diff --git a/Makefile b/Makefile
index 1496c7f..50ea96b 100644
--- a/Makefile
+++ b/Makefile
@@ -21,6 +21,7 @@ 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)
@@ -44,7 +45,8 @@ KERNEL_OBJ := \
 	kern/libkern/panic.o \
 	kern/libkern/memory.o \
 	kern/libkern/spinlock.o \
-	kern/libkern/mini-printf.o
+	kern/libkern/mini-printf.o \
+	kern/libkern/stdio.o
 
 kern/kernel.elf: $(KERNEL_OBJ)
 	@echo LD $@
diff --git a/kern/kalloc.c b/kern/kalloc.c
index 17d65c7..88238ee 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 f0df22e..3671b04 100644
--- a/kern/libkern/panic.c
+++ b/kern/libkern/panic.c
@@ -1,8 +1,18 @@
+#include "stdbool.h"
+#include <mini-printf.h>
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdio.h>
 #include <uart.h>
-volatile int panicked;
 
-void panic(char *s) {
-    panicked = 1;
-    uart_puts(s);
-    while (1);
+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");
 }
diff --git a/kern/libkern/panic.h b/kern/libkern/panic.h
index 77cd640..53dc283 100644
--- a/kern/libkern/panic.h
+++ b/kern/libkern/panic.h
@@ -1,6 +1,8 @@
 #ifndef KERNEL_PANIC_H
 #define KERNEL_PANIC_H
 
-void panic(char *s);
+#define PANIC(fmt, ...) __panic("[%s:%d %s] \n" fmt, __FILE__, __LINE__, __func__)
+
+void __panic(const char *restrict fmt, ...);
 
 #endif
diff --git a/kern/libkern/proc.c b/kern/libkern/proc.c
index 66bca67..bebf1a5 100644
--- a/kern/libkern/proc.c
+++ b/kern/libkern/proc.c
@@ -1,6 +1,6 @@
 #include <proc.h>
 
-struct Cpu cpus[NCPU];
+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.
  */
-struct Cpu *mycpu(void) {
-    int         id = cpuid();
-    struct Cpu *c = &cpus[id];
+Cpu *mycpu(void) {
+    int  id = cpuid();
+    Cpu *c = &cpus[id];
     return c;
 }
diff --git a/kern/libkern/spinlock.c b/kern/libkern/spinlock.c
index f3b194a..7dc6658 100644
--- a/kern/libkern/spinlock.c
+++ b/kern/libkern/spinlock.c
@@ -3,8 +3,6 @@
  * (Not mutexes as these are spinning locks).
  */
 
-// #include <lib/stdio.h>
-#include "string.h"
 #include <panic.h>
 #include <proc.h>
 #include <riscv.h>
@@ -41,86 +39,42 @@
  * On RISC-V, this emits a fence instruction.
  */
 
-/** Initialize Spinlock */
-void initlock(struct Spinlock *lk, char *name) {
-    lk->name = name;
-    lk->locked = 0;
-    lk->cpu = 0;
-}
-
-/**
- * Acquire the lock.
- * Loops (spins) until the lock is acquired.
- * Panics if the lock is already held by this cpu.
+/*
+ * 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.
  */
-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();
+uint32_t push_off(void) {
+    int  old = intr_get();
+    Cpu *cpu = mycpu();
 
     intr_off();
-    if (mycpu()->noff == 0)
-        mycpu()->intena = old;
 
-    mycpu()->noff += 1;
+    if (cpu->noff == 0)
+        cpu->intena = old;
+
+    cpu->noff += 1;
+    return cpu->noff;
 }
 
-void pop_off(void) {
-    struct Cpu *c = mycpu();
+uint32_t pop_off(void) {
+    Cpu *cpu = mycpu();
+
     if (intr_get())
-        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)
+        PANIC("pop_off - interruptible");
+
+    if (cpu->noff < 1)
+        PANIC("pop_off");
+
+    cpu->noff -= 1;
+
+    if (cpu->noff == 0 && cpu->intena)
         intr_on();
+
+    return cpu->noff;
 }
 
 void spinlock_init(spinlock_t *l) {
@@ -135,34 +89,47 @@ __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();
 }
 
-// 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.
+/**
+ * Test-and-test-and-set acquire with polite spinning + exponential backoff.
+ */
 void spin_lock(spinlock_t *l) {
-    unsigned backoff = 1;
+    uint32_t 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 (unsigned i = 0; i < backoff; ++i) cpu_relax();
+            for (uint32_t i = 0; i < backoff; ++i)
+                __asm__ volatile("nop"); /* NOTE: Pause can be used here if supported */
             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 5561b98..de6091e 100644
--- a/kern/libkern/spinlock.h
+++ b/kern/libkern/spinlock.h
@@ -1,60 +1,22 @@
 #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
new file mode 100644
index 0000000..ab8a2b5
--- /dev/null
+++ b/kern/libkern/stddef.h
@@ -0,0 +1,9 @@
+#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
new file mode 100644
index 0000000..d02e59b
--- /dev/null
+++ b/kern/libkern/stdio.c
@@ -0,0 +1,20 @@
+#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
new file mode 100644
index 0000000..2602c87
--- /dev/null
+++ b/kern/libkern/stdio.h
@@ -0,0 +1,18 @@
+#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 bb753f3..df0af5d 100644
--- a/kern/libkern/uart.c
+++ b/kern/libkern/uart.c
@@ -4,7 +4,3 @@
 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 953c5f6..8f08f1a 100644
--- a/kern/libkern/uart.h
+++ b/kern/libkern/uart.h
@@ -4,7 +4,4 @@
 /** 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 37ab333..ac5a46d 100644
--- a/kern/proc.h
+++ b/kern/proc.h
@@ -1,6 +1,8 @@
+#ifndef PROC_H
+#define PROC_H
+
 #include <config.h>
 #include <riscv.h>
-#include <spinlock.h>
 #include <stdint.h>
 
 typedef enum {
@@ -13,7 +15,7 @@ typedef enum {
 } ProcessState;
 
 /** Saved registers for kernel context switches. */
-struct Context {
+typedef struct Context {
     uint64_t ra;
     uint64_t sp;
 
@@ -30,18 +32,18 @@ struct Context {
     uint64_t s9;
     uint64_t s10;
     uint64_t s11;
-};
+} Context;
 
 /** Per-CPU state. */
-struct Cpu {
+typedef struct cpu_t {
     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 {
+typedef struct TrapFrame_t {
     /*   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()
@@ -78,11 +80,13 @@ typedef struct {
     /* 264 */ uint64_t t4;
     /* 272 */ uint64_t t5;
     /* 280 */ uint64_t t6;
-} TrapFrame_t;
+} TrapFrame;
 
-struct Cpu *mycpu(void);
+Cpu *mycpu(void);
 
-extern struct Cpu cpus[NCPU];
+extern Cpu cpus[NCPU];
 
 /** Per-process state */
 struct Proc {};
+
+#endif
diff --git a/kern/start.c b/kern/start.c
index e717064..15a224f 100644
--- a/kern/start.c
+++ b/kern/start.c
@@ -1,10 +1,12 @@
 #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>
 
 /**
@@ -37,30 +39,24 @@ 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);
-        uart_puts("Core count: ");
-        uart_putc(max_hart + '0');
-        uart_putc('\n');
-        if (max_hart == NCPU) {
-            uart_puts("All cores up!");
-            uart_putc('\n');
-        }
+        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");
+
         spin_unlock(&sl);
     }