1/*	$OpenBSD: srt0.S,v 1.4 2022/12/08 01:25:45 guenther Exp $	*/
2
3/*
4 * Copyright (c) 1997 Michael Shalayeff
5 * All rights reserved.
6 *
7 * Redistribution and use in source and binary forms, with or without
8 * modification, are permitted provided that the following conditions
9 * are met:
10 * 1. Redistributions of source code must retain the above copyright
11 *    notice, this list of conditions and the following disclaimer.
12 * 2. Redistributions in binary form must reproduce the above copyright
13 *    notice, this list of conditions and the following disclaimer in the
14 *    documentation and/or other materials provided with the distribution.
15 *
16 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
17 * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
18 * WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
19 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
20 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
21 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
22 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
23 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
24 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
25 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
26 * SUCH DAMAGE.
27 *
28 */
29#include <machine/asm.h>
30#include <assym.h>
31
32#define BOOTSTACK 0xfffc
33
34	.globl	end
35	.globl	edata
36	.globl	boot
37	.globl	_rtt
38	.globl	bios_bootdev
39	.globl	pmm_init
40	.globl	Gdtr
41
42	.text
43	.code16
44	.globl	_start
45_start:
46#ifdef DEBUG
47	movl	$0xb80a0, %ebx
48	addr32 movl $0x07420742, (%ebx)
49#endif
50
51/* Clobbers %ax, maybe more */
52#define	putc(c)		movb	$c, %al;	call	Lchr
53
54	/*
55	 * We are loaded by the PXE loader at location 0x7C00.  Like
56	 * the standard /boot, we are linked to run at 0x40120 (load
57	 * address 0x40000), so we relocate to there.
58	 *
59	 * From 0x7C00 to 0x40000 is 0x38400 (230400) bytes, so we don't
60	 * have to worry about an overlapping copy until pxeboot is
61	 * over 225 KB.
62	 *
63	 * PXE loads us with a stack that grows down from 0x80000 (512 KB).
64	 * While it is unlikely that this will clash with our code that
65	 * we're copying up, we create a temporary stack just below us
66	 * before the relocate.  We also set the entire %esp register, to
67	 * be on the safe side.
68	 */
69#define PXEBOOTADDR	0x7c00		/* Address where we are loaded by PXE */
70	xorw	%ax, %ax
71	movw	%ax, %ss		/* CPU disables interrupts till... */
72	movl	$PXEBOOTADDR-4, %esp	/* after this instruction */
73
74	pushl	%edx			/* Preserve the drive number. */
75
76	movw	$(PXEBOOTADDR >> 4), %ax /* Reloc from %ds = 0x7c0. */
77	movw	$(LINKADDR >> 4), %bx	 /* Reloc to %es = 0x4012. */
78
79	movl	$end, %edx
80	subl	$_start, %edx		/* How big are we? */
81
82	/*
83	 * Relocate in blocks that are a maximum of 32KB in size, incrementing
84	 * the segment registers after each block. The 'rep; movsb' instruction
85	 * uses %cx, which limits us to a maximum block size of 0xfff0, even
86	 * though we can address the full 64KB within a single segment.
87	 */
88#define RELOC_BLOCK_SIZE 0x8000
89reloc_loop:
90	movl	%edx, %ecx
91	jcxz	reloc_done
92	cmpl	$RELOC_BLOCK_SIZE, %ecx
93	jbe	reloc_notrunc
94	movl	$RELOC_BLOCK_SIZE, %ecx
95reloc_notrunc:
96	subl	%ecx, %edx
97
98	movw	%ax, %ds		/* Where we're coming from */
99	xorw	%si, %si
100
101	movw	%bx, %es		/* Where we're going to */
102	xorw	%di, %di
103
104	cld
105	rep; movsb			/* Copy into place */
106
107	addw	$(RELOC_BLOCK_SIZE >> 4), %ax
108	addw	$(RELOC_BLOCK_SIZE >> 4), %bx
109
110	jmp	reloc_loop
111
112reloc_done:
113	popl	%edx
114	jmpl	$(LINKADDR >> 4), $(relocated-_start)	/* Now relocate */
115
116relocated:
117	/*
118	 * In 16-bit mode, we have segment registers == 0x4012, and
119	 * offsets work from here, with offset(_start) == 0.
120	 *
121	 * In 32-bit mode, we have a flat memory model, where
122	 * offset(_start) == 0x40120.  This is how we're linked.
123	 *
124	 * Now transition to protected mode.
125	 *
126	 * First, initialise the global descriptor table.
127	 */
128	cli
129	push	%cs
130	pop	%ds
131	addr32 data32 lgdt (Gdtr - LINKADDR)
132
133	movl	%cr0, %eax
134	orl	$CR0_PE, %eax
135	data32 movl %eax, %cr0
136	data32 ljmp $8, $1f		/* Seg sel 0x08 is flat 32-bit code */
1371:
138	.code32
139	movl	$0x10, %eax		/* Seg sel 0x10 is flat 32-bit data */
140	mov	%ax, %ds
141	mov	%ax, %es
142	mov	%ax, %fs
143	mov	%ax, %gs
144	mov	%ax, %ss
145	movl	$BOOTSTACK, %esp
146#ifdef DEBUG
147	movl	$0xb8000, %ebx
148	movl	$0x07420742, (%ebx)
149#endif
150
151	xorl	%edx, %edx
152	movl	%edx, bios_bootdev
153	pushl	%edx			/* boot() takes this as a parameter */
154
155#ifdef DEBUG
156	movl	$0xb80a4, %ebx
157	movl	$0x07520752, (%ebx)
158#endif
159
160	/* Zero .bss */
161	xorl	%eax, %eax
162	movl	$end, %ecx
163	subl	$edata, %ecx
164	movl	$edata, %edi
165	cld
166	rep;	stosb
167
168	/* Set up an interrupt descriptor table for protected mode. */
169	call	pmm_init
170
171	/* Set our program name ("PXEBOOT", not "BOOT"). */
172	movl	$pxe_progname, %eax
173	movl	%eax, progname
174
175	/*
176	 * Now call "main()".
177	 *
178	 * We run in flat 32-bit protected mode, with no address mapping.
179	 */
180#ifdef DEBUG
181	movl	$0xb8004, %ebx
182	movl	$0x07410741, (%ebx)
183#endif
184	call	boot
185
186	/* boot() should not return.  If it does, reset computer. */
187	jmp	_rtt
188
189ENTRY(debugchar)
190	pushl	%ebx
191	movl	8(%esp), %ebx
192	addl	%ebx, %ebx
193	addl	$0xb8000, %ebx
194
195	xorl	%eax, %eax
196	movb	12(%esp), %al
197
198	andl	$0xfffffffe, %ebx
199	movb	%al, (%ebx)
200	popl	%ebx
201	ret
202
203	.code16
204
205/*
206 * Write out value in %ax in hex
207 */
208hex_word:
209	pushw	%ax
210	mov	%ah, %al
211	call	hex_byte
212	popw	%ax
213	/* fall thru */
214/*
215 * Write out value in %al in hex
216 */
217hex_byte:
218	pushw	%ax
219	shrb	$4, %al
220	call	hex_nibble
221	popw	%ax
222	/* fall thru */
223
224/* Write out nibble in %al */
225hex_nibble:
226	and	$0x0F, %al
227	add	$'0', %al
228	cmpb	$'9', %al
229	jbe	Lchr
230	addb	$'A'-'9'-1, %al
231	/* fall thru to Lchr */
232/*
233 *	Lchr: write the character in %al to console
234 */
235Lchr:
236	pushw	%bx
237	movb	$0x0e, %ah
238	xorw	%bx, %bx
239	incw	%bx		/* movw $0x01, %bx */
240	int	$0x10
241	popw	%bx
242	ret
243
244pxe_progname:
245	.asciz	"PXEBOOT"
246
247	.end
248