2 ; Boot sector for GeekOS
3 ; Copyright (c) 2001,2004 David H. Hovemeyer <daveho@cs.umd.edu>
4 ; Copyright (c) 2003, Jeffrey K. Hollingsworth <hollings@cs.umd.edu>
7 ; This is free software. You are permitted to use,
8 ; redistribute, and modify it as specified in the file "COPYING".
10 ; Loads setup code and a program image from sectors 1..n of a floppy
11 ; and executes the setup code (which will in turn execute
14 ; Some of this code is adapted from Kernel Toolkit 0.2
15 ; and Linux version 2.2.x, so the following copyrights apply:
17 ; Copyright (C) 1991, 1992 Linus Torvalds
18 ; modified by Drew Eckhardt
19 ; modified by Bruce Evans (bde)
20 ; adapted for Kernel Toolkit by Luigi Sgro
24 ; Pad to desired offset from start symbol.
25 ; Usage: Pad_From_Symbol offset, symbol
26 %macro Pad_From_Symbol 2
27 times (%1 - ($ - %2)) db 0
30 ; ----------------------------------------------------------------------
32 ; ----------------------------------------------------------------------
37 BeginText: ; needed to calculate padding bytes to fill the sector
39 ; Copy the boot sector into INITSEG.
41 mov ds, ax ; source segment for string copy
42 xor si, si ; source index for string copy
44 mov es, ax ; destination segment for string copy
45 xor di, di ; destination index for string copy
46 cld ; clear direction flag
47 mov cx, 256 ; number of words to copy
48 rep movsw ; copy 512 bytes
50 jmp INITSEG:after_move
53 ; Now we're executing in INITSEG
56 ; We want the data segment to refer to INITSEG
57 ; (since we've defined variables in the same place as the code)
58 mov ds, ax ; ax still contains INITSEG
60 ; Put the stack in the place where we were originally loaded.
61 ; By definition, there is nothing important there now.
64 mov sp, (BOOTSEG << 4) + 512 - 2
68 ; Load the setup code.
69 mov ax, word [setupStart]
70 mov word [sec_count], ax
72 mov word [max_sector], ax
75 push ax ; 1st param to ReadSector (log sec num)
76 push word SETUPSEG ; 2nd param to ReadSector (seg base)
77 sub ax, [setupStart] ; convert to 0-indexed
78 shl ax, 9 ; multiply by 512
79 push ax ; ...to get 3rd param (byte offset)
81 ; read the sector from the floppy
83 add sp, 6 ; clear 3 word params
89 mov bx, word [max_sector]
90 cmp word [sec_count], bx
94 ; Load the kernel image from sectors KERN_START_SEC..n of the
95 ; floppy into memory at KERNSEG. Note that there are 128 sectors
96 ; per 64K segment. So, when figuring out which segment to
97 ; load the sector into, we shift right by 7 bits (which is
98 ; equivalent to dividing by 128).
100 ; Figure out start sector and max sector
102 mov ax, word [kernelStart]
103 mov word [sec_count], ax
104 add ax, word [kernelSize]
105 mov word [max_sector], ax
107 mov ax, [sec_count] ; logical sector on the floppy
111 push ax ; 1st param to ReadSector (log sec num)
112 sub ax, [kernelStart] ; convert to 0-indexed
113 mov cx, ax ; save in cx
114 shr ax, 7 ; divide by 128
115 shl ax, 12 ; ...and multiply by 0x1000
116 add ax, KERNSEG ; ...to get base relative to KERNSEG
120 push ax ; 2nd param to ReadSector (seg base)
121 and cx, 0x7f ; mod sector by 128
122 shl cx, 9 ; ...and multiply by 512
123 push cx ; to get offset in segment (3rd parm)
129 ; read the sector from the floppy
131 add sp, 6 ; clear 3 word params
137 ; have we loaded all of the sectors?
138 mov bx, word [max_sector]
139 cmp word [sec_count], bx
143 ; Load the guest image starting at 1MB
144 ; floppy into memory at KERNSEG. Note that there are 128 sectors
145 ; per 64K segment. So, when figuring out which segment to
146 ; load the sector into, we shift right by 7 bits (which is
147 ; equivalent to dividing by 128).
149 ; Figure out start sector and max sector
150 mov ax, word [vmStart]
151 mov word [sec_count], ax
152 add ax, word [vmSize]
153 mov word [max_sector], ax
156 mov ax, [sec_count] ; logical sector on the floppy
157 push ax ; 1st param to ReadSector (log sec num)
163 mov ax, VMSEG ; ...to get base relative to VMSEG
164 push ax ; 2nd param to ReadSector (seg base)
170 mov ax, 2000h ; Always write at the start of the segment
171 push ax ; 3rd parameter
177 ; read the sector from the floppy
179 add sp, 6 ; clear 3 word params
184 ; mov dx, word [es:2000h] ;
223 ; update the low->high copy table for the bios
224 ; mov ax, word [bootsect_src_base] ;
228 ; adc byte [bootsect_src_base+2], 0
229 ; mov word [bootsect_src_base],ax
231 mov ax, word [bootsect_dst_base] ;
233 adc byte [bootsect_dst_base+2], 0
234 mov word [bootsect_dst_base],ax
236 ; have we loaded all of the sectors?
238 mov bx, word [max_sector]
239 cmp word [sec_count], bx
247 ; Now we've loaded the setup code and the kernel image.
248 ; Jump to setup code.
251 ; Read a sector from the floppy drive.
252 ; This code (and the rest of this boot sector) will have to
253 ; be re-written at some point so it reads more than one
257 ; - "logical" sector number [bp+8]
258 ; - destination segment [bp+6]
259 ; - destination offset [bp+4]
261 push bp ; set up stack frame
263 pusha ; save all registers
278 ; Sector = log_sec % SECTORS_PER_TRACK
279 ; Head = (log_sec / SECTORS_PER_TRACK) % HEADS
280 mov ax, [bp+8] ; get logical sector number from stack
281 xor dx, dx ; dx is high part of dividend (== 0)
282 mov bx, SECTORS_PER_TRACK ; divisor
283 div bx ; do the division
284 mov [sec], dx ; sector is the remainder
285 and ax, 1 ; same as mod by HEADS==2 (slight hack)
288 ; Track = log_sec / (SECTORS_PER_TRACK*HEADS)
289 mov ax, [bp+8] ; get logical sector number again
290 xor dx, dx ; dx is high part of dividend
291 mov bx, SECTORS_PER_TRACK*2 ; divisor
292 div bx ; do the division
293 mov [track], ax ; track is quotient
308 ; Now, try to actually read the sector from the floppy,
309 ; retrying up to 3 times.
311 mov [num_retries], byte 0
314 mov ax, [bp+6] ; dest segment...
315 mov es, ax ; goes in es
316 mov ax, (0x02 << 8) | 1 ; function = 02h in ah,
318 mov bx, [track] ; track number...
319 mov ch, bl ; goes in ch
320 mov bx, [sec] ; sector number...
321 mov cl, bl ; goes in cl...
322 inc cl ; but it must be 1-based, not 0-based
323 mov bx, [head] ; head number...
324 mov dh, bl ; goes in dh
325 xor dl, dl ; hard code drive=0
326 mov bx, [bp+4] ; offset goes in bx
327 ; (es:bx points to buffer)
328 ; Call the BIOS Read Diskette Sectors service
331 ; If the carry flag is NOT set, then there was no error
335 ; Error - code stored in ah
338 inc byte [num_retries]
339 cmp byte [num_retries], 3
342 ; If we got here, we failed thrice, so we give up
348 popa ; restore all regisiters
349 pop bp ; leave stack frame
352 ; Include utility routines:
354 ; REPLACED WITH FOLLOWING WHICH MUST BE COMPILED 16 FOR USE IN THIS CODE
357 mov cx, 4 ; 4 hex digits
359 rol dx, 4 ; rotate so that lowest 4 bits are used
360 mov ax, 0E0Fh ; ah = request, al = mask for nybble
362 add al, 90h ; convert al to ascii hex (four instructions)
363 daa ; I've spent 1 hour to understand how it works..
372 PrintNL: ; print CR and NL
383 ; ----------------------------------------------------------------------
385 ; ----------------------------------------------------------------------
387 ; These are used by ReadSector
393 ; Used for loops reading sectors from floppy
406 db 0,0x20,0x08 ; ! base = 0x082000
408 dw 0 ; ! limit16,base24 =0
412 db 0,0,0x10 ; ! base = 0x100000
414 dw 0 ; ! limit16,base24 =0
415 dw 0,0,0,0 ; ! BIOS CS
416 dw 0,0,0,0 ; ! BIOS DS
419 ; Padding to make the PFAT Boot Record sit just before the BIOS signature.
420 ;Pad_From_Symbol PFAT_BOOT_RECORD_OFFSET, BeginText
423 ; Describes how to load the setup program and kernel.
424 ; The default values are appropriate for creating a boot
425 ; floppy by concatenating the boot sector, setup program,
426 ; and kernel image. The buildFat program will change
427 ; these values if the boot floppy is formatted as a PFAT
444 ;; part of pfat boot record
446 dw 1 ; by default, setup is at first sector
448 ;; part of pfat boot record
450 dw NUM_SETUP_SECTORS ; number of sectors in setup
452 ;; part of pfat boot record
454 dw 1+NUM_SETUP_SECTORS ; default start sector for kernel
456 ;; part of pfat boot record
460 ;; part of pfat boot record
462 dw 1+NUM_SETUP_SECTORS+NUM_KERN_SECTORS
463 ;; part of pfat boot record
466 dw NUM_VM_KERNEL_SECTORS
469 ; Finish by writing the BIOS signature to mark this as
470 ; a valid boot sector.
471 Pad_From_Symbol BIOS_SIGNATURE_OFFSET, BeginText
472 Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector