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1.1 root 1: /*
2: Copyright (C) 2000, Entity Cyber, Inc.
3:
4: Authors: Gary Byers ([email protected])
5: Marty Connor ([email protected])
6:
7: This software may be used and distributed according to the terms
8: of the GNU Public License (GPL), incorporated herein by reference.
9:
10: Description:
11:
12: This is just a little bit of code and data that can get prepended
13: to a ROM image in order to allow bootloaders to load the result
14: as if it were a Linux kernel image.
15:
16: A real Linux kernel image consists of a one-sector boot loader
17: (to load the image from a floppy disk), followed a few sectors
18: of setup code, followed by the kernel code itself. There's
19: a table in the first sector (starting at offset 497) that indicates
20: how many sectors of setup code follow the first sector and which
21: contains some other parameters that aren't interesting in this
22: case.
23:
24: When a bootloader loads the sectors that comprise a kernel image,
25: it doesn't execute the code in the first sector (since that code
26: would try to load the image from a floppy disk.) The code in the
27: first sector below doesn't expect to get executed (and prints an
28: error message if it ever -is- executed.)
29:
30: We don't require much in the way of setup code. Historically, the
31: Linux kernel required at least 4 sectors of setup code.
32: Therefore, at least 4 sectors must be present even though we don't
33: use them.
34:
35: */
36:
37: FILE_LICENCE ( GPL_ANY )
38:
39: #define SETUPSECS 4 /* Minimal nr of setup-sectors */
40: #define PREFIXSIZE ((SETUPSECS+1)*512)
41: #define PREFIXPGH (PREFIXSIZE / 16 )
42: #define BOOTSEG 0x07C0 /* original address of boot-sector */
43: #define INITSEG 0x9000 /* we move boot here - out of the way */
44: #define SETUPSEG 0x9020 /* setup starts here */
45: #define SYSSEG 0x1000 /* system loaded at 0x10000 (65536). */
46:
47: .text
48: .code16
49: .arch i386
50: .org 0
51: .section ".prefix", "ax", @progbits
52: .globl _lkrn_start
53: _lkrn_start:
54: /*
55: This is a minimal boot sector. If anyone tries to execute it (e.g., if
56: a .lilo file is dd'ed to a floppy), print an error message.
57: */
58:
59: bootsector:
60: jmp $BOOTSEG, $1f /* reload cs:ip to match relocation addr */
61: 1:
62: movw $0x2000, %di /* 0x2000 is arbitrary value >= length
63: of bootsect + room for stack */
64:
65: movw $BOOTSEG, %ax
66: movw %ax,%ds
67: movw %ax,%es
68:
69: cli
70: movw %ax, %ss /* put stack at BOOTSEG:0x2000. */
71: movw %di,%sp
72: sti
73:
74: movw $why_end-why, %cx
75: movw $why, %si
76:
77: movw $0x0007, %bx /* page 0, attribute 7 (normal) */
78: movb $0x0e, %ah /* write char, tty mode */
79: prloop:
80: lodsb
81: int $0x10
82: loop prloop
83: freeze: jmp freeze
84:
85: why: .ascii "This image cannot be loaded from a floppy disk.\r\n"
86: why_end:
87:
88:
89: /*
90: The following header is documented in the Linux source code at
91: Documentation/x86/boot.txt
92: */
93: .org 497
94: setup_sects:
95: .byte SETUPSECS
96: root_flags:
97: .word 0
98: syssize:
99: .long -PREFIXPGH
100:
101: .section ".zinfo.fixup", "a", @progbits /* Compressor fixups */
102: .ascii "ADDL"
103: .long syssize
104: .long 16
105: .long 0
106: .previous
107:
108: ram_size:
109: .word 0
110: vid_mode:
111: .word 0
112: root_dev:
113: .word 0
114: boot_flag:
115: .word 0xAA55
116: jump:
117: /* Manually specify a two-byte jmp instruction here rather
118: * than leaving it up to the assembler. */
119: .byte 0xeb
120: .byte setup_code - header
121: header:
122: .byte 'H', 'd', 'r', 'S'
123: version:
124: .word 0x0207 /* 2.07 */
125: realmode_swtch:
126: .long 0
127: start_sys:
128: .word 0
129: kernel_version:
130: .word 0
131: type_of_loader:
132: .byte 0
133: loadflags:
134: .byte 0
135: setup_move_size:
136: .word 0
137: code32_start:
138: .long 0
139: ramdisk_image:
140: .long 0
141: ramdisk_size:
142: .long 0
143: bootsect_kludge:
144: .long 0
145: heap_end_ptr:
146: .word 0
147: pad1:
148: .word 0
149: cmd_line_ptr:
150: .long 0
151: initrd_addr_max:
152: /* We don't use an initrd but some bootloaders (e.g. SYSLINUX) have
153: * been known to require this field. Set the value to 2 GB. This
154: * value is also used by the Linux kernel. */
155: .long 0x7fffffff
156: kernel_alignment:
157: .long 0
158: relocatable_kernel:
159: .byte 0
160: pad2:
161: .byte 0, 0, 0
162: cmdline_size:
163: .long 0
164: hardware_subarch:
165: .long 0
166: hardware_subarch_data:
167: .byte 0, 0, 0, 0, 0, 0, 0, 0
168:
169: /*
170: We don't need to do too much setup.
171:
172: This code gets loaded at SETUPSEG:0. It wants to start
173: executing the image that's loaded at SYSSEG:0 and
174: whose entry point is SYSSEG:0.
175: */
176: setup_code:
177: /* We expect to be contiguous in memory once loaded. The Linux image
178: * boot process requires that setup code is loaded separately from
179: * "non-real code". Since we don't need any information that's left
180: * in the prefix, it doesn't matter: we just have to ensure that
181: * %cs:0000 is where the start of the image *would* be.
182: */
183: ljmp $(SYSSEG-(PREFIXSIZE/16)), $run_ipxe
184:
185:
186: .org PREFIXSIZE
187: /*
188: We're now at the beginning of the kernel proper.
189: */
190: run_ipxe:
191: /* Set up stack just below 0x7c00 */
192: xorw %ax, %ax
193: movw %ax, %ss
194: movw $0x7c00, %sp
195:
196: /* Retrieve command-line pointer */
197: movl %es:cmd_line_ptr, %edx
198:
199: /* Install iPXE */
200: call install
201:
202: /* Set up real-mode stack */
203: movw %bx, %ss
204: movw $_estack16, %sp
205:
206: /* Jump to .text16 segment */
207: pushw %ax
208: pushw $1f
209: lret
210: .section ".text16", "awx", @progbits
211: 1:
212: /* Set up %ds for access to .data16 */
213: movw %bx, %ds
214:
215: /* Store command-line pointer */
216: movl %edx, cmdline_phys
217:
218: /* Run iPXE */
219: pushl $main
220: pushw %cs
221: call prot_call
222: popl %ecx /* discard */
223:
224: /* Uninstall iPXE */
225: call uninstall
226:
227: /* Boot next device */
228: int $0x18
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