diff --git a/boot.asm b/boot.asm
index bea5f84..874ed1f 100644
--- a/boot.asm
+++ b/boot.asm
@@ -11,7 +11,7 @@ boot:
 	mov [disk],dl
 
 	mov ah, 0x2    ;read sectors
-	mov al, 6      ;sectors to read
+	mov al, 64     ;sectors to read
 	mov ch, 0      ;cylinder idx
 	mov dh, 0      ;head idx
 	mov cl, 2      ;sector idx
@@ -30,22 +30,37 @@ boot:
 	mov gs, ax
 	mov ss, ax
 	jmp CODE_SEG:boot2
+
+; access(8)
+;    P:present(1) DPL:descriptor_privilege_level(2)
+;    S:descriptor_type(1) E:executable(1) DC:direction/conforming(1)
+;    RW:readable/writable(1) A:accessed(1)
+; flags(4)
+;    G:granularity(1) DB:size(1) L:long_mode_code(1) Reserved(1)
+
 gdt_start:
-	dq 0x0
+	dq 0x0000000000000000 ; null descriptor
+
 gdt_code:
-	dw 0xFFFF
-	dw 0x0
-	db 0x0
-	db 10011010b
-	db 11001111b
-	db 0x0
+	dw 0xFFFF    ; limit_0_15(16)
+	dw 0x0000    ; base_0_15(16)
+	db 0x00      ; base_16_23(8)
+	db 10011010b ; P(1)=1=present DPL(2)=00=ring0 S(1)=1=non_system E(1)=1=code
+	             ; DC(1)=0=same_ring RW(1)=1=readable A(1)=0=not_accessed
+	db 11001111b ; G(1)=1=page, DB(1)=1=32b, L(1)=0=non_64b Reserved(1)=0=_
+	             ; limit_16_19(4)=1111
+	db 0x00      ; base_24_31(8)
+
 gdt_data:
-	dw 0xFFFF
-	dw 0x0
-	db 0x0
-	db 10010010b
-	db 11001111b
-	db 0x0
+	dw 0xFFFF    ; limit_0_15(16)
+	dw 0x0000    ; base_0_15(16)
+	db 0x00      ; base_16_23(8)
+	db 10010010b ; P(1)=1=present DPL(2)=00=ring0 S(1)=1=non_system E(1)=0=data
+	             ; DC(1)=0=grows_up RW(1)=1=rw A(1)=0=not_accessed
+	db 11001111b ; G(1)=1=page DB(1)=1=32b_sp L(1)=0=_ Reserved(1)=0=_
+	             ; limit_16_19(4)=1111
+	db 0x00      ; base_24_31(8)
+
 gdt_end:
 gdt_pointer:
 	dw gdt_end - gdt_start
@@ -82,4 +97,4 @@ section .bss
 align 4
 kernel_stack_bottom: equ $
 	resb 16384 ; 16 KB
-kernel_stack_top:
+kernel_stack_top:
\ No newline at end of file
diff --git a/do.sh b/do.sh
new file mode 100644
index 0000000..e655f26
--- /dev/null
+++ b/do.sh
@@ -0,0 +1,29 @@
+#!/bin/sh
+set -e
+
+# TODO: Check the compiler does x86
+
+if [ "$1" = "build" ]; then
+    shift
+
+    printf "Assembling bootloader...\n"
+    nasm -f elf32 boot.asm -o boot.o
+
+    printf "Compiling...\n"
+    gcc \
+        -save-temps \
+        -std=c99 -m32 \
+        -fno-pic \
+        -mgeneral-regs-only \
+        -ffreestanding -nostdlib \
+        -Wall -Wextra -Wpedantic \
+        kernel.c boot.o \
+        -o kernel.bin \
+        -T linker.ld
+fi
+
+if [ "$1" = "boot" ]; then
+    printf "Booting...\n\n"
+    MACHINE="-machine pc -cpu 486"
+    qemu-system-i386 $MACHINE -net none -serial stdio -drive file=kernel.bin,index=0,if=floppy,format=raw
+fi
\ No newline at end of file
diff --git a/kernel.c b/kernel.c
index 8d1bdfd..1d66482 100644
--- a/kernel.c
+++ b/kernel.c
@@ -1,13 +1,488 @@
+#include <stdbool.h>
+#include <stddef.h>
 #include <stdint.h>
 
+bool terminate = false;
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+static inline void outb(uint16_t port, uint8_t val)
+{
+    __asm__ volatile ("outb %b0, %w1" : : "a"(val), "Nd"(port) : "memory");
+}
+
+static inline uint8_t inb(uint16_t port)
+{
+    uint8_t ret;
+    __asm__ volatile ("inb %w1, %b0" : "=a"(ret) : "Nd"(port) : "memory");
+    return ret;
+}
+
+static inline void io_wait(void)
+{
+    outb(0x80, 0);
+}
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+#define PORT 0x3f8          // COM1
+
+void init_serial(void)
+{
+    // Enable "data available" interrupt.
+    outb(PORT + 1, 0x01);
+}
+
+// Use only when there _is_ something to read.
+char read_serial()
+{
+    return inb(PORT);
+}
+
+int is_transmit_empty()
+{
+    return inb(PORT + 5) & 0x20;
+}
+
+void write_serial(char a)
+{
+    while (is_transmit_empty() == 0);
+    outb(PORT, a);
+}
+
+#undef PORT
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+/*
+ * Convert numbers to hexadecimal chars.
+ */
+
+char nibble_to_hex(int n)
+{
+    if   (n>=0 && n<10) return (n+48);
+    else if (n>=10 && n<16) return (n+55+32);
+    else return '?';
+}
+
+void int_to_hex(char *hex, uint64_t a, size_t size)
+{
+    int i = 0;
+    int divisor = size << 3;
+    while (divisor != 0) {
+        divisor -= 4;
+        hex[i] = nibble_to_hex((a >> divisor) & 0xF);
+        i += 1;
+    }
+}
+
+void u8_to_hex (char *s, uint8_t  a) { int_to_hex(s, a, 1); }
+void u16_to_hex(char *s, uint16_t a) { int_to_hex(s, a, 2); }
+void u32_to_hex(char *s, uint32_t a) { int_to_hex(s, a, 4); }
+void u64_to_hex(char *s, uint64_t a) { int_to_hex(s, a, 8); }
+
+/*
+ * Logging functions.
+ */
+
+// Use serial for log output.
+#define PUTCHAR write_serial
+
+// Sends buffer of a given size to the log.
+void klog(char *buff, int size)
+{
+    for (int i=0; i<size; ++i) PUTCHAR(buff[i]);
+}
+
+// Like klog but also sends a newline character.
+void klogl(char *buff, int size)
+{
+    klog(buff, size); PUTCHAR('\n');
+}
+
+// Sends a NUL-terminated string to the log (not including the NUL).
+void klogs(const char *str)
+{
+    while(*str) PUTCHAR(*str++);
+}
+
+#undef PUTCHAR
+
+// Functions that send integers to the log, in hex (without 0x prefix).
+#define B hexbuff
+#define D char B[16] = {0}
+void klog_u8 (uint8_t  a) { D; u8_to_hex (B, a); klog(B, 2) ; }
+void klog_u16(uint16_t a) { D; u16_to_hex(B, a); klog(B, 4) ; }
+void klog_u32(uint32_t a) { D; u32_to_hex(B, a); klog(B, 8) ; }
+void klog_u64(uint64_t a) { D; u64_to_hex(B, a); klog(B, 16); }
+
+// These send a newline character too.
+void klogl_u8 (uint8_t  a) { D; u8_to_hex (B, a); klogl(B, 2) ; }
+void klogl_u16(uint16_t a) { D; u16_to_hex(B, a); klogl(B, 4) ; }
+void klogl_u32(uint32_t a) { D; u32_to_hex(B, a); klogl(B, 8) ; }
+void klogl_u64(uint64_t a) { D; u64_to_hex(B, a); klogl(B, 16); }
+#undef D
+#undef B
+
+void demo_klog_functions(void)
+{
+    klogs("\nCheck; one, two, one, two.\n");
+    klogl_u8(0x01);
+    klogl_u16(0x2345);
+    klogl_u32(0x6789abcd);
+    klogl_u64(0xef0123456789abcd);
+    klog_u8(0x01);
+    klog_u16(0x2345);
+    klog_u32(0x6789abcd);
+    klog_u64(0xef0123456789abcd);
+    klogs("\n0123456789abcdef0123456789abcd\n");
+}
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+#define PIC1            0x20     /* IO base address for master PIC */
+#define PIC2            0xA0     /* IO base address for slave PIC */
+#define PIC1_COMMAND    PIC1
+#define PIC1_DATA       (PIC1+1)
+#define PIC2_COMMAND    PIC2
+#define PIC2_DATA       (PIC2+1)
+
+#define ICW1_ICW4       0x01     /* Indicates that ICW4 will be present */
+#define ICW1_INIT       0x10     /* Initialization - required! */
+
+#define ICW4_8086       0x01     /* 8086/88 (MCS-80/85) mode */
+
+void pic_remap(int offset)
+{
+    uint8_t a1, a2;
+    // save masks
+    a1 = inb(PIC1_DATA); a2 = inb(PIC2_DATA);
+    // starts the initialization sequence (in cascade mode)
+    outb(PIC1_COMMAND, ICW1_INIT | ICW1_ICW4); io_wait();
+    outb(PIC2_COMMAND, ICW1_INIT | ICW1_ICW4); io_wait();
+    // ICW2: Master PIC vector offset
+    outb(PIC1_DATA, offset); io_wait();
+    // ICW2: Slave PIC vector offset
+    outb(PIC2_DATA, offset+8); io_wait();
+    // ICW3: tell Master PIC that there is a slave PIC at IRQ2 (0000 0100)
+    outb(PIC1_DATA, 4); io_wait();
+    // ICW3: tell Slave PIC its cascade identity (0000 0010)
+    outb(PIC2_DATA, 2); io_wait();
+    // ICW4: have the PICs use 8086 mode (and not 8080 mode)
+    outb(PIC1_DATA, ICW4_8086); io_wait();
+    outb(PIC2_DATA, ICW4_8086); io_wait();
+    // restore saved masks.
+    outb(PIC1_DATA, a1); outb(PIC2_DATA, a2);
+}
+
+void pic_end_of_irq(int irq)
+{
+    if (irq >= 8) outb(PIC2_COMMAND, 0x20);
+    outb(PIC1_COMMAND, 0x20);
+}
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+struct gdt_descriptor
+{
+    uint16_t limit;
+    uint32_t base;
+} __attribute__((packed));
+
+void sgdt(struct gdt_descriptor *gdtd)
+{
+    __asm__ volatile ("sgdt %0" : : "m"(*gdtd) : "memory");
+}
+
+void klog_info_about_gdt(void)
+{
+    struct gdt_descriptor gdtd;
+    sgdt(&gdtd);
+
+    klogs("**\n GDT info:\n");
+    klogl_u32(gdtd.base);
+    klogl_u16(gdtd.limit);
+
+    // For each (8 bytes long) entry in the table...
+    for (uint32_t b = gdtd.base; b < gdtd.base + gdtd.limit; b += 8) {
+        // ... log each byte, in hex.
+        for (uint32_t i = b; i < b + 8; ++i) {
+            klog_u8(*(uint8_t*)i);
+        }
+        klogs("\n");
+    }
+}
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+struct idt_gate_descriptor
+{
+    uint16_t offset_1;        // offset bits 0..15
+    uint16_t selector;        // a code segment selector in GDT or LDT
+    uint8_t  zero;            // unused, set to 0
+    uint8_t  type_attributes; // gate type, dpl, and p fields
+    uint16_t offset_2;        // offset bits 16..31
+} __attribute__((packed));
+
+#define PIC_REMAP_OFFSET 0x20
+
+// The IDT begins with gates for the exceptions; we remap the PIC to start at
+// some point after them. For the PIC IRQs that's 16 gates more. If we define
+// our software interrupts... we'll have to make room.
+#define IDT_NUM_GATES (PIC_REMAP_OFFSET + 16)
+
+__attribute__((aligned(0x10)))
+struct idt_gate_descriptor idt[IDT_NUM_GATES] = {0};
+
+static inline void lidt(void)
+{
+    struct {
+        uint16_t limit;
+        void     *base;
+    } __attribute__((packed)) src = { IDT_NUM_GATES * 8 - 1, idt };
+    __asm__ ("lidt %0" : : "m"(src) );
+}
+
+struct interrupt_frame; // Not defined yet, but we need a pointer to it.
+
+#define GATE_F          void (*f)(struct interrupt_frame *)
+#define GATE_WITH_ERR_F void (*f)(struct interrupt_frame *, uint32_t)
+#define FOR_INTR        0x8e /* Present, DPL 0, 32b interrupt gate */
+#define FOR_TRAP        0x8f /* Present, DPL 0, 32b trap gate */
+
+struct idt_gate_descriptor gate(uint8_t type_attributes, uint32_t f)
+{
+    return (struct idt_gate_descriptor){
+        .type_attributes = type_attributes,
+        .selector = 0x0008, // Our code segment in GDT, ring0 requested.
+        .offset_1 = (uint16_t)(f & 0xffff),
+        .offset_2 = (uint16_t)((f>>16) & 0xffff),
+        .zero = 0x00
+    };
+}
+
+void klog_idt_gate_update(int n)
+{
+    klogs(" "); klog_u8(n);
+}
+
+void set_irq_handler(size_t irq, GATE_F)
+{
+    idt[irq + PIC_REMAP_OFFSET] = gate(FOR_INTR, (uint32_t)f);
+    klog_idt_gate_update(irq);
+}
+
+void set_exception_with_err_handler(size_t number, GATE_WITH_ERR_F)
+{
+    idt[number] = gate(FOR_TRAP, (uint32_t)f);
+    klog_idt_gate_update(number);
+}
+
+void set_exception_handler(size_t number, GATE_F)
+{
+    idt[number] = gate(FOR_TRAP, (uint32_t)f);
+    klog_idt_gate_update(number);
+}
+
+#undef FOR_TRAP
+#undef FOR_INTR
+#undef GATE_WITH_ERR_F
+#undef GATE_F
+
+void pic_clear_irq(uint8_t irq)
+{
+    uint16_t port;
+    uint8_t value;
+    if(irq < 8) {
+        port = PIC1_DATA;
+    } else {
+        port = PIC2_DATA; irq -= 8;
+    }
+    value = inb(port) & ~(1 << irq);
+    outb(port, value);
+}
+
+void pic_mask_all_irqs(void)
+{
+    outb(PIC1_DATA, 0xff); outb(PIC2_DATA, 0xff);
+}
+
+static inline void sti(void)
+{
+    __asm__ volatile ("sti");
+}
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+void panic(void)
+{
+    klogs("Panic: halt.");
+    __asm__ volatile ("cli; hlt;");
+}
+
+__attribute__((interrupt))
+void exception_with_err_handler(struct interrupt_frame *frame, uint32_t error_code)
+{
+    (void)frame;
+    klogs("\n(generic handler) Exception with err "); klogl_u32(error_code);
+    panic();
+}
+
+__attribute__((interrupt))
+void exception_handler(struct interrupt_frame *frame)
+{
+    (void)frame;
+    klogs("\n(generic handler) Exception\n");
+    panic();
+}
+
+__attribute__((interrupt))
+void exception_de(struct interrupt_frame *frame)
+{
+    (void)frame;
+    klogs("\nException: Divide Error.\n");
+    panic();
+}
+
+__attribute__((interrupt))
+void exception_gp(struct interrupt_frame *frame, uint32_t error_code)
+{
+    (void)frame;
+    klogs("\nException: General Protection; error code "); klogl_u32(error_code);
+    panic();
+}
+
+__attribute__((interrupt))
+void irq1_handler(struct interrupt_frame *frame)
+{
+    (void)frame;
+    uint8_t k = inb(0x60);
+    klogs("Key: "); klogl_u8(k);
+    if (k ==  /* q */ 0x90) terminate = true;
+    pic_end_of_irq(1);
+}
+
+__attribute__((interrupt))
+void irq4_handler(struct interrupt_frame *frame)
+{
+    (void)frame;
+    uint8_t c = read_serial();
+    write_serial(c); // Echo.
+
+    // Testing stuff when receiving some chars.
+    if (c == 'q') terminate = true;
+    if (c == '0')
+        __asm__ volatile ("mov $0, %bl; div %bl"); // Division by zero.
+    if (c == 'f')
+        __asm__ volatile ("int $13;");             // General Protection fault.
+
+    pic_end_of_irq(4);
+}
+
+void init_interrupts(void)
+{
+    klogs("**\n  Init interrupts\n");
+
+    klogs("Preparing PIC.\n");
+    pic_mask_all_irqs();
+    pic_remap(PIC_REMAP_OFFSET);
+
+    klogs("Init exceptions.\n");
+#define A(n) set_exception_handler(n, &exception_handler)
+#define B(n) set_exception_with_err_handler(n, &exception_with_err_handler)
+    set_exception_handler(0, &exception_de);
+    A(1); A(2); A(3); A(4); A(5); A(6); A(7); B(8); A(9);
+    B(10); B(11); B(12);
+    set_exception_with_err_handler(13, &exception_gp);
+    B(14); A(15); A(16); B(17); A(18); A(19);
+    A(20); B(21); A(28); B(29); B(30);
+#undef B
+#undef A
+
+    klogs("\nInit IRQs.\n");
+
+    // Assign handlers to hardware interrupts.
+    // Use the IRQ number here (the offset is applied elsewhere).
+    set_irq_handler(1, &irq1_handler);  // Keyboard.
+    set_irq_handler(4, &irq4_handler);  // Serial.
+    klogs("\n");
+
+    // Make our IDT the active one.
+    lidt();
+
+    // Unmask hardware interrupts that we're ready to handle.
+    pic_clear_irq(1);
+    pic_clear_irq(4);
+
+    // Start accepting IRQs (that is... from the PIC).
+    sti();
+}
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
+//////////////////////////////////////////////////////////////////////
+
 void kmain(void)
 {
-    const uint16_t color = 0x0F00;
-    const char *hello = "Hello C world!";
-    volatile uint16_t *vga = (volatile uint16_t *)0xb8000;
+    init_serial();
+    klogs("Hello from kmain!\n");
+    demo_klog_functions();
 
-    for (int i = 0; i < 16; ++i)
-    {
+    init_interrupts();
+
+    const uint16_t color = 0x7e00;
+    volatile uint16_t *vga = (volatile uint16_t *)0xb8000;
+    const char *hello = "Hello! Please, see serial output.";
+
+    for (int i = 0; hello[i] != 0; ++i) {
         vga[i + 80] = color | (uint16_t)hello[i];
     }
-}
+
+    klog_info_about_gdt();
+
+    klogs("\n\n**\n  Ready!\n");
+    klogs(
+      "(accepting input from keyboard)\n"
+      " type q to return from kmain\n"
+      " key info sent to serial\n"
+      "(accepting input from serial, with echo)\n"
+      " send q to return from kmain\n"
+      " send f to invoke GPF\n"
+      " send 0 to trigger divide by zero\n"
+    );
+    while (!terminate) {
+        __asm__ volatile ("hlt");
+    }
+
+    klogs("\nkmain returning now... o/\n");
+}
\ No newline at end of file