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
37 ; Copy the boot sector into INITSEG.
39 mov ds, ax ; source segment for string copy
40 xor si, si ; source index for string copy
42 mov es, ax ; destination segment for string copy
43 xor di, di ; destination index for string copy
44 cld ; clear direction flag
45 mov cx, 256 ; number of words to copy
46 rep movsw ; copy 512 bytes
48 jmp INITSEG:after_move
51 ; 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
66 ; Load the setup code.
67 mov ax, word [setupStart]
68 mov word [sec_count], ax
70 mov word [max_sector], ax
73 push ax ; 1st param to ReadSector (log sec num)
74 push word SETUPSEG ; 2nd param to ReadSector (seg base)
75 sub ax, [setupStart] ; convert to 0-indexed
76 shl ax, 9 ; multiply by 512
77 push ax ; ...to get 3rd param (byte offset)
79 ; read the sector from the floppy
81 add sp, 6 ; clear 3 word params
87 mov bx, word [max_sector]
88 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
110 mov ax, [sec_count] ; logical sector on the floppy
111 push ax ; 1st param to ReadSector (log sec num)
117 mov ax, VMSEG ; ...to get base relative to VMSEG
118 push ax ; 2nd param to ReadSector (seg base)
124 mov ax, 2000h ; Always write at the start of the segment
125 push ax ; 3rd parameter
131 ; read the sector from the floppy
133 add sp, 6 ; clear 3 word params
138 ; mov dx, word [es:2000h] ;
177 ; update the low->high copy table for the bios
178 ; mov ax, word [bootsect_src_base] ;
182 ; adc byte [bootsect_src_base+2], 0
183 ; mov word [bootsect_src_base],ax
185 mov ax, word [bootsect_dst_base] ;
187 adc byte [bootsect_dst_base+2], 0
188 mov word [bootsect_dst_base],ax
190 ; have we loaded all of the sectors?
192 mov bx, word [max_sector]
193 cmp word [sec_count], bx
201 ; Now we've loaded the setup code and the kernel image.
202 ; Jump to setup code.
205 ; Read a sector from the floppy drive.
206 ; This code (and the rest of this boot sector) will have to
207 ; be re-written at some point so it reads more than one
211 ; - "logical" sector number [bp+8]
212 ; - destination segment [bp+6]
213 ; - destination offset [bp+4]
215 push bp ; set up stack frame
217 pusha ; save all registers
232 ; Sector = log_sec % SECTORS_PER_TRACK
233 ; Head = (log_sec / SECTORS_PER_TRACK) % HEADS
234 mov ax, [bp+8] ; get logical sector number from stack
235 xor dx, dx ; dx is high part of dividend (== 0)
236 mov bx, SECTORS_PER_TRACK ; divisor
237 div bx ; do the division
238 mov [sec], dx ; sector is the remainder
239 and ax, 1 ; same as mod by HEADS==2 (slight hack)
242 ; Track = log_sec / (SECTORS_PER_TRACK*HEADS)
243 mov ax, [bp+8] ; get logical sector number again
244 xor dx, dx ; dx is high part of dividend
245 mov bx, SECTORS_PER_TRACK*2 ; divisor
246 div bx ; do the division
247 mov [track], ax ; track is quotient
262 ; Now, try to actually read the sector from the floppy,
263 ; retrying up to 3 times.
265 mov [num_retries], byte 0
268 mov ax, [bp+6] ; dest segment...
269 mov es, ax ; goes in es
270 mov ax, (0x02 << 8) | 1 ; function = 02h in ah,
272 mov bx, [track] ; track number...
273 mov ch, bl ; goes in ch
274 mov bx, [sec] ; sector number...
275 mov cl, bl ; goes in cl...
276 inc cl ; but it must be 1-based, not 0-based
277 mov bx, [head] ; head number...
278 mov dh, bl ; goes in dh
279 xor dl, dl ; hard code drive=0
280 mov bx, [bp+4] ; offset goes in bx
281 ; (es:bx points to buffer)
282 ; Call the BIOS Read Diskette Sectors service
285 ; If the carry flag is NOT set, then there was no error
289 ; Error - code stored in ah
292 inc byte [num_retries]
293 cmp byte [num_retries], 3
296 ; If we got here, we failed thrice, so we give up
302 popa ; restore all regisiters
303 pop bp ; leave stack frame
306 ; Include utility routines:
308 ; REPLACED WITH FOLLOWING WHICH MUST BE COMPILED 16 FOR USE IN THIS CODE
311 mov cx, 4 ; 4 hex digits
313 rol dx, 4 ; rotate so that lowest 4 bits are used
314 mov ax, 0E0Fh ; ah = request, al = mask for nybble
316 add al, 90h ; convert al to ascii hex (four instructions)
317 daa ; I've spent 1 hour to understand how it works..
326 PrintNL: ; print CR and NL
337 ; ----------------------------------------------------------------------
339 ; ----------------------------------------------------------------------
341 ; These are used by ReadSector
347 ; Used for loops reading sectors from floppy
360 db 0,0x20,0x08 ; ! base = 0x082000
362 dw 0 ; ! limit16,base24 =0
366 db 0,0,0x10 ; ! base = 0x100000
368 dw 0 ; ! limit16,base24 =0
369 dw 0,0,0,0 ; ! BIOS CS
370 dw 0,0,0,0 ; ! BIOS DS
373 ; Padding to make the PFAT Boot Record sit just before the BIOS signature.
374 ;Pad_From_Symbol PFAT_BOOT_RECORD_OFFSET, BeginText
377 ; Describes how to load the setup program and kernel.
378 ; The default values are appropriate for creating a boot
379 ; floppy by concatenating the boot sector, setup program,
380 ; and kernel image. The buildFat program will change
381 ; these values if the boot floppy is formatted as a PFAT
398 ;; part of pfat boot record
400 dw 1 ; by default, setup is at first sector
402 ;; part of pfat boot record
404 dw NUM_SETUP_SECTORS ; number of sectors in setup
406 ;; part of pfat boot record
408 dw 1+NUM_SETUP_SECTORS ; default start sector for kernel
410 ;; part of pfat boot record
417 ; Finish by writing the BIOS signature to mark this as
418 ; a valid boot sector.
419 Pad_From_Symbol BIOS_SIGNATURE_OFFSET, BeginText
420 Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector