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in start_kernel, bootldr gives us an entry point to the kernel (start of loaded .text):
copy first 15 words (vectors and their displacement constants) of kern text segment to
the beginning of physical memory.
this beautiful trick allows us to load kernel at anywhere we want and to let bootldr
be at 0x00000000 (yes, we are rewriting ourselfes, part of bootldr :).

/*-
 * Copyright (c) 2005, Kohsuke Ohtani
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. Neither the name of the author nor the names of any co-contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 */

/*
 * head.S - low level platform support
 *
 * This file contains the code from crt0.S which is released under
 * public domain by Jeff Frohwein.
 */

/*
 * I know that I'm a bit slacky, but this is the memory map for cats.
 * GXemul> device list
 * 0:                       ram @ 0x00041000000, len = 0x40000 (DYNTRANS R+W)
 * 1:                footbridge @ 0x00042000000, len = 0x400
 * 2:                       ram @ 0x00050000000, len = 0x10000 (DYNTRANS R+W)
 * 3:            footbridge_isa @ 0x00079000000, len = 0x8
 * 4:            footbridge_pci @ 0x0007b000000, len = 0x1000000
 * 5:                      8259 @ 0x0007c000020, len = 0x2
 * 6:                      8253 @ 0x0007c000040, len = 0x4
 * 7:                     pckbc @ 0x0007c000060, len = 0x10
 * 8:                    pccmos @ 0x0007c000070, len = 0x4
 * 9:          8259 [secondary] @ 0x0007c0000a0, len = 0x2
 * 10:            ns16550 [tty1] @ 0x0007c0002f8, len = 0x8
 * 11:                 lpt [lpt] @ 0x0007c000378, len = 0x3
 * 12:                  vga_ctrl @ 0x0007c0003c0, len = 0x20
 * 13:            ns16550 [tty0] @ 0x0007c0003f8, len = 0x8
 * 14: dec21143 [10:20:30:00:00:10] @ 0x0007c010000, len = 0x100
 * 15:                       ram @ 0x00080000000, len = 0x1000 (DYNTRANS R+W)
 * 16:              ram [mirror] @ 0x00080010000, len = 0x100 ()
 * 17:                   vga_gfx @ 0x000800a0000, len = 0x18000 ()
 * 18:             vga_charcells @ 0x000800b8000, len = 0x8000 (DYNTRANS R+W)
 * 19:              ram [mirror] @ 0x000c0000000, len = 0x20000000 (DYNTRANS R+W)
 * 20:              ram [mirror] @ 0x000f0000000, len = 0x1000000 (DYNTRANS R+W)
 * 21:                  fb [VGA] @ 0x01c00000000, len = 0xbb800 (DYNTRANS R+W)
 * 22:                  mc146818 @ 0x01d00000000, len = 0x2
 *
 * Firmware loads us at 0xf0000000 with MMU on and TTB @0x006f8000.
 */

#include <conf/config.h>
#include <platform.h>

#define ENTRY(x) .global x; .align; x##:

	.section ".text","ax"
	.code 32
/*
 * Kernel start point
 */
ENTRY(boot_entry)
	b	start_vector

stack_end:	.word	BOOT_STACK+0xf00
ENTRY(ebsabootaddr)
	.word	0

start_vector:
	mov	r1, #0xd3		/* Enter SVC mode, Disable IRQ,FIQ */
	msr	cpsr_c, r1

	mrc p15, 0, r1, c1, c0
	bic r1, r1, #1	/* disable MMU */
	mcr p15, 0, r1, c1, c0

	/* setup bootldr stack */
	ldr	sp, stack_end

	/* grab firmware bootinfo */
	ldr	r1, =ebsabootaddr
	str r0, [r1]

	/* initialize boot console */
	bl	bootcons_init

	ldr	r0, =loader_main
	bx	r0		/* Change to ARM mode */

/*
 * Start kernel
 */
ENTRY(start_kernel)
	/*
	 * XXX Copy kernel vectors and their displacement constants
	 * XXX to the beginning of physical memory.
	 * NOTE: it shouldn't hurt if we replace first 15 words of ourselfes.
	 */
	mov r2, #0	/* counter */
	mov r3, #15	/* num. of current insn */
	mov r4, #0	/* save addr */
	mov r5, r0	/* load addr */
	mov r6, #0	/* data */

copy_insn:	
	ldr r6, [r5], #4	/* load insn */
	str r6, [r4], #4	/* save insn */
	add r2, r2, #1		/* increment counter */
	tst r2, r3
	bne copy_insn

	bx	r0

		.section .tail,"ax"
dummy:
	.byte	0xff

.end