1 ; Boot sector for GeekOS
2 ; Copyright (c) 2001,2004 David H. Hovemeyer <daveho@cs.umd.edu>
3 ; Copyright (c) 2003, Jeffrey K. Hollingsworth <hollings@cs.umd.edu>
6 ; This is free software. You are permitted to use,
7 ; redistribute, and modify it as specified in the file "COPYING".
9 ; Loads setup code and a program image from sectors 1..n of a floppy
10 ; and executes the setup code (which will in turn execute
13 ; Some of this code is adapted from Kernel Toolkit 0.2
14 ; and Linux version 2.2.x, so the following copyrights apply:
16 ; Copyright (C) 1991, 1992 Linus Torvalds
17 ; modified by Drew Eckhardt
18 ; modified by Bruce Evans (bde)
19 ; adapted for Kernel Toolkit by Luigi Sgro
23 ; Pad to desired offset from start symbol.
24 ; Usage: Pad_From_Symbol offset, symbol
25 %macro Pad_From_Symbol 2
26 times (%1 - ($ - %2)) db 0
29 ; ----------------------------------------------------------------------
31 ; ----------------------------------------------------------------------
36 BeginText: ; needed to calculate padding bytes to fill the sector
38 ; Copy the boot sector into INITSEG.
40 mov ds, ax ; source segment for string copy
41 xor si, si ; source index for string copy
43 mov es, ax ; destination segment for string copy
44 xor di, di ; destination index for string copy
45 cld ; clear direction flag
46 mov cx, 256 ; number of words to copy
47 rep movsw ; copy 512 bytes
49 jmp INITSEG:after_move
52 ; Now we're executing in INITSEG
54 ; We want the data segment to refer to INITSEG
55 ; (since we've defined variables in the same place as the code)
56 mov ds, ax ; ax still contains INITSEG
58 ; Put the stack in the place where we were originally loaded.
59 ; By definition, there is nothing important there now.
62 mov sp, (BOOTSEG << 4) + 512 - 2
65 ; Load the setup code.
66 mov ax, word [setupStart]
67 mov word [sec_count], ax
69 mov word [max_sector], ax
72 push ax ; 1st param to ReadSector (log sec num)
73 push word SETUPSEG ; 2nd param to ReadSector (seg base)
74 sub ax, [setupStart] ; convert to 0-indexed
75 shl ax, 9 ; multiply by 512
76 push ax ; ...to get 3rd param (byte offset)
78 ; read the sector from the floppy
80 add sp, 6 ; clear 3 word params
86 mov bx, word [max_sector]
87 cmp word [sec_count], bx
91 ; Load the kernel image from sectors KERN_START_SEC..n of the
92 ; floppy into memory at KERNSEG. Note that there are 128 sectors
93 ; per 64K segment. So, when figuring out which segment to
94 ; load the sector into, we shift right by 7 bits (which is
95 ; equivalent to dividing by 128).
97 ; Figure out start sector and max sector
98 mov ax, word [kernelStart]
99 mov word [sec_count], ax
100 add ax, word [kernelSize]
101 mov word [max_sector], ax
103 mov ax, [sec_count] ; logical sector on the floppy
104 push ax ; 1st param to ReadSector (log sec num)
105 sub ax, [kernelStart] ; convert to 0-indexed
106 mov cx, ax ; save in cx
107 shr ax, 7 ; divide by 128
108 shl ax, 12 ; ...and multiply by 0x1000
109 add ax, KERNSEG ; ...to get base relative to KERNSEG
110 push ax ; 2nd param to ReadSector (seg base)
111 and cx, 0x7f ; mod sector by 128
112 shl cx, 9 ; ...and multiply by 512
113 push cx ; to get offset in segment (3rd parm)
115 ; read the sector from the floppy
117 add sp, 6 ; clear 3 word params
122 ; have we loaded all of the sectors?
123 mov bx, word [max_sector]
124 cmp word [sec_count], bx
127 ; Now we've loaded the setup code and the kernel image.
128 ; Jump to setup code.
131 ; Read a sector from the floppy drive.
132 ; This code (and the rest of this boot sector) will have to
133 ; be re-written at some point so it reads more than one
137 ; - "logical" sector number [bp+8]
138 ; - destination segment [bp+6]
139 ; - destination offset [bp+4]
141 push bp ; set up stack frame
143 pusha ; save all registers
158 ; Sector = log_sec % SECTORS_PER_TRACK
159 ; Head = (log_sec / SECTORS_PER_TRACK) % HEADS
160 mov ax, [bp+8] ; get logical sector number from stack
161 xor dx, dx ; dx is high part of dividend (== 0)
162 mov bx, SECTORS_PER_TRACK ; divisor
163 div bx ; do the division
164 mov [sec], dx ; sector is the remainder
165 and ax, 1 ; same as mod by HEADS==2 (slight hack)
168 ; Track = log_sec / (SECTORS_PER_TRACK*HEADS)
169 mov ax, [bp+8] ; get logical sector number again
170 xor dx, dx ; dx is high part of dividend
171 mov bx, SECTORS_PER_TRACK*2 ; divisor
172 div bx ; do the division
173 mov [track], ax ; track is quotient
188 ; Now, try to actually read the sector from the floppy,
189 ; retrying up to 3 times.
191 mov [num_retries], byte 0
194 mov ax, [bp+6] ; dest segment...
195 mov es, ax ; goes in es
196 mov ax, (0x02 << 8) | 1 ; function = 02h in ah,
198 mov bx, [track] ; track number...
199 mov ch, bl ; goes in ch
200 mov bx, [sec] ; sector number...
201 mov cl, bl ; goes in cl...
202 inc cl ; but it must be 1-based, not 0-based
203 mov bx, [head] ; head number...
204 mov dh, bl ; goes in dh
205 xor dl, dl ; hard code drive=0
206 mov bx, [bp+4] ; offset goes in bx
207 ; (es:bx points to buffer)
209 ; Call the BIOS Read Diskette Sectors service
212 ; If the carry flag is NOT set, then there was no error
216 ; Error - code stored in ah
219 inc byte [num_retries]
220 cmp byte [num_retries], 3
223 ; If we got here, we failed thrice, so we give up
229 popa ; restore all regisiters
230 pop bp ; leave stack frame
233 ; Include utility routines
236 ; ----------------------------------------------------------------------
238 ; ----------------------------------------------------------------------
240 ; These are used by ReadSector
246 ; Used for loops reading sectors from floppy
250 ; Padding to make the PFAT Boot Record sit just before the BIOS signature.
251 Pad_From_Symbol PFAT_BOOT_RECORD_OFFSET, BeginText
254 ; Describes how to load the setup program and kernel.
255 ; The default values are appropriate for creating a boot
256 ; floppy by concatenating the boot sector, setup program,
257 ; and kernel image. The buildFat program will change
258 ; these values if the boot floppy is formatted as a PFAT
275 ;; part of pfat boot record
277 dw 1 ; by default, setup is at first sector
279 ;; part of pfat boot record
281 dw NUM_SETUP_SECTORS ; number of sectors in setup
283 ;; part of pfat boot record
285 dw 1+NUM_SETUP_SECTORS ; default start sector for kernel
287 ;; part of pfat boot record
289 dw NUM_KERN_SECTORS+2*NUM_BIOS_SECTORS+NUM_VGA_BIOS_SECTORS+NUM_VMXASSIST_SECTORS
292 ; Finish by writing the BIOS signature to mark this as
293 ; a valid boot sector.
294 Pad_From_Symbol BIOS_SIGNATURE_OFFSET, BeginText
295 Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector