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+/*
+ * Copyright (c) 2017 Richard Braun.
+ * Copyright (c) 2017 Jerko Lenstra.
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a
+ * copy of this software and associated documentation files (the "Software"),
+ * to deal in the Software without restriction, including without limitation
+ * the rights to use, copy, modify, merge, publish, distribute, sublicense,
+ * and/or sell copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+ * DEALINGS IN THE SOFTWARE.
+ */
+
+#include "boot.h"
+
+/*
+ * These are values used in the OS image header, as defined by the multiboot
+ * specification.
+ *
+ * See https://www.gnu.org/software/grub/manual/multiboot/multiboot.html.
+ */
+#define BOOT_HDR_MAGIC  0x1BADB002
+#define BOOT_HDR_CHECK  0x2BADB002
+#define BOOT_HDR_FLAGS  0x0
+
+/*
+ * The .section directive tells the assembler which section the following
+ * instructions should go into.
+ *
+ * The "a" flag makes the section allocatable, meaning memory will be
+ * allocated for that section at load time.
+ *
+ * See https://sourceware.org/binutils/docs-2.29/as/Section.html#Section.
+ */
+.section .hdr, "a"
+
+/* Generate code for i386 */
+.code32
+
+/*
+ * The .int directive is used to emit verbatim machine words. Here, the
+ * third word is the checksum of the first two, defined as "a 32-bit
+ * unsigned value which, when added to the other magic fields (i.e.
+ * ‘magic’ and ‘flags’), must have a 32-bit unsigned sum of zero".
+ * Intuitively, adding the two first words and making the result negative
+ * gives a value that, when added to the other fields, gives 0, despite
+ * the word being unsigned. This trick works because values use two's
+ * complement representation.
+ *
+ * See https://en.wikipedia.org/wiki/Two%27s_complement.
+ */
+.int BOOT_HDR_MAGIC
+.int BOOT_HDR_FLAGS
+.int -(BOOT_HDR_FLAGS + BOOT_HDR_MAGIC)
+
+/*
+ * Put the following instructions into the .text section, which is
+ * allocatable and executable.
+ */
+.section .text, "ax"
+
+/*
+ * This symbol is the entry point, i.e. the first instruction that should
+ * be run when control is passed to the kernel. The address of this symbol
+ * is what the following command returns :
+ *   readelf -aW x1 | grep "Entry point"
+ *
+ * The .global directive tells the assembler to make the symbol global,
+ * i.e. to make it visible to other compilation units.
+ *
+ * When this code is run, the machine state should comply with what the
+ * multiboot specification defines.
+ */
+.global boot_start
+boot_start:
+  cmp $BOOT_HDR_CHECK, %eax     /* Compare EAX against the expected value */
+  jne .                         /* If not equal, jump to the current address.
+                                   This is an infinite loop. */
+  mov $boot_stack, %esp         /* Set up a stack */
+  add $BOOT_STACK_SIZE, %esp    /* On x86, stacks grow downwards, so start
+                                   at the top */
+  jmp main                      /* Jump to the C main function */
+
+loop:
+  hlt                           /* Never reached, for safety */
+  jmp loop