start16.S
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/*
* U-Boot - x86 Startup Code
*
* (C) Copyright 2008-2011
* Graeme Russ, <graeme.russ@gmail.com>
*
* (C) Copyright 2002,2003
* Daniel Engström, Omicron Ceti AB, <daniel@omicron.se>
*
* SPDX-License-Identifier: GPL-2.0+
*/
#include <asm/global_data.h>
#include <asm/processor-flags.h>
#define BOOT_SEG 0xffff0000 /* linear segment of boot code */
#define a32 .byte 0x67;
#define o32 .byte 0x66;
.section .start16, "ax"
.code16
.globl start16
start16:
/* Save BIST */
movl %eax, %ecx
/* Set the Cold Boot / Hard Reset flag */
movl $GD_FLG_COLD_BOOT, %ebx
xorl %eax, %eax
movl %eax, %cr3 /* Invalidate TLB */
/* Turn off cache (this might require a 486-class CPU) */
movl %cr0, %eax
orl $(X86_CR0_NW | X86_CR0_CD), %eax
movl %eax, %cr0
wbinvd
/* load the temporary Global Descriptor Table */
o32 cs lidt idt_ptr
o32 cs lgdt gdt_ptr
/* Now, we enter protected mode */
movl %cr0, %eax
orl $X86_CR0_PE, %eax
movl %eax, %cr0
/* Flush the prefetch queue */
jmp ff
ff:
/* Finally restore BIST and jump to the 32-bit initialization code */
movw $code32start, %ax
movw %ax, %bp
movl %ecx, %eax
o32 cs ljmp *(%bp)
/* 48-bit far pointer */
code32start:
.long _start /* offset */
.word 0x10 /* segment */
idt_ptr:
.word 0 /* limit */
.long 0 /* base */
/*
* The following Global Descriptor Table is just enough to get us into
* 'Flat Protected Mode' - It will be discarded as soon as the final
* GDT is setup in a safe location in RAM
*/
gdt_ptr:
.word 0x1f /* limit (31 bytes = 4 GDT entries - 1) */
.long BOOT_SEG + gdt_rom /* base */
/* Some CPUs are picky about GDT alignment... */
.align 16
.globl gdt_rom
gdt_rom:
/*
* The GDT table ...
*
* Selector Type
* 0x00 NULL
* 0x08 Unused
* 0x10 32bit code
* 0x18 32bit data/stack
*/
/* The NULL Desciptor - Mandatory */
.word 0x0000 /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x00 /* access */
.byte 0x00 /* flags + limit_high */
.byte 0x00 /* base_high */
/* Unused Desciptor - (matches Linux) */
.word 0x0000 /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x00 /* access */
.byte 0x00 /* flags + limit_high */
.byte 0x00 /* base_high */
/*
* The Code Segment Descriptor:
* - Base = 0x00000000
* - Size = 4GB
* - Access = Present, Ring 0, Exec (Code), Readable
* - Flags = 4kB Granularity, 32-bit
*/
.word 0xffff /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x9b /* access */
.byte 0xcf /* flags + limit_high */
.byte 0x00 /* base_high */
/*
* The Data Segment Descriptor:
* - Base = 0x00000000
* - Size = 4GB
* - Access = Present, Ring 0, Non-Exec (Data), Writable
* - Flags = 4kB Granularity, 32-bit
*/
.word 0xffff /* limit_low */
.word 0x0000 /* base_low */
.byte 0x00 /* base_middle */
.byte 0x93 /* access */
.byte 0xcf /* flags + limit_high */
.byte 0x00 /* base_high */