1#
2# Copyright (c) 1998 Robert Nordier
3# All rights reserved.
4#
5# Redistribution and use in source and binary forms are freely
6# permitted provided that the above copyright notice and this
7# paragraph and the following disclaimer are duplicated in all
8# such forms.
9#
10# This software is provided "AS IS" and without any express or
11# implied warranties, including, without limitation, the implied
12# warranties of merchantability and fitness for a particular
13# purpose.
14#
15
16# $Id: boot1.s,v 1.7 1999/01/10 13:29:51 peter Exp $
17
18 .set MEM_REL,0x700 # Relocation address
19 .set MEM_ARG,0x900 # Arguments
20 .set MEM_ORG,0x7c00 # Origin
21 .set MEM_BUF,0x8c00 # Load area
22 .set MEM_BTX,0x9000 # BTX start
23 .set MEM_JMP,0x9010 # BTX entry point
24 .set MEM_USR,0xa000 # Client start

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34 .globl xread
35
36start: jmp main # Start recognizably
37
38 .org 0x4,0x90
39
40# External read from disk
41
42xread: pushl %ecx # Set
43 pushl %eax # LBA
44 pushl %es # Set transfer
45 pushl %ebx # buffer
46 pushl %edx # Set count:drive
47 callwi(read) # Read from disk
48 popl %edx # Pop all
49 popl %ebx # registers
50 popl %es # pushed, but
51 popl %ecx # preserve
52 popl %ecx # AX
53 lret # To far caller
54
55# Bootstrap
56
57main: cld # String ops inc
58 xorl %ecx,%ecx # Zero
59 movl %cx,%es # Address
60 movl %cx,%ds # data

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122 ret # To caller
123
124# Local read from disk
125
126nread: movwir(MEM_BUF,_bx) # Transfer buffer
127 movw1r(0x8,_si_,_ax) # Get
128 movw1r(0xa,_si_,_cx) # LBA
129 pushl %cs # Read from
130 callwi(xread) # disk
131 jnc return # If success
132 movwir(msg_read,_si) # Message
133
134# Error exit
135
136error: callwi(putstr) # Display message
137 movwir(prompt,_si) # Display
138 callwi(putstr) # prompt

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150 jne putstr.0 # No
151
152ereturn: movb $0x1,%ah # Invalid
153 stc # argument
154return: ret # To caller
155
156# Read from disk
157
158read: movl %esp,%ebp # Address stack frame
159 pushl %edx # Save
160 movb $0x8,%ah # BIOS: Get drive
161 int $0x13 # parameters
162 movb %dh,%ch # Max head number
163 popl %edx # Restore
164 jc return # If error
165 andb $0x3f,%cl # Sectors per track
166 jz ereturn # If zero
167 cli # Disable interrupts

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184 ja ereturn # No
185 xchgb %al,%ah # Set up cylinder
186 rorb $0x2,%al # number
187 orb %ch,%al # Merge
188 incl %eax # sector
189 xchgl %eax,%ecx # number
190 movb %bh,%dh # Head number
191 subb %ah,%al # Sectors this track
192 movb1r(0x3,_bp_,_ah) # Blocks to read
193 cmpb %ah,%al # To read
194 jb read.1 # this
195 movb %ah,%al # track
196read.1: movwir(0x5,_di) # Try count
197read.2: lesw1r(0x4,_bp_,_bx) # Transfer buffer
198 pushl %eax # Save
199 movb $0x2,%ah # BIOS: Read
200 int $0x13 # from disk
201 popl %ebx # Restore
202 jnc read.3 # If success
203 decl %edi # Retry?
204 jz read.5 # No
205 xorb %ah,%ah # BIOS: Reset
206 int $0x13 # disk system
207 xchgl %ebx,%eax # Block count
208 jmp read.2 # Continue
209read.3: movzbl %bl,%eax # Sectors read
210 addwr1(_ax,0x8,_bp_) # Adjust
211 jnc read.4 # LBA,
212 incw1(0xa,_bp_) # transfer
213read.4: shlb %bl # buffer
214 addbr1(_bl,0x5,_bp_) # pointer,
215 subbr1(_al,0x3,_bp_) # block count
216 ja read # If not done
217read.5: ret # To caller
218
219# Messages
220
221msg_read: .asciz "Read"
222msg_part: .asciz "Boot"
223
224prompt: .asciz " error\r\n"
225
226 .org PRT_OFF,0x90
227
228# Partition table
229
230 .fill 0x30,0x1,0x0
231part4: .byte 0x80, 0x00, 0x01, 0x00
232 .byte 0xa5, 0xff, 0xff, 0xff
233 .byte 0x00, 0x00, 0x00, 0x00
234 .byte 0x50, 0xc3, 0x00, 0x00
235
236 .word 0xaa55 # Magic number