diff --git a/Makefile b/Makefile
index 50ea96b..1496c7f 100644
--- a/Makefile
+++ b/Makefile
@@ -21,7 +21,6 @@ CFLAGS += -ffreestanding
 CFLAGS += -fno-common
 CFLAGS += -nostdlib
 CFLAGS += -mno-relax
-CFLAGS += -std=gnu99
 
 CFLAGS += -fno-stack-protector # Prevents code that needs libc / runtime support
 CFLAGS += -MD # Generate header dependency files (.d)
@@ -45,8 +44,7 @@ KERNEL_OBJ := \
 	kern/libkern/panic.o \
 	kern/libkern/memory.o \
 	kern/libkern/spinlock.o \
-	kern/libkern/mini-printf.o \
-	kern/libkern/stdio.o
+	kern/libkern/mini-printf.o
 
 kern/kernel.elf: $(KERNEL_OBJ)
 	@echo LD $@
diff --git a/kern/kalloc.c b/kern/kalloc.c
index 88238ee..17d65c7 100644
--- a/kern/kalloc.c
+++ b/kern/kalloc.c
@@ -44,7 +44,7 @@ void kfree(void *pa) {
     // Assert that page is a ligned to a page boundary and that its correctly
     // sized
     if (((u64)pa % PGSIZE) != 0 || (char *)pa < kernel_end || (u64)pa >= PHYSTOP)
-        PANIC("kfree");
+        panic("kfree");
 
     // Fill with junk to catch dangling refs.
     memset(pa, 1, PGSIZE);
diff --git a/kern/libkern/panic.c b/kern/libkern/panic.c
index 3671b04..f0df22e 100644
--- a/kern/libkern/panic.c
+++ b/kern/libkern/panic.c
@@ -1,18 +1,8 @@
-#include "stdbool.h"
-#include <mini-printf.h>
-#include <stdarg.h>
-#include <stddef.h>
-#include <stdio.h>
 #include <uart.h>
+volatile int panicked;
 
-volatile int panicked = false;
-
-__attribute__((visibility("hidden")))
-void __panic(const char *restrict fmt, ...) {
-    va_list ap;
-    va_start(ap, fmt);
-    (void)mini_vpprintf(stdout_puts, NULL, fmt, ap);
-    va_end(ap);
-    panicked = true;
-    while (true) asm volatile("wfi");
+void panic(char *s) {
+    panicked = 1;
+    uart_puts(s);
+    while (1);
 }
diff --git a/kern/libkern/panic.h b/kern/libkern/panic.h
index 53dc283..77cd640 100644
--- a/kern/libkern/panic.h
+++ b/kern/libkern/panic.h
@@ -1,8 +1,6 @@
 #ifndef KERNEL_PANIC_H
 #define KERNEL_PANIC_H
 
-#define PANIC(fmt, ...) __panic("[%s:%d %s] \n" fmt, __FILE__, __LINE__, __func__)
-
-void __panic(const char *restrict fmt, ...);
+void panic(char *s);
 
 #endif
diff --git a/kern/libkern/proc.c b/kern/libkern/proc.c
index bebf1a5..66bca67 100644
--- a/kern/libkern/proc.c
+++ b/kern/libkern/proc.c
@@ -1,6 +1,6 @@
 #include <proc.h>
 
-Cpu cpus[NCPU];
+struct Cpu cpus[NCPU];
 
 /**
  * Must be called with interrupts disabled, to prevent race with process being
@@ -14,8 +14,8 @@ int cpuid() {
 /**
  * Return this CPU's cpu struct. Interrupts must be disabled.
  */
-Cpu *mycpu(void) {
-    int  id = cpuid();
-    Cpu *c = &cpus[id];
+struct Cpu *mycpu(void) {
+    int         id = cpuid();
+    struct Cpu *c = &cpus[id];
     return c;
 }
diff --git a/kern/libkern/spinlock.c b/kern/libkern/spinlock.c
index 7dc6658..f3b194a 100644
--- a/kern/libkern/spinlock.c
+++ b/kern/libkern/spinlock.c
@@ -3,6 +3,8 @@
  * (Not mutexes as these are spinning locks).
  */
 
+// #include <lib/stdio.h>
+#include "string.h"
 #include <panic.h>
 #include <proc.h>
 #include <riscv.h>
@@ -39,42 +41,86 @@
  * On RISC-V, this emits a fence instruction.
  */
 
-/*
- * These are from the original xv6 implementation, with only slight modifications on their return type.
- *
- * push_off/pop_off are like intr_off()/intr_on() except that they are matched:
- * it takes two pop_off()s to undo two push_off()s.  Also, if interrupts
- * are initially off, then push_off, pop_off leaves them off.
- */
-
-uint32_t push_off(void) {
-    int  old = intr_get();
-    Cpu *cpu = mycpu();
-
-    intr_off();
-
-    if (cpu->noff == 0)
-        cpu->intena = old;
-
-    cpu->noff += 1;
-    return cpu->noff;
+/** Initialize Spinlock */
+void initlock(struct Spinlock *lk, char *name) {
+    lk->name = name;
+    lk->locked = 0;
+    lk->cpu = 0;
 }
 
-uint32_t pop_off(void) {
-    Cpu *cpu = mycpu();
+/**
+ * Acquire the lock.
+ * Loops (spins) until the lock is acquired.
+ * Panics if the lock is already held by this cpu.
+ */
+void acquire(struct Spinlock *lk) {
+    push_off(); // disable interrupts to avoid deadlock.
 
+    if (holding(lk)) // If the lock is already held, panic.
+        panic("acquire");
+
+    // Spin until aquired. See file header for details
+    while (__sync_lock_test_and_set(&lk->locked, 1) != 0);
+
+    __sync_synchronize(); // No loads/stores after this point
+
+    // Record info about lock acquisition for holding() and debugging.
+    lk->cpu = mycpu();
+}
+
+/**
+ * Release the lock.
+ * Panics if the lock is not held.
+ */
+void release(struct Spinlock *lk) {
+    if (!holding(lk)) // If the lock is not held, panic.
+        panic("release");
+
+    lk->cpu = 0;                      // 0 means unheld
+    __sync_synchronize();             // No loads/stores after this point
+    __sync_lock_release(&lk->locked); // Essentially lk->locked = 0
+
+    pop_off();
+}
+
+// Check whether this cpu is holding the lock.
+// Interrupts must be off.
+int holding(struct Spinlock *lk) {
+    int r;
+    r = (lk->locked && lk->cpu == mycpu());
+    return r;
+}
+
+// push_off/pop_off are like intr_off()/intr_on() except that they are matched:
+// it takes two pop_off()s to undo two push_off()s.  Also, if interrupts
+// are initially off, then push_off, pop_off leaves them off.
+
+void push_off(void) {
+    int old = intr_get();
+
+    intr_off();
+    if (mycpu()->noff == 0)
+        mycpu()->intena = old;
+
+    mycpu()->noff += 1;
+}
+
+void pop_off(void) {
+    struct Cpu *c = mycpu();
     if (intr_get())
-        PANIC("pop_off - interruptible");
-
-    if (cpu->noff < 1)
-        PANIC("pop_off");
-
-    cpu->noff -= 1;
-
-    if (cpu->noff == 0 && cpu->intena)
+        panic("pop_off - interruptible");
+    if (c->noff < 1) {
+        {
+            // TODO: Remove this block when fixed
+            char amt[100];
+            itoa(c->noff, amt, 10);
+            uart_puts(amt);
+        }
+        panic("pop_off");
+    }
+    c->noff -= 1;
+    if (c->noff == 0 && c->intena)
         intr_on();
-
-    return cpu->noff;
 }
 
 void spinlock_init(spinlock_t *l) {
@@ -89,47 +135,34 @@ __attribute__((warn_unused_result)) bool spin_trylock(spinlock_t *l) {
 }
 
 void spin_unlock(spinlock_t *l) {
-    // if (!spin_is_holding(l))
-    //     panic("spin_unlock");
-
-    l->cpu = 0;
-
     // Release: store 0 with .rl ordering.
     uint32_t dummy;
     __asm__ volatile("amoswap.w.rl %0, %2, (%1)\n" : "=&r"(dummy) : "r"(&l->v), "r"(0u) : "memory");
-
-    // __sync_synchronize();             // No loads/stores after this point
-    // __sync_lock_release(&lk->locked); // Essentially lk->locked = 0
-
-    // pop_off();
 }
 
-/**
- * Test-and-test-and-set acquire with polite spinning + exponential backoff.
- */
+// Optional: tiny pause/backoff (works even if Zihintpause isn't present).
+// See: https://github.com/riscv/riscv-isa-manual/blob/main/src/zihintpause.adoc
+static inline void cpu_relax(void) {
+#if defined(__riscv_zihintpause)
+    __asm__ volatile("pause");
+#else
+    __asm__ volatile("nop");
+#endif
+}
+
+// Test-and-test-and-set acquire with polite spinning + exponential backoff.
 void spin_lock(spinlock_t *l) {
-    uint32_t backoff = 1;
+    unsigned backoff = 1;
     for (;;) {
         if (spin_trylock(l))
             return;
 
+        // Contended: spin on plain loads (no AMO) until it looks free.
         while (__atomic_load_n(&l->v, __ATOMIC_RELAXED) != 0) {
-            for (uint32_t i = 0; i < backoff; ++i)
-                __asm__ volatile("nop"); /* NOTE: Pause can be used here if supported */
+            for (unsigned i = 0; i < backoff; ++i) cpu_relax();
             if (backoff < 1u << 12)
                 backoff <<= 1;
         }
+        // Try again; loop.
     }
-
-    l->cpu = mycpu();
-}
-
-/**
- * Check whether this cpu is holding the lock.
- * Interrupts must be off.
- */
-bool spin_is_holding(spinlock_t *l) {
-    int r;
-    r = (l->v && l->cpu == mycpu());
-    return r;
 }
diff --git a/kern/libkern/spinlock.h b/kern/libkern/spinlock.h
index de6091e..5561b98 100644
--- a/kern/libkern/spinlock.h
+++ b/kern/libkern/spinlock.h
@@ -1,22 +1,60 @@
 #ifndef KERNEL_Spinlock_H
 #define KERNEL_Spinlock_H
 
-#include <proc.h>
-#include <stdbool.h>
 #include <stdint.h>
 
+/** Mutual exclusion spin lock */
+struct Spinlock {
+    u32 locked; // Is the lock held?
+
+    // NOTE: Perhaps feature gate this?
+
+    // For debugging:
+    char       *name; // Name of lock.
+    struct Cpu *cpu;  // The cpu holding the lock.
+};
+
+/**
+ * Acquire the lock.
+ * Loops (spins) until the lock is acquired.
+ * Panics if the lock is already held by this cpu.
+ */
+void acquire(struct Spinlock *);
+
+/**
+ * Check whether this cpu is holding the lock.
+ * Interrupts must be off.
+ */
+int holding(struct Spinlock *);
+
+/**
+ * Initialize Spinlock
+ */
+void initlock(struct Spinlock *, char *);
+
+/**
+ * Release the lock.
+ * Panics if the lock is not held.
+ */
+void release(struct Spinlock *);
+
+/**
+ * @brief push_off/pop_off are like intr_off()/intr_on() except that they are
+ * matched: it takes two pop_off()s to undo two push_off()s.  Also, if
+ * interrupts are initially off, then push_off, pop_off leaves them off.
+ */
+void push_off(void);
+
+/** @copydoc pop_off */
+void pop_off(void);
+
 typedef struct {
     volatile uint32_t v; // 0 = unlocked, 1 = locked
-    Cpu              *cpu;
 } spinlock_t;
 
-uint32_t push_off(void);
-uint32_t pop_off(void);
-
 void spinlock_init(spinlock_t *l);
 bool spin_trylock(spinlock_t *l);
 void spin_unlock(spinlock_t *l);
-bool spin_is_holding(spinlock_t *l);
 void spin_lock(spinlock_t *l);
 
 #endif
diff --git a/kern/libkern/stddef.h b/kern/libkern/stddef.h
deleted file mode 100644
index ab8a2b5..0000000
--- a/kern/libkern/stddef.h
+++ /dev/null
@@ -1,9 +0,0 @@
-#ifndef STDDEF_H
-#define STDDEF_H
-
-#ifndef NULL
-#define NULL ((void*)0)
-#endif
-
-#endif // STDDEF_H
-
diff --git a/kern/libkern/stdio.c b/kern/libkern/stdio.c
deleted file mode 100644
index d02e59b..0000000
--- a/kern/libkern/stdio.c
+++ /dev/null
@@ -1,20 +0,0 @@
-#include <uart.h>
-#include <mini-printf.h>
-#include <stddef.h>
-
-int stdout_puts(char *s, int len, void *unused) {
-    (void)unused;
-    // Example: UART write loop
-    for (int i = 0; i < len; i++) {
-        uart_putc(s[i]);   // <-- your low-level "put char" routine
-    }
-    return len;
-}
-
-int kprintf(const char *restrict fmt, ...) {
-    va_list ap;
-    va_start(ap, fmt);
-    int ret = mini_vpprintf(stdout_puts, NULL, fmt, ap);
-    va_end(ap);
-    return ret;
-}
diff --git a/kern/libkern/stdio.h b/kern/libkern/stdio.h
deleted file mode 100644
index 2602c87..0000000
--- a/kern/libkern/stdio.h
+++ /dev/null
@@ -1,18 +0,0 @@
-#ifndef STDIO_H
-#define STDIO_H
-
-int stdout_puts(char *s, int len, void *unused);
-int kprintf(const char *restrict format, ...);
-
-// int fprintf(FILE *restrict stream, const char *restrict format, ...);
-// int dprintf(int fd, const char *restrict format, ...);
-// int sprintf(char *restrict str, const char *restrict format, ...);
-// int snprintf(char str[restrict.size], size_t size, const char *restrict format, ...);
-
-// int vprintf(const char *restrict format, va_list ap);
-// int vfprintf(FILE *restrict stream, const char *restrict format, va_list ap);
-// int vdprintf(int fd, const char *restrict format, va_list ap);
-// int vsprintf(char *restrict str, const char *restrict format, va_list ap);
-// int vsnprintf(char str[restrict.size], size_t size, const char *restrict format, va_list ap);
-
-#endif // STDIO_H
diff --git a/kern/libkern/uart.c b/kern/libkern/uart.c
index df0af5d..bb753f3 100644
--- a/kern/libkern/uart.c
+++ b/kern/libkern/uart.c
@@ -4,3 +4,7 @@
 void uart_putc(char c) {
     *UART_BASE = c;
 }
+
+void uart_puts(const char *s) {
+    while (*s) uart_putc(*s++);
+}
diff --git a/kern/libkern/uart.h b/kern/libkern/uart.h
index 8f08f1a..953c5f6 100644
--- a/kern/libkern/uart.h
+++ b/kern/libkern/uart.h
@@ -4,4 +4,7 @@
 /** Send a single character to the UART device */
 void uart_putc(char c);
 
+/** Send a **NULL TERMINATED** string to the UART device */
+void uart_puts(const char *s);
+
 #endif
diff --git a/kern/proc.h b/kern/proc.h
index ac5a46d..37ab333 100644
--- a/kern/proc.h
+++ b/kern/proc.h
@@ -1,8 +1,6 @@
-#ifndef PROC_H
-#define PROC_H
-
 #include <config.h>
 #include <riscv.h>
+#include <spinlock.h>
 #include <stdint.h>
 
 typedef enum {
@@ -15,7 +13,7 @@ typedef enum {
 } ProcessState;
 
 /** Saved registers for kernel context switches. */
-typedef struct Context {
+struct Context {
     uint64_t ra;
     uint64_t sp;
 
@@ -32,18 +30,18 @@ typedef struct Context {
     uint64_t s9;
     uint64_t s10;
     uint64_t s11;
-} Context;
+};
 
 /** Per-CPU state. */
-typedef struct cpu_t {
+struct Cpu {
     struct Process *proc;    // The process running on this cpu, or null.
     struct Context  context; // swtch() here to enter scheduler().
     int             noff;    // Depth of push_off() nesting.
     int             intena;  // Were interrupts enabled before push_off()?
-} Cpu;
+};
 
 /** Saved registers for kernel context switches. */
-typedef struct TrapFrame_t {
+typedef struct {
     /*   0 */ uint64_t kernel_satp;   // kernel page table
     /*   8 */ uint64_t kernel_sp;     // top of process's kernel stack
     /*  16 */ uint64_t kernel_trap;   // usertrap()
@@ -80,13 +78,11 @@ typedef struct TrapFrame_t {
     /* 264 */ uint64_t t4;
     /* 272 */ uint64_t t5;
     /* 280 */ uint64_t t6;
-} TrapFrame;
+} TrapFrame_t;
 
-Cpu *mycpu(void);
+struct Cpu *mycpu(void);
 
-extern Cpu cpus[NCPU];
+extern struct Cpu cpus[NCPU];
 
 /** Per-process state */
 struct Proc {};
-
-#endif
diff --git a/kern/start.c b/kern/start.c
index 15a224f..e717064 100644
--- a/kern/start.c
+++ b/kern/start.c
@@ -1,12 +1,10 @@
 #include <config.h>
 #include <kalloc.h>
 #include <memory.h>
-#include <panic.h>
 #include <proc.h>
 #include <riscv.h>
 #include <spinlock.h>
 #include <stdint.h>
-#include <stdio.h>
 #include <uart.h>
 
 /**
@@ -39,24 +37,30 @@ void start() {
     if (id == 0) {
         /* Here we will do a bunch of initialization steps */
         kalloc_init();
+        uart_puts("Hello Neptune!\n");
         spinlock_init(&sl);
-        kprintf("Hello Neptune!\n");
-
-        __sync_synchronize();
         hold = 0;
     } else {
         while (hold);
     }
 
+    // spin_lock(&sl);
+    //
+    // uart_puts("Hart number: ");
+    // uart_putc(id + '0');
+    // uart_putc('\n');
+    //
+    // spin_unlock(&sl);
+
     if (id == 0) {
         spin_lock(&sl);
-        kprintf("Core count: %d\n", max_hart);
-
-        if (max_hart == NCPU)
-            kprintf("All cores up!\n");
-        else
-            PANIC("Some cores seem to have been enumerated incorrectly!\n");
-
+        uart_puts("Core count: ");
+        uart_putc(max_hart + '0');
+        uart_putc('\n');
+        if (max_hart == NCPU) {
+            uart_puts("All cores up!");
+            uart_putc('\n');
+        }
         spin_unlock(&sl);
     }