2 * U-boot - x86 Startup Code
4 * (C) Copyright 2008-2011
5 * Graeme Russ, <graeme.russ@gmail.com>
7 * (C) Copyright 2002,2003
8 * Daniel Engström, Omicron Ceti AB, <daniel@omicron.se>
10 * SPDX-License-Identifier: GPL-2.0+
13 #include <asm/global_data.h>
14 #include <asm/processor-flags.h>
16 #define BOOT_SEG 0xffff0000 /* linear segment of boot code */
17 #define a32 .byte 0x67;
18 #define o32 .byte 0x66;
20 .section .start16, "ax"
27 /* Set the Cold Boot / Hard Reset flag */
28 movl $GD_FLG_COLD_BOOT, %ebx
31 movl %eax, %cr3 /* Invalidate TLB */
33 /* Turn off cache (this might require a 486-class CPU) */
35 orl $(X86_CR0_NW | X86_CR0_CD), %eax
39 /* load the temporary Global Descriptor Table */
43 /* Now, we enter protected mode */
48 /* Flush the prefetch queue */
52 /* Finally restore BIST and jump to the 32bit initialization code */
53 movw $code32start, %ax
58 /* 48-bit far pointer */
60 .long _start /* offset */
61 .word 0x10 /* segment */
68 * The following Global Descriptor Table is just enough to get us into
69 * 'Flat Protected Mode' - It will be discarded as soon as the final
70 * GDT is setup in a safe location in RAM
73 .word 0x1f /* limit (31 bytes = 4 GDT entries - 1) */
74 .long BOOT_SEG + gdt /* base */
76 /* Some CPUs are picky about GDT alignment... */
86 * 0x18 32bit data/stack
88 /* The NULL Desciptor - Mandatory */
89 .word 0x0000 /* limit_low */
90 .word 0x0000 /* base_low */
91 .byte 0x00 /* base_middle */
92 .byte 0x00 /* access */
93 .byte 0x00 /* flags + limit_high */
94 .byte 0x00 /* base_high */
96 /* Unused Desciptor - (matches Linux) */
97 .word 0x0000 /* limit_low */
98 .word 0x0000 /* base_low */
99 .byte 0x00 /* base_middle */
100 .byte 0x00 /* access */
101 .byte 0x00 /* flags + limit_high */
102 .byte 0x00 /* base_high */
105 * The Code Segment Descriptor:
106 * - Base = 0x00000000
108 * - Access = Present, Ring 0, Exec (Code), Readable
109 * - Flags = 4kB Granularity, 32-bit
111 .word 0xffff /* limit_low */
112 .word 0x0000 /* base_low */
113 .byte 0x00 /* base_middle */
114 .byte 0x9b /* access */
115 .byte 0xcf /* flags + limit_high */
116 .byte 0x00 /* base_high */
119 * The Data Segment Descriptor:
120 * - Base = 0x00000000
122 * - Access = Present, Ring 0, Non-Exec (Data), Writable
123 * - Flags = 4kB Granularity, 32-bit
125 .word 0xffff /* limit_low */
126 .word 0x0000 /* base_low */
127 .byte 0x00 /* base_middle */
128 .byte 0x93 /* access */
129 .byte 0xcf /* flags + limit_high */
130 .byte 0x00 /* base_high */