diff --git a/build.zig b/build.zig
index 1d53b09..eacdb1e 100644
--- a/build.zig
+++ b/build.zig
@@ -7,13 +7,12 @@ pub fn build(b: *std.Build) void {
         .os_tag = .freestanding,
         .abi = .none
     });
-    const optimize = .Debug; // .ReleaseSmall
 
     const exe = b.addExecutable(.{
         .name = "stm32",
         .root_source_file = b.path("src/main.zig"),
         .target = target,
-        .optimize = optimize,
+        .optimize = .Debug, // .ReleaseSmall,
         .linkage = .static,
     });
 
diff --git a/src/gpio.zig b/src/gpio.zig
index 1d713e6..cbb20fa 100644
--- a/src/gpio.zig
+++ b/src/gpio.zig
@@ -7,8 +7,7 @@ const registers = packed struct {
     odr: u32,
     bsrr: u32,
     lckr: u32,
-    afrl: u32,
-    afrh: u32,
+    afr: u64,
     brr: u32,
     ascr: u32,
 };
@@ -16,30 +15,79 @@ const registers = packed struct {
 const driver = struct {
     port: *registers,
 
-    const mode = enum {
+    const mode = enum(u32) {
+        alternate_0 = 0,
+        alternate_1,
+        alternate_2,
+        alternate_3,
+        alternate_4,
+        alternate_5,
+        alternate_6,
+        alternate_7,
+        alternate_8,
+        alternate_9,
+        alternate_10,
+        alternate_11,
+        alternate_12,
+        alternate_13,
+        alternate_14,
+        alternate_15,
         input,
         output,
-        alternate,
         analog,
     };
 
     pub fn set_mode(self: driver, pin: u4, m: mode) void {
-        self.port.moder &= ~(@as(u32, 0x3) << (2 * pin));
+        const offset = 2 * @as(u5, pin);
+        const moder = self.port.moder & ~(@as(u32, 3) << offset);
 
         const mask: u32 = switch (m) {
             .input => 0,
             .output => 1,
-            .alternate => 2,
-            .analog => 3
+            .analog => 3,
+            else => 2,
         };
 
-        self.port.moder |= mask << (2 * pin);
+        self.port.moder = moder | (mask << offset);
+
+        if (mask == 2) {
+            const afr_offset = (4 * @as(u6, pin));
+            const afr = self.port.afr & ~(@as(u64, 0xF) << afr_offset);
+            self.port.afr = afr | (@as(u64, @intFromEnum(m)) << afr_offset);
+        }
     }
 
     pub fn toggle(self: driver, pin: u4) void {
         self.port.odr ^= @as(u32, 1) << pin;
     }
+
+    pub fn set(self: driver, pin: u4) void {
+        self.port.odr |= @as(u32, 1) << pin;
+    }
+
+    pub fn clear(self: driver, pin: u4) void {
+        self.port.odr &= ~(@as(u32, 1) << pin);
+    }
+
+    pub fn write(self: driver, pin: u4, val: bool) void {
+        if (val) {
+            self.set(pin);
+        } else {
+            self.clear(pin);
+        }
+    }
+
+    pub fn read(self: driver, pin: u4) bool {
+        return (self.port.idr & @as(u32, 1) << pin) != 0;
+    }
 };
 
 pub const gpioa = driver { .port = @ptrFromInt(0x48000000) };
+pub const gpiob = driver { .port = @ptrFromInt(0x48000400) };
+pub const gpioc = driver { .port = @ptrFromInt(0x48000800) };
+pub const gpiod = driver { .port = @ptrFromInt(0x48000C00) };
+pub const gpioe = driver { .port = @ptrFromInt(0x48001000) };
+pub const gpiof = driver { .port = @ptrFromInt(0x48001400) };
+pub const gpiog = driver { .port = @ptrFromInt(0x48001800) };
+pub const gpioh = driver { .port = @ptrFromInt(0x48001C00) };
 
diff --git a/src/main.zig b/src/main.zig
index 8ca6ada..3891e52 100644
--- a/src/main.zig
+++ b/src/main.zig
@@ -3,15 +3,18 @@ const gpio = @import("gpio.zig");
 
 const rcc: *[39]u32 = @ptrFromInt(0x40021000);
 const gpioa = gpio.gpioa;
+const gpioc = gpio.gpioc;
 
 export fn _start() callconv(.C) noreturn {
     cpu.interrupt_disable();
 
-    rcc[19] |= 1; // gpioaen
+    rcc[19] |= 5; // gpio a and c
     gpioa.set_mode(5, .output);
+    gpioc.set_mode(13, .input);
 
     while (true) {
-        gpioa.toggle(5);
+        const state = gpioc.read(13);
+        gpioa.write(5, state);
     }
 }