1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
|
!
! This is a 'Master Boot Record' following the MSDOS 'standards'.
! This BB successfully boots MSDOS or Linux.
!
! In addition it has the facility to load and execute a small program
! (of 8 extents) before the boot blocks are checked.
!
! Lowest available is $0500, MSDOS appears to use $0600 ... I wonder why?
ORGADDR=$0500
preboot=0 ! Include the pre-boot loader ?
! Include standard layout
org ORGADDR
include sysboot.s
public partition_1
public partition_2
public partition_3
public partition_4
org ORGADDR+$3
.ascii "ELKS MBR Copyright 1996, Robert de Bath"
! Start after dos fsstat data, not strictly required.
org codestart
cli ! Assume _nothing_!
cld
mov bx,#$7C00 ! Pointer to start of BB.
xor ax,ax ! Segs all to zero
mov ds,ax
mov es,ax
mov ss,ax
mov sp,bx ! SP Just below BB
mov cx,#$100 ! Move 256 words
mov si,bx ! From default BB
mov di,#ORGADDR ! To the correct address.
rep
movsw
jmpi cont,#0 ! Set CS:IP correct.
cont:
sti ! Let the interrupts back in.
!
! Next check for a pre-boot load.
if preboot
push bx
mov si,#pre_boot_table
lodsw
mov di,ax ! First word is execute address
more_boot:
lodsw
test ax,ax
jz load_done
mov bx,ax ! word 1 address
lodsw
mov cx,ax ! word 2 CX, head/sector
lodsw
mov dx,ax ! word 3 DX, drive, cylinder
lodsw ! word 4 AX, $02, sector count
int $13
jnc more_boot ! This doesn't retry, with a HD it shouldn't be bad.
jc disk_error
load_done:
call di
exec_done:
pop bx
endif
! Now check the partition table, must use SI as pointer cause that's what the
! partition boot blocks expect.
mov si,#partition_1
check_active:
cmp byte [si],#$80 ! Flag for activated partition
jz found_active
bad_boot:
add si,#partition_2-partition_1
cmp si,#bootblock_magic
jnz check_active
mov si,#no_bootpart ! Message & boot
jmp no_boot
found_active:
mov di,#6 ! Max retries, int list says 3 ... double it
mov dx,[si] ! dh = Drive head, dl = $80 ie HD drive 0
mov cx,[si+2] ! cx = Sector & head encoded for int $13
! bx is correct
retry:
movb [$7DFE],#0 ! Clear magic for dosemu
mov ax,#$0201 ! Read 1 sector
int $13 ! Disk read.
jnc sector_loaded
! Error, reset and retry
xor ax,ax
int $13 ! Disk reset
dec di
jnz retry ! Try again
disk_error:
mov si,#disk_read_error
jmp no_boot ! Sorry it ain't gonna work.
sector_loaded:
mov di,#$7DFE ! End of sector loaded
cmp [di],#$AA55 ! Check for magic
jnz bad_boot ! No? Try next partition.
mov bp,si ! LILO says some BBs use bp rather than si
jmpi #$7C00,#0 ! Go!
! Fatal errors ...........
no_boot: ! SI now has pointer to error message
lodsb
cmp al,#0
jz EOS
mov bx,#7
mov ah,#$E ! Can't use $13 cause that's AT+ only!
int $10
jmp no_boot
EOS:
cmp si,#press_end ! After msg output 'press key' message
jz keyboot
mov si,#press_key
jmp no_boot
keyboot: ! Wait for a key then reboot
xor ax,ax
int $16
int $19 ! This should be OK as we haven't touched anything.
jmpi $0,$FFFF ! Wam! Try or die!
no_bootpart:
.asciz "No bootable partition"
disk_read_error:
.asciz "Error loading system"
press_key:
.asciz "\r\nPress a key to reboot"
press_end:
if preboot
return:
ret
export pre_boot_table
pre_boot_table:
! Example: Load rest of H0,C0 into mem at $7C00 (8k).
! .word $7C00, $7C00,$0002,$0000,$0210, $0000
.word return
.word 0
endif
! Now make sure this isn't to big!
if *>partition_1
fail! Partition overlap
endif
!THE END
|
