diff --git a/arch/x86/apic.c b/arch/x86/apic.c
index e50f3beb..9ff8c97b 100644
--- a/arch/x86/apic.c
+++ b/arch/x86/apic.c
@@ -115,7 +115,9 @@ void init_apic(unsigned int cpu_id, apic_mode_t mode) {
     apic_spiv_t spiv;
     cpu_t *cpu;
 
-    BUG_ON(mode < APIC_MODE_DISABLED);
+    BUG_ON(mode == APIC_MODE_UNKNOWN);
+    if (mode <= APIC_MODE_DISABLED)
+        return;
 
     apic_base.reg = rdmsr(MSR_APIC_BASE);
 
@@ -162,7 +164,7 @@ void init_apic(unsigned int cpu_id, apic_mode_t mode) {
         vmap_4k(apic_get_base(apic_base), apic_base.base, L1_PROT_NOCACHE);
 
     spiv.reg = apic_read(APIC_SPIV);
-    spiv.vector = 0xFF;
+    spiv.vector = APIC_SPI_VECTOR;
     spiv.apic_enable = 1;
     apic_write(APIC_SPIV, spiv.reg);
 }
diff --git a/arch/x86/boot/multiboot.c b/arch/x86/boot/multiboot.c
index f1c245ea..2a727d78 100644
--- a/arch/x86/boot/multiboot.c
+++ b/arch/x86/boot/multiboot.c
@@ -26,6 +26,7 @@
 #include <acpi_ktf.h>
 #include <console.h>
 #include <drivers/fb.h>
+#include <errno.h>
 #include <mm/pmm.h>
 #include <multiboot2.h>
 
@@ -227,23 +228,12 @@ int mbi_get_avail_memory_range(unsigned index, addr_range_t *r) {
     }
 
     if (multiboot_mmap_num == 0) {
-        if (index == 0) {
-            BUG_ON(multiboot_mem_lower == 0);
-
-            r->start = 0x0;
-            r->end = _ptr(multiboot_mem_lower);
-            return 0;
-        }
-        else if (index == 1) {
-            BUG_ON(multiboot_mem_upper == 0);
-
-            r->start = _ptr(MB(1));
-            r->end = r->start + (multiboot_mem_upper);
-            return 0;
-        }
+        r->start = _ptr(index == 0 ? 0x0 : MB(1));
+        r->end = r->start + (index == 0 ? multiboot_mem_lower : multiboot_mem_upper);
+        return r->end > r->start ? 0 : -ENOENT;
     }
 
-    return -1;
+    return -ENOMEM;
 }
 
 int mbi_get_memory_range(paddr_t pa, addr_range_t *r) {
@@ -260,16 +250,12 @@ int mbi_get_memory_range(paddr_t pa, addr_range_t *r) {
     }
 
     if (multiboot_mmap_num == 0) {
-        BUG_ON(multiboot_mem_lower == 0);
-
         _start = 0x0;
         _end = multiboot_mem_lower;
 
         if (pa < _end)
             goto found;
 
-        BUG_ON(multiboot_mem_upper == 0);
-
         _start = MB(1);
         _end = _start + multiboot_mem_upper;
 
@@ -277,7 +263,7 @@ int mbi_get_memory_range(paddr_t pa, addr_range_t *r) {
             goto found;
     }
 
-    return -1;
+    return -ENOMEM;
 
 found:
     if (r) {
diff --git a/arch/x86/entry.S b/arch/x86/entry.S
index 5432716b..f3a6d368 100644
--- a/arch/x86/entry.S
+++ b/arch/x86/entry.S
@@ -115,6 +115,10 @@ ENTRY(asm_interrupt_handler_\sym)
 END_FUNC(asm_interrupt_handler_\sym)
 .endm
 
+ENTRY(asm_interrupt_handler_dummy)
+    IRET
+END_FUNC(asm_interrupt_handler_dummy)
+
 ENTRY(handle_exception)
     SAVE_ALL_REGS
 
diff --git a/arch/x86/traps.c b/arch/x86/traps.c
index a44d2609..bd76b384 100644
--- a/arch/x86/traps.c
+++ b/arch/x86/traps.c
@@ -43,6 +43,7 @@ extern void asm_interrupt_handler_uart1(void);
 extern void asm_interrupt_handler_uart2(void);
 extern void asm_interrupt_handler_keyboard(void);
 extern void asm_interrupt_handler_timer(void);
+extern void asm_interrupt_handler_dummy(void);
 
 extern void terminate_user_task(void);
 
@@ -112,6 +113,7 @@ static void init_gdt(percpu_t *percpu) {
     barrier();
     lgdt(&percpu->gdt_ptr);
 
+    write_ss(__KERN_DS);
     write_gs(GDT_PERCPU << 3);
     wrmsr(MSR_KERNEL_GS_BASE, 0x0);
 
@@ -164,6 +166,8 @@ void init_traps(const cpu_t *cpu) {
                   _ul(asm_interrupt_handler_keyboard), GATE_DPL0, GATE_PRESENT, 0);
     set_intr_gate(&percpu->idt[APIC_TIMER_IRQ], __KERN_CS,
                   _ul(asm_interrupt_handler_timer), GATE_DPL0, GATE_PRESENT, 0);
+    set_intr_gate(&percpu->idt[APIC_SPI_VECTOR], __KERN_CS,
+                  _ul(asm_interrupt_handler_dummy), GATE_DPL0, GATE_PRESENT, 0);
 
     barrier();
     lidt(&percpu->idt_ptr);
diff --git a/common/usermode.c b/common/usermode.c
index 8a0d311e..af16f035 100644
--- a/common/usermode.c
+++ b/common/usermode.c
@@ -22,6 +22,7 @@
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
+#include <errno.h>
 #include <lib.h>
 #include <mm/vmm.h>
 #include <pagetable.h>
@@ -51,9 +52,11 @@ long syscall_handler(long syscall_nr, long arg1, long arg2, long arg3, long arg4
         frame_t *frame;
 
         frame = get_free_frames(order);
+        if (!frame)
+            return -ENOMEM;
+
         if (!va)
             va = mfn_to_virt_user(frame->mfn);
-
         va = vmap_user(va, frame->mfn, order, L4_PROT_USER, L3_PROT_USER, L2_PROT_USER,
                        L1_PROT_USER);
         return _ul(va);
diff --git a/drivers/pic.c b/drivers/pic.c
index 2419c0f6..781f4f6d 100644
--- a/drivers/pic.c
+++ b/drivers/pic.c
@@ -53,18 +53,20 @@ void init_pic(void) {
     outb(PIC2_PORT_DATA, 0xFF);
 }
 
-void pic_enable_irq(pic_device_sel_t pic, uint8_t irq) {
+static void pic_toggle_irq(pic_device_sel_t pic, uint8_t irq, bool enable) {
     BUG_ON((pic != PIC1_DEVICE_SEL && pic != PIC2_DEVICE_SEL) || irq >= PIC_IRQ_MAX);
+
     uint8_t port = (pic == PIC1_DEVICE_SEL ? PIC1_PORT_DATA : PIC2_PORT_DATA);
+    uint8_t mask = 1 << irq;
     uint8_t unmasked_irqs = inb(port);
 
-    outb(port, (~(1 << irq)) & unmasked_irqs);
+    outb(port, enable ? (~mask & unmasked_irqs) : (mask | unmasked_irqs));
 }
 
-void pic_disable_irq(pic_device_sel_t pic, uint8_t irq) {
-    BUG_ON((pic != PIC1_DEVICE_SEL && pic != PIC2_DEVICE_SEL) || irq >= PIC_IRQ_MAX);
-    uint8_t port = (pic == PIC1_DEVICE_SEL ? PIC1_PORT_DATA : PIC2_PORT_DATA);
-    uint8_t unmasked_irqs = inb(port);
+void pic_enable_irq(pic_device_sel_t pic, uint8_t irq) {
+    pic_toggle_irq(pic, irq, true);
+}
 
-    outb(port, (1 << irq) | unmasked_irqs);
+void pic_disable_irq(pic_device_sel_t pic, uint8_t irq) {
+    pic_toggle_irq(pic, irq, false);
 }
diff --git a/include/arch/x86/pagetable.h b/include/arch/x86/pagetable.h
index b67ab009..f30de58a 100644
--- a/include/arch/x86/pagetable.h
+++ b/include/arch/x86/pagetable.h
@@ -269,6 +269,16 @@ static inline pdpe_t *get_pdpe(const void *va) {
     return (pdpe_t *) &(l3e->entry);
 }
 
+/* Return the virtual address of the PML4 entry mapping this virtual address
+ * in the page tables specified by a CR3 pointer.
+ * The entry is assumed to be mapped
+ */
+static inline pml4_t *get_pml4(cr3_t *cr3_ptr, const void *va) {
+    pml4_t *l4t = mfn_to_virt_kern(cr3_ptr->mfn);
+    pml4_t *l4e = l4_table_entry(l4t, va);
+    return (pml4_t *) &(l4e->entry);
+}
+
 static inline void set_pgentry(pgentry_t *e, mfn_t mfn, unsigned long flags) {
     *e = pgentry_from_mfn(mfn, flags);
     barrier();
diff --git a/include/arch/x86/traps.h b/include/arch/x86/traps.h
index 431b0604..604212ca 100644
--- a/include/arch/x86/traps.h
+++ b/include/arch/x86/traps.h
@@ -48,6 +48,8 @@
 #define KB_PORT2_IRQ    KEYBOARD_PORT2_IRQ_VECTOR
 #define APIC_TIMER_IRQ  APIC_TIMER_IRQ_VECTOR
 
+#define APIC_SPI_VECTOR 0xFF
+
 extern void init_traps(const cpu_t *cpu);
 extern void init_boot_traps(void);
 extern void init_rmode_traps(void);
diff --git a/include/lib.h b/include/lib.h
index bce6f233..d4bef3d8 100644
--- a/include/lib.h
+++ b/include/lib.h
@@ -239,6 +239,10 @@ static inline unsigned long read_gs(void) {
     return gs;
 }
 
+static inline void write_ss(unsigned long ss) {
+    asm volatile("mov %0, %%ss" ::"r"(ss));
+}
+
 static inline void write_gs(unsigned long gs) {
     asm volatile("mov %0, %%gs" ::"r"(gs));
 }
diff --git a/include/mm/regions.h b/include/mm/regions.h
index 65930a69..114758e3 100644
--- a/include/mm/regions.h
+++ b/include/mm/regions.h
@@ -85,9 +85,7 @@ extern unsigned regions_num;
 
 extern void display_memory_map(void);
 
-extern addr_range_t get_memory_range(paddr_t pa);
-extern paddr_t get_memory_range_start(paddr_t pa);
-extern paddr_t get_memory_range_end(paddr_t pa);
+extern int get_memory_range(paddr_t pa, addr_range_t *r);
 
 extern int get_avail_memory_range(unsigned index, addr_range_t *r);
 extern bool has_memory_range(paddr_t pa);
@@ -130,6 +128,7 @@ static inline bool in_kernel_section(const void *addr) {
            (addr >= _ptr(__start_data) && addr < _ptr(__end_data)) ||
            (addr >= _ptr(__start_bss) && addr < _ptr(__end_bss)) ||
            (addr >= _ptr(__start_rodata) && addr < _ptr(__end_rodata)) ||
+           (addr >= _ptr(__start_extables) && addr < _ptr(__end_extables)) ||
            (addr >= _ptr(__start_symbols) && addr < _ptr(__end_symbols));
 }
 
diff --git a/include/mm/slab.h b/include/mm/slab.h
index 2dc25f3e..bbe51059 100644
--- a/include/mm/slab.h
+++ b/include/mm/slab.h
@@ -92,26 +92,9 @@ struct meta_slab {
 
 typedef struct meta_slab meta_slab_t;
 
-static inline void increment_slab_allocs(meta_slab_t *slab) {
-    BUG_ON(slab == NULL);
-    BUG_ON((slab->slab_allocs >= (slab->slab_len / slab->slab_size)));
+/* External declarations */
 
-    slab->slab_allocs++;
-}
-
-static inline void decrement_slab_allocs(meta_slab_t *slab) {
-    BUG_ON(slab == NULL);
-    BUG_ON((slab->slab_allocs == 0));
-
-    slab->slab_allocs--;
-}
-
-static inline bool slab_is_empty(meta_slab_t *slab) {
-    BUG_ON(slab == NULL);
-    return (slab->slab_allocs == 0);
-}
-
-int init_slab(void);
+extern int init_slab(void);
 extern void *kmalloc(size_t size);
 extern void *kzalloc(size_t size);
 extern void kfree(void *ptr);
diff --git a/mm/regions.c b/mm/regions.c
index 4dbbf356..3cbac2a5 100644
--- a/mm/regions.c
+++ b/mm/regions.c
@@ -23,6 +23,7 @@
  * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 #include <console.h>
+#include <errno.h>
 #include <multiboot.h>
 
 #include <mm/regions.h>
@@ -79,27 +80,12 @@ void display_memory_map(void) {
     }
 }
 
-addr_range_t get_memory_range(paddr_t pa) {
-    addr_range_t r;
+int get_memory_range(paddr_t pa, addr_range_t *r) {
+    memset(r, 0, sizeof(*r));
 
-    memset(&r, 0, sizeof(r));
-    if (mbi_get_memory_range(pa, &r) < 0)
-        /* FIXME: e820_lower_memory_bound() */
-        panic("Unable to get memory range for: 0x%016lx", pa);
-
-    return r;
-}
-
-paddr_t get_memory_range_start(paddr_t pa) {
-    addr_range_t r = get_memory_range(pa);
-
-    return _paddr(r.start);
-}
-
-paddr_t get_memory_range_end(paddr_t pa) {
-    addr_range_t r = get_memory_range(pa);
-
-    return _paddr(r.end);
+    if (mbi_get_memory_range(pa, r) < 0)
+        return -ENOENT;
+    return 0;
 }
 
 int get_avail_memory_range(unsigned index, addr_range_t *r) {
diff --git a/mm/slab.c b/mm/slab.c
index e85925ea..ba391c14 100644
--- a/mm/slab.c
+++ b/mm/slab.c
@@ -88,7 +88,7 @@ static int initialize_slab(meta_slab_t *slab) {
 static void *slab_alloc(meta_slab_t *slab) {
     slab_t *next_free = NULL;
 
-    if (NULL == slab) {
+    if (!slab) {
         dprintk("failed, NULL slab param\n");
         return NULL;
     }
@@ -99,7 +99,9 @@ static void *slab_alloc(meta_slab_t *slab) {
     /* TODO: Below should be done in thread-safe manner */
     next_free = list_first_entry(&slab->slab_head, slab_t, list);
     list_unlink(&next_free->list);
-    increment_slab_allocs(slab);
+
+    BUG_ON(slab->slab_allocs >= (slab->slab_len / slab->slab_size));
+    slab->slab_allocs++;
     return next_free;
 }
 
@@ -113,7 +115,9 @@ static void slab_free(meta_slab_t *slab, void *ptr) {
     new_slab = (slab_t *) ptr;
     /* TODO: eventually below should be done in thread-safe manner */
     list_add_tail(&new_slab->list, &slab->slab_head);
-    decrement_slab_allocs(slab);
+
+    BUG_ON(slab->slab_allocs == 0);
+    slab->slab_allocs--;
 }
 
 meta_slab_t *slab_meta_alloc() {
@@ -271,6 +275,10 @@ void *kzalloc(size_t size) {
     return ptr;
 }
 
+static inline bool slab_is_empty(meta_slab_t *slab) {
+    return slab->slab_allocs == 0;
+}
+
 /*
  * Loop through all the orders and check where does this memory belong
  * Then link it back into free list. Not a O(1) of implementation but we're looking for
@@ -285,6 +293,7 @@ static void ktf_free(void *ptr) {
     for (alloc_order = SLAB_ORDER_16; alloc_order < SLAB_ORDER_MAX; alloc_order++) {
         /* Go through list of meta_slab_t and try to allocate a free slab */
         list_for_each_entry (slab, &meta_slab_list[alloc_order], list) {
+            BUG_ON(!slab);
             if ((_ul(ptr) >= (_ul(slab->slab_base))) &&
                 (_ul(ptr) < (_ul(slab->slab_base) + _ul(slab->slab_len)))) {
                 slab_free(slab, ptr);
diff --git a/smp/mptables.c b/smp/mptables.c
index 1d44ad8e..65ac1bcd 100644
--- a/smp/mptables.c
+++ b/smp/mptables.c
@@ -84,7 +84,11 @@ static mpf_t *get_mpf_addr(void) {
     if (ptr)
         return ptr;
 
-    sysm_addr = kmap_4k(paddr_to_mfn(get_memory_range_end(KB(512)) - KB(1)), L1_PROT_RO);
+    addr_range_t range;
+    if (get_memory_range(KB(512), &range) < 0)
+        return NULL;
+
+    sysm_addr = kmap_4k(paddr_to_mfn(_paddr(range.end) - KB(1)), L1_PROT_RO);
     ptr = find_mpf(sysm_addr, sysm_addr + KB(1));
     if (ptr)
         return ptr;
diff --git a/smp/smp.c b/smp/smp.c
index 7cd96643..fc0ef959 100644
--- a/smp/smp.c
+++ b/smp/smp.c
@@ -60,6 +60,7 @@ void __noreturn ap_startup(void) {
     smp_wmb();
 
     init_timers(cpu);
+    sti();
 
     if (opt_fpu)
         enable_fpu();