main() anyerror!void {
 
             if (verb > 100) {
                 std.debug.warn("failed!\n");
-            } else {
-                std.debug.warn(".");
             }
         }
     }
diff --git a/05/src/main.zig b/05/src/main.zig
index dee0a3a..18353e7 100644
--- a/05/src/main.zig
+++ b/05/src/main.zig
@@ -14,19 +14,20 @@ pub fn main() anyerror!void {
     var m1 = try intcode.Machine.init(alloc, p1);
     var m2 = try intcode.Machine.init(alloc, p2);
 
-    std.debug.warn("Part 1\n");
-    try m1.input.append(1); // Air conditioner unit
+    try m1.writeToInput(1); // Air conditioner unit
     try m1.run();
 
-    for (m1.output.toSlice()) |value, idx| {
-        std.debug.warn("Test {}: {}\n", idx, value);
+    var out: i32 = 0;
+    while (m1.readFromOutput(false)) |item| {
+        out = item;
+    } else |err| {
+        std.debug.assert(err == std.os.ReadError.WouldBlock);
     }
 
-    std.debug.warn("\nPart 2\n");
-    try m2.input.append(5); // Thermal radiator controller
+    std.debug.warn("Day 5, Part 1: {}\n", out);
+
+    try m2.writeToInput(5); // Thermal radiator controller
     try m2.run();
 
-    for (m2.output.toSlice()) |value, idx| {
-        std.debug.warn("Test {}: {}\n", idx, value);
-    }
+    std.debug.warn("Day 5, Part 2: {}\n", try m2.readFromOutput(false));
 }
diff --git a/07/src/main.zig b/07/src/main.zig
index 630266d..a3ef2cb 100644
--- a/07/src/main.zig
+++ b/07/src/main.zig
@@ -3,7 +3,7 @@ const std = @import("std");
 const intcode = @import("intcode");
 
 // TODO: it would be awesome if we could make this a comptime function
-const permutations = [120][5]i32{
+const perm1 = [120][5]i32{
     .{ 0, 1, 2, 3, 4 },
     .{ 0, 1, 2, 4, 3 },
     .{ 0, 1, 3, 2, 4 },
@@ -138,7 +138,7 @@ pub fn main() anyerror!void {
 
     // 120 phase permutations. Find the highest.
     var max: i32 = 0;
-    for (permutations) |inputs| {
+    for (perm1) |inputs| {
         for (machines) |*machine| {
             var copy = try alloc.alloc(i32, program.len);
             std.mem.copy(i32, copy, program);
@@ -148,22 +148,19 @@ pub fn main() anyerror!void {
 
         // Phase input
         for (inputs) |input, i| {
-            try machines[i].input.append(input);
+            try machines[i].writeToInput(input);
         }
 
         // amplifier input
         for (machines) |*machine, i| {
             var ampInput: i32 = 0;
-            if (i > 0) {
-                try machine.input.append(machines[i - 1].output.at(0));
-            } else {
-                try machine.input.append(0);
-            }
+            if (i > 0) ampInput = try machines[i - 1].readFromOutput(false);
 
+            try machine.writeToInput(ampInput);
             try machine.run();
         }
 
-        const final = machines[4].output.at(0);
+        const final = try machines[4].readFromOutput(false);
         if (final > max)
             max = final;
 
@@ -174,5 +171,7 @@ pub fn main() anyerror!void {
         }
     }
 
-    std.debug.warn("Part 1: {}\n", max);
+    std.debug.warn("Day 7, Part 1: {}\n", max);
+
+    // In part 2, the machines need to be wired into a feedback loop
 }
diff --git a/lib/intcode/intcode.zig b/lib/intcode/intcode.zig
index 9897b71..2a795eb 100644
--- a/lib/intcode/intcode.zig
+++ b/lib/intcode/intcode.zig
@@ -1,7 +1,6 @@
 const std = @import("std");
 
-const CharDev = std.ArrayList(i32);
-const CharDevLimit = 32;
+const CharDev = std.atomic.Queue(i32);
 
 pub fn loadFromStream(alloc: *std.mem.Allocator, stream: *std.fs.File.InStream.Stream) anyerror![]i32 {
     var program = try alloc.alloc(i32, 1024);
@@ -91,6 +90,8 @@ const Instruction = struct {
 };
 
 pub const Machine = struct {
+    alloc: *std.mem.Allocator,
+
     memory: []i32,
 
     ip: usize = 0,
@@ -108,9 +109,10 @@ pub const Machine = struct {
 
     pub fn init(alloc: *std.mem.Allocator, program: []i32) anyerror!Machine {
         return Machine{
+            .alloc = alloc,
             .memory = program,
-            .input = try CharDev.initCapacity(alloc, CharDevLimit),
-            .output = try CharDev.initCapacity(alloc, CharDevLimit),
+            .input = CharDev.init(),
+            .output = CharDev.init(),
         };
     }
 
@@ -119,8 +121,47 @@ pub const Machine = struct {
     }
 
     pub fn deinit(self: *Machine) void {
-        self.input.deinit();
-        self.output.deinit();
+        // TODO
+    }
+
+    pub fn readFromInput(self: *Machine, blocking: bool) !i32 {
+        return self.__in(&self.input, blocking);
+    }
+
+    pub fn writeToInput(self: *Machine, val: i32) !void {
+        return self.__out(&self.input, val);
+    }
+
+    pub fn readFromOutput(self: *Machine, blocking: bool) !i32 {
+        return self.__in(&self.output, blocking);
+    }
+
+    pub fn writeToOutput(self: *Machine, val: i32) !void {
+        return self.__out(&self.output, val);
+    }
+
+    inline fn __in(self: *Machine, dev: *CharDev, blocking: bool) !i32 {
+        var node = dev.get();
+
+        if (node == null) {
+            if (!blocking) return std.os.ReadError.WouldBlock;
+            // Just busy-wait for now
+            while (node == null) node = dev.get();
+        }
+
+        if (node) |n| {
+            defer self.alloc.destroy(n);
+            return n.data;
+        }
+
+        unreachable;
+    }
+
+    inline fn __out(self: *Machine, dev: *CharDev, val: i32) !void {
+        var node = try self.alloc.create(CharDev.Node);
+        node.data = val;
+
+        dev.put(node);
     }
 
     /// Read an immediate or position value from memory. Parameter determines
@@ -159,11 +200,11 @@ pub const Machine = struct {
             },
 
             .IN => {
-                self.__write(0, self.input.orderedRemove(0));
+                self.__write(0, try self.readFromInput(true)); // blocking read
             },
 
             .OUT => {
-                try self.output.append(self.__read(0, insn.modes[0]));
+                try self.writeToOutput(self.__read(0, insn.modes[0]));
             },
 
             .JNZ => {
@@ -256,12 +297,11 @@ test "day 5 example 1" {
     var after: [5]i32 = .{ 666, 0, 4, 0, 99 };
     var machine = try Machine.init(test_allocator, before[0..before.len]);
 
-    try machine.input.append(666);
+    try machine.writeToInput(666);
     try machine.run();
 
     assert(std.mem.eql(i32, before[0..before.len], after[0..after.len]));
-    assert(machine.output.len == 1);
-    assert(machine.output.at(0) == 666);
+    assert(666 == try machine.readFromOutput(false));
 }
 
 test "Instruction#decode ADD" {
diff --git a/run-intcodes b/run-intcodes
index 0b27af2..ac04d79 100755
--- a/run-intcodes
+++ b/run-intcodes
@@ -1,5 +1,5 @@
 #!/bin/sh
-zig test lib/intcode/intcode.zig
-zig build --build-file 02/build.zig run < 02/input
-zig build --build-file 05/build.zig run < 05/input
-zig build --build-file 07/build.zig run < 07/input
+zig test lib/intcode/intcode.zig &&
+	zig build --build-file 02/build.zig run < 02/input &&
+	zig build --build-file 05/build.zig run < 05/input &&
+	zig build --build-file 07/build.zig run < 07/input