diff --git a/src/obkrnl/src/config/mod.rs b/src/obkrnl/src/config/mod.rs
index ff075b142..2d2b01c75 100644
--- a/src/obkrnl/src/config/mod.rs
+++ b/src/obkrnl/src/config/mod.rs
@@ -8,11 +8,15 @@ pub use self::arch::*;
 #[cfg_attr(target_arch = "x86_64", path = "x86_64.rs")]
 mod arch;
 
+/// # Interupt safety
+/// This function is interupt safe.
 pub fn boot_env() -> &'static BootEnv {
     // SAFETY: This is safe because the setup() requirements.
     unsafe { &*BOOT_ENV }
 }
 
+/// # Interupt safety
+/// This function is interupt safe.
 pub fn config() -> &'static Config {
     // SAFETY: This is safe because the setup() requirements.
     unsafe { &*CONFIG }
diff --git a/src/obkrnl/src/console/mod.rs b/src/obkrnl/src/console/mod.rs
index d632c1c64..2732af9ea 100644
--- a/src/obkrnl/src/console/mod.rs
+++ b/src/obkrnl/src/console/mod.rs
@@ -12,6 +12,10 @@ mod vm;
 /// performance critical path.
 ///
 /// The LF character will be automatically appended.
+///
+/// # Interupt safety
+/// This macro is interupt safe as long as [`Display`] implementation on all arguments are interupt
+/// safe (e.g. no heap allocation).
 #[macro_export]
 macro_rules! info {
     ($($args:tt)*) => {
@@ -19,6 +23,9 @@ macro_rules! info {
     };
 }
 
+/// # Interupt safety
+/// This function is interupt safe as long as [`Display`] implementation on `msg` are interupt safe
+/// (e.g. no heap allocation).
 #[inline(never)]
 pub fn info(file: &str, line: u32, msg: impl Display) {
     print(
@@ -33,6 +40,9 @@ pub fn info(file: &str, line: u32, msg: impl Display) {
     );
 }
 
+/// # Interupt safety
+/// This function is interupt safe as long as [`Display`] implementation on `msg` are interupt safe
+/// (e.g. no heap allocation).
 #[inline(never)]
 pub fn error(file: &str, line: u32, msg: impl Display) {
     print(
@@ -47,6 +57,9 @@ pub fn error(file: &str, line: u32, msg: impl Display) {
     )
 }
 
+/// # Interupt safety
+/// This function is interupt safe as long as [`Display`] implementation on `msg` are interupt safe
+/// (e.g. no heap allocation).
 fn print(vty: MsgType, msg: impl Display) {
     match boot_env() {
         BootEnv::Vm(env) => self::vm::print(env, vty, msg),
@@ -64,6 +77,7 @@ struct Log<'a, M: Display> {
 
 impl<'a, M: Display> Display for Log<'a, M> {
     fn fmt(&self, f: &mut Formatter<'_>) -> core::fmt::Result {
+        // This implementation must be interupt safe.
         writeln!(
             f,
             "{}++++++++++++++++++ {} {}:{}{0:#}",
diff --git a/src/obkrnl/src/console/vm.rs b/src/obkrnl/src/console/vm.rs
index b17de3310..680c77990 100644
--- a/src/obkrnl/src/console/vm.rs
+++ b/src/obkrnl/src/console/vm.rs
@@ -3,6 +3,9 @@ use core::ptr::{addr_of_mut, write_volatile};
 use obconf::Vm;
 use obvirt::console::{Memory, MsgType};
 
+/// # Interupt safety
+/// This function is interupt safe as long as [`Display`] implementation on `msg` are interupt safe
+/// (e.g. no heap allocation).
 pub fn print(env: &Vm, ty: MsgType, msg: impl Display) {
     let c = env.console as *mut Memory;
     let mut w = Writer(c);
@@ -16,6 +19,7 @@ struct Writer(*mut Memory);
 
 impl Write for Writer {
     fn write_str(&mut self, s: &str) -> core::fmt::Result {
+        // This implementation must be interupt safe.
         unsafe { write_volatile(addr_of_mut!((*self.0).msg_len), s.len()) };
         unsafe { write_volatile(addr_of_mut!((*self.0).msg_addr), s.as_ptr() as usize) };
         Ok(())
diff --git a/src/obkrnl/src/context/mod.rs b/src/obkrnl/src/context/mod.rs
index d4a967ec2..fd25e0086 100644
--- a/src/obkrnl/src/context/mod.rs
+++ b/src/obkrnl/src/context/mod.rs
@@ -26,6 +26,9 @@ impl Context {
         }
     }
 
+    /// # Interupt safety
+    /// This function is interupt safe.
+    #[inline(never)]
     pub fn thread() -> Arc<Thread> {
         // It does not matter if we are on a different CPU after we load the Context::thread because
         // it is going to be the same one since it represent the current thread.
@@ -38,15 +41,19 @@ impl Context {
         unsafe { Arc::from_raw(td) }
     }
 
+    /// Pin the calling thread to one CPU.
+    ///
+    /// This thread will never switch to a different CPU until the returned [`PinnedContext`] is
+    /// dropped (but it is allowed to sleep).
+    ///
     /// See `critical_enter` and `critical_exit` on the PS4 for a reference.
+    #[inline(never)]
     pub fn pin() -> PinnedContext {
-        // TODO: Verify if memory ordering here is correct. We need a call to self::arch::current()
-        // to execute after the thread is in a critical section. The CPU must not reorder this. Our
-        // current implementation follow how Drop on Arc is implemented.
+        // Relax ordering should be enough here since this increment will be checked by the same CPU
+        // when an interupt happens.
         let td = unsafe { self::arch::thread() };
 
-        unsafe { (*td).critical_sections().fetch_add(1, Ordering::Release) };
-        core::sync::atomic::fence(Ordering::Acquire);
+        unsafe { (*td).critical_sections().fetch_add(1, Ordering::Relaxed) };
 
         // Once the thread is in a critical section it will never be switch a CPU so it is safe to
         // keep a pointer to a context here.
@@ -83,10 +90,11 @@ impl PinnedContext {
 
 impl Drop for PinnedContext {
     fn drop(&mut self) {
-        // TODO: Verify if memory ordering here is correct.
+        // Relax ordering should be enough here since this decrement will be checked by the same CPU
+        // when an interupt happens.
         let td = unsafe { (*self.0).thread.load(Ordering::Relaxed) };
 
-        unsafe { (*td).critical_sections().fetch_sub(1, Ordering::Release) };
+        unsafe { (*td).critical_sections().fetch_sub(1, Ordering::Relaxed) };
 
         // TODO: Implement td_owepreempt.
     }
diff --git a/src/obkrnl/src/context/x86_64.rs b/src/obkrnl/src/context/x86_64.rs
index d3d59532b..a0c449025 100644
--- a/src/obkrnl/src/context/x86_64.rs
+++ b/src/obkrnl/src/context/x86_64.rs
@@ -52,7 +52,8 @@ pub unsafe fn thread() -> *const Thread {
 }
 
 pub unsafe fn current() -> *const Context {
-    // Load current GS.
+    // Load current GS. Although the "rdmsr" does not read or write to any memory but it need to
+    // synchronize with a critical section.
     let mut edx: u32;
     let mut eax: u32;
 
@@ -61,7 +62,7 @@ pub unsafe fn current() -> *const Context {
         in("ecx") 0xc0000101u32,
         out("edx") edx,
         out("eax") eax,
-        options(pure, nomem, preserves_flags, nostack)
+        options(preserves_flags, nostack)
     );
 
     // Combine EDX and EAX.
diff --git a/src/obkrnl/src/main.rs b/src/obkrnl/src/main.rs
index af3bdded3..aec1d352f 100644
--- a/src/obkrnl/src/main.rs
+++ b/src/obkrnl/src/main.rs
@@ -74,6 +74,8 @@ fn main() -> ! {
     }
 }
 
+/// # Interupt safety
+/// This function is interupt safe.
 #[allow(dead_code)]
 #[cfg_attr(target_os = "none", panic_handler)]
 fn panic(i: &PanicInfo) -> ! {
diff --git a/src/obkrnl/src/malloc/stage2.rs b/src/obkrnl/src/malloc/stage2.rs
index 33125f21e..24eb312f6 100644
--- a/src/obkrnl/src/malloc/stage2.rs
+++ b/src/obkrnl/src/malloc/stage2.rs
@@ -66,6 +66,8 @@ impl Stage2 {
         Self { zones, stats }
     }
 
+    /// Returns null on failure.
+    ///
     /// See `malloc` on the PS4 for a reference.
     ///
     /// # Safety
diff --git a/src/obkrnl/src/panic/mod.rs b/src/obkrnl/src/panic/mod.rs
index 11bfbebc1..537030542 100644
--- a/src/obkrnl/src/panic/mod.rs
+++ b/src/obkrnl/src/panic/mod.rs
@@ -1,6 +1,9 @@
 use core::arch::asm;
 
 /// Perform panic after printing the panic message.
+///
+/// # Interupt safety
+/// This function is interupt safe.
 pub fn panic() -> ! {
     loop {
         #[cfg(target_arch = "aarch64")]
diff --git a/src/obkrnl/src/proc/thread.rs b/src/obkrnl/src/proc/thread.rs
index e1d4cdbc7..1a0aa04b2 100644
--- a/src/obkrnl/src/proc/thread.rs
+++ b/src/obkrnl/src/proc/thread.rs
@@ -4,6 +4,9 @@ use core::sync::atomic::AtomicU32;
 ///
 /// All thread **must** run to completion once execution has been started otherwise resource will be
 /// leak if the thread is dropped while its execution currently in the kernel space.
+///
+/// We subtitute `TDP_NOSLEEPING` with `td_intr_nesting_level` since the only cases the thread
+/// should not allow to sleep is when it being handle an interupt.
 pub struct Thread {
     critical_sections: AtomicU32, // td_critnest
     active_interrupts: usize,     // td_intr_nesting_level
@@ -23,7 +26,12 @@ impl Thread {
         }
     }
 
-    pub fn critical_sections(&self) -> &AtomicU32 {
+    /// See [`crate::context::Context::pin()`] for a safe wrapper.
+    ///
+    /// # Safety
+    /// This is a counter. Each increment must paired with a decrement. Failure to do so will cause
+    /// the whole system to be in an undefined behavior.
+    pub unsafe fn critical_sections(&self) -> &AtomicU32 {
         &self.critical_sections
     }
 
diff --git a/src/obkrnl/src/uma/bucket.rs b/src/obkrnl/src/uma/bucket.rs
new file mode 100644
index 000000000..a0150f810
--- /dev/null
+++ b/src/obkrnl/src/uma/bucket.rs
@@ -0,0 +1,10 @@
+/// Implementation of `uma_bucket` structure.
+pub struct UmaBucket {
+    len: usize, // ub_cnt
+}
+
+impl UmaBucket {
+    pub fn len(&self) -> usize {
+        self.len
+    }
+}
diff --git a/src/obkrnl/src/uma/cache.rs b/src/obkrnl/src/uma/cache.rs
new file mode 100644
index 000000000..1894eaf88
--- /dev/null
+++ b/src/obkrnl/src/uma/cache.rs
@@ -0,0 +1,13 @@
+use super::bucket::UmaBucket;
+
+/// Implementation of `uma_cache` structure.
+#[derive(Default)]
+pub struct UmaCache {
+    alloc: Option<UmaBucket>, // uc_allocbucket
+}
+
+impl UmaCache {
+    pub fn alloc(&self) -> Option<&UmaBucket> {
+        self.alloc.as_ref()
+    }
+}
diff --git a/src/obkrnl/src/uma/mod.rs b/src/obkrnl/src/uma/mod.rs
index 90eae17c0..f4c2dd2e7 100644
--- a/src/obkrnl/src/uma/mod.rs
+++ b/src/obkrnl/src/uma/mod.rs
@@ -1,15 +1,34 @@
+use self::cache::UmaCache;
+use crate::config::config;
+use crate::context::Context;
 use alloc::borrow::Cow;
+use alloc::vec::Vec;
+
+mod bucket;
+mod cache;
 
 /// Implementation of `uma_zone` structure.
 pub struct UmaZone {
-    size: usize, // uz_size
+    size: usize,           // uz_size
+    caches: Vec<UmaCache>, // uz_cpu
 }
 
 impl UmaZone {
     /// See `uma_zcreate` on the PS4 for a reference.
     pub fn new(_: Cow<'static, str>, size: usize, _: usize) -> Self {
-        // TODO: Check if size is allowed to be zero. If not, change it to NonZero<usize>.
-        Self { size }
+        // Ths PS4 allocate a new uma_zone from masterzone_z but we don't have that. This method
+        // basically an implementation of zone_ctor.
+        let len = config().max_cpu.get();
+        let mut caches = Vec::with_capacity(len);
+
+        for _ in 0..len {
+            caches.push(UmaCache::default());
+        }
+
+        Self {
+            size, // TODO: Check if size is allowed to be zero. If not, change it to NonZero<usize>.
+            caches,
+        }
     }
 
     pub fn size(&self) -> usize {
@@ -18,6 +37,26 @@ impl UmaZone {
 
     /// See `uma_zalloc_arg` on the PS4 for a reference.
     pub fn alloc(&self) -> *mut u8 {
+        // Our implementation imply M_WAITOK and M_ZERO.
+        let td = Context::thread();
+
+        if td.active_interrupts() != 0 {
+            panic!("heap allocation in an interrupt handler is not supported");
+        }
+
+        // Try to allocate from per-CPU cache.
+        let cx = Context::pin();
+        let cache = &self.caches[cx.cpu()];
+        let bucket = cache.alloc();
+
+        while let Some(bucket) = bucket {
+            if bucket.len() != 0 {
+                todo!()
+            }
+
+            todo!()
+        }
+
         todo!()
     }
 }