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
55 ; We want the data segment to refer to INITSEG
56 ; (since we've defined variables in the same place as the code)
57 mov ds, ax ; ax still contains INITSEG
59 ; Put the stack in the place where we were originally loaded.
60 ; By definition, there is nothing important there now.
63 mov sp, (BOOTSEG << 4) + 512 - 2
67 ; Load the setup code.
68 mov ax, word [setupStart]
69 mov word [sec_count], ax
71 mov word [max_sector], ax
74 push ax ; 1st param to ReadSector (log sec num)
75 push word SETUPSEG ; 2nd param to ReadSector (seg base)
76 sub ax, [setupStart] ; convert to 0-indexed
77 shl ax, 9 ; multiply by 512
78 push ax ; ...to get 3rd param (byte offset)
80 ; read the sector from the floppy
82 add sp, 6 ; clear 3 word params
88 mov bx, word [max_sector]
89 cmp word [sec_count], bx
95 ; Load the kernel image from sectors KERN_START_SEC..n of the
96 ; floppy into memory at KERNSEG. Note that there are 128 sectors
97 ; per 64K segment. So, when figuring out which segment to
98 ; load the sector into, we shift right by 7 bits (which is
99 ; equivalent to dividing by 128).
101 ; Figure out start sector and max sector
103 mov ax, word [kernelStart]
104 mov word [sec_count], ax
105 add ax, word [kernelSize]
106 mov word [max_sector], ax
111 mov ax, [sec_count] ; logical sector on the floppy
112 push ax ; 1st param to ReadSector (log sec num)
118 mov ax, VMSEG ; ...to get base relative to VMSEG
119 push ax ; 2nd param to ReadSector (seg base)
125 mov ax, 2000h ; Always write at the start of the segment
126 push ax ; 3rd parameter
132 ; read the sector from the floppy
134 add sp, 6 ; clear 3 word params
139 ; mov dx, word [es:2000h] ;
178 ; update the low->high copy table for the bios
179 ; mov ax, word [bootsect_src_base] ;
183 ; adc byte [bootsect_src_base+2], 0
184 ; mov word [bootsect_src_base],ax
186 mov ax, word [bootsect_dst_base] ;
188 adc byte [bootsect_dst_base+2], 0
189 mov word [bootsect_dst_base],ax
191 ; have we loaded all of the sectors?
193 mov bx, word [max_sector]
194 cmp word [sec_count], bx
202 ; Now we've loaded the setup code and the kernel image.
203 ; Jump to setup code.
206 ; Read a sector from the floppy drive.
207 ; This code (and the rest of this boot sector) will have to
208 ; be re-written at some point so it reads more than one
212 ; - "logical" sector number [bp+8]
213 ; - destination segment [bp+6]
214 ; - destination offset [bp+4]
216 push bp ; set up stack frame
218 pusha ; save all registers
233 ; Sector = log_sec % SECTORS_PER_TRACK
234 ; Head = (log_sec / SECTORS_PER_TRACK) % HEADS
235 mov ax, [bp+8] ; get logical sector number from stack
236 xor dx, dx ; dx is high part of dividend (== 0)
237 mov bx, SECTORS_PER_TRACK ; divisor
238 div bx ; do the division
239 mov [sec], dx ; sector is the remainder
240 and ax, 1 ; same as mod by HEADS==2 (slight hack)
243 ; Track = log_sec / (SECTORS_PER_TRACK*HEADS)
244 mov ax, [bp+8] ; get logical sector number again
245 xor dx, dx ; dx is high part of dividend
246 mov bx, SECTORS_PER_TRACK*2 ; divisor
247 div bx ; do the division
248 mov [track], ax ; track is quotient
263 ; Now, try to actually read the sector from the floppy,
264 ; retrying up to 3 times.
266 mov [num_retries], byte 0
269 mov ax, [bp+6] ; dest segment...
270 mov es, ax ; goes in es
271 mov ax, (0x02 << 8) | 1 ; function = 02h in ah,
273 mov bx, [track] ; track number...
274 mov ch, bl ; goes in ch
275 mov bx, [sec] ; sector number...
276 mov cl, bl ; goes in cl...
277 inc cl ; but it must be 1-based, not 0-based
278 mov bx, [head] ; head number...
279 mov dh, bl ; goes in dh
280 xor dl, dl ; hard code drive=0
281 mov bx, [bp+4] ; offset goes in bx
282 ; (es:bx points to buffer)
283 ; Call the BIOS Read Diskette Sectors service
286 ; If the carry flag is NOT set, then there was no error
290 ; Error - code stored in ah
293 inc byte [num_retries]
294 cmp byte [num_retries], 3
297 ; If we got here, we failed thrice, so we give up
303 popa ; restore all regisiters
304 pop bp ; leave stack frame
307 ; Include utility routines:
309 ; REPLACED WITH FOLLOWING WHICH MUST BE COMPILED 16 FOR USE IN THIS CODE
312 mov cx, 4 ; 4 hex digits
314 rol dx, 4 ; rotate so that lowest 4 bits are used
315 mov ax, 0E0Fh ; ah = request, al = mask for nybble
317 add al, 90h ; convert al to ascii hex (four instructions)
318 daa ; I've spent 1 hour to understand how it works..
327 PrintNL: ; print CR and NL
338 ; ----------------------------------------------------------------------
340 ; ----------------------------------------------------------------------
342 ; These are used by ReadSector
348 ; Used for loops reading sectors from floppy
361 db 0,0x20,0x08 ; ! base = 0x082000
363 dw 0 ; ! limit16,base24 =0
367 db 0,0,0x10 ; ! base = 0x100000
369 dw 0 ; ! limit16,base24 =0
370 dw 0,0,0,0 ; ! BIOS CS
371 dw 0,0,0,0 ; ! BIOS DS
374 ; Padding to make the PFAT Boot Record sit just before the BIOS signature.
375 ;Pad_From_Symbol PFAT_BOOT_RECORD_OFFSET, BeginText
378 ; Describes how to load the setup program and kernel.
379 ; The default values are appropriate for creating a boot
380 ; floppy by concatenating the boot sector, setup program,
381 ; and kernel image. The buildFat program will change
382 ; these values if the boot floppy is formatted as a PFAT
399 ;; part of pfat boot record
401 dw 1 ; by default, setup is at first sector
403 ;; part of pfat boot record
405 dw NUM_SETUP_SECTORS ; number of sectors in setup
407 ;; part of pfat boot record
409 dw 1+NUM_SETUP_SECTORS ; default start sector for kernel
411 ;; part of pfat boot record
418 ; Finish by writing the BIOS signature to mark this as
419 ; a valid boot sector.
420 Pad_From_Symbol BIOS_SIGNATURE_OFFSET, BeginText
421 Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector