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 ;; temporalily for io performance test
46 ;; mov eax, 0x12345678
53 ; Copy the boot sector into INITSEG.
55 mov ds, ax ; source segment for string copy
56 xor si, si ; source index for string copy
58 mov es, ax ; destination segment for string copy
59 xor di, di ; destination index for string copy
60 cld ; clear direction flag
61 mov cx, 256 ; number of words to copy
62 rep movsw ; copy 512 bytes
64 jmp INITSEG:after_move
67 ; Now we're executing in INITSEG
70 ; We want the data segment to refer to INITSEG
71 ; (since we've defined variables in the same place as the code)
72 mov ds, ax ; ax still contains INITSEG
74 ; Put the stack in the place where we were originally loaded.
75 ; By definition, there is nothing important there now.
78 mov sp, (BOOTSEG << 4) + 512 - 2
82 ; Load the setup code.
83 mov ax, word [setupStart]
84 mov word [sec_count], ax
86 mov word [max_sector], ax
89 push ax ; 1st param to ReadSector (log sec num)
90 push word SETUPSEG ; 2nd param to ReadSector (seg base)
91 sub ax, [setupStart] ; convert to 0-indexed
92 shl ax, 9 ; multiply by 512
93 push ax ; ...to get 3rd param (byte offset)
95 ; read the sector from the floppy
97 add sp, 6 ; clear 3 word params
103 mov bx, word [max_sector]
104 cmp word [sec_count], bx
110 ; Load the kernel image from sectors KERN_START_SEC..n of the
111 ; floppy into memory at KERNSEG. Note that there are 128 sectors
112 ; per 64K segment. So, when figuring out which segment to
113 ; load the sector into, we shift right by 7 bits (which is
114 ; equivalent to dividing by 128).
116 ; Figure out start sector and max sector
118 mov ax, word [kernelStart]
119 mov word [sec_count], ax
120 add ax, word [kernelSize]
121 mov word [max_sector], ax
126 mov ax, [sec_count] ; logical sector on the floppy
127 push ax ; 1st param to ReadSector (log sec num)
133 mov ax, VMSEG ; ...to get base relative to VMSEG
134 push ax ; 2nd param to ReadSector (seg base)
140 mov ax, 2000h ; Always write at the start of the segment
141 push ax ; 3rd parameter
147 ; read the sector from the floppy
149 add sp, 6 ; clear 3 word params
154 ; mov dx, word [es:2000h] ;
193 ; update the low->high copy table for the bios
194 ; mov ax, word [bootsect_src_base] ;
198 ; adc byte [bootsect_src_base+2], 0
199 ; mov word [bootsect_src_base],ax
201 mov ax, word [bootsect_dst_base] ;
203 adc byte [bootsect_dst_base+2], 0
204 mov word [bootsect_dst_base],ax
206 ; have we loaded all of the sectors?
208 mov bx, word [max_sector]
209 cmp word [sec_count], bx
217 ; Now we've loaded the setup code and the kernel image.
218 ; Jump to setup code.
221 ; Read a sector from the floppy drive.
222 ; This code (and the rest of this boot sector) will have to
223 ; be re-written at some point so it reads more than one
227 ; - "logical" sector number [bp+8]
228 ; - destination segment [bp+6]
229 ; - destination offset [bp+4]
231 push bp ; set up stack frame
233 pusha ; save all registers
248 ; Sector = log_sec % SECTORS_PER_TRACK
249 ; Head = (log_sec / SECTORS_PER_TRACK) % HEADS
250 mov ax, [bp+8] ; get logical sector number from stack
251 xor dx, dx ; dx is high part of dividend (== 0)
252 mov bx, SECTORS_PER_TRACK ; divisor
253 div bx ; do the division
254 mov [sec], dx ; sector is the remainder
255 and ax, 1 ; same as mod by HEADS==2 (slight hack)
258 ; Track = log_sec / (SECTORS_PER_TRACK*HEADS)
259 mov ax, [bp+8] ; get logical sector number again
260 xor dx, dx ; dx is high part of dividend
261 mov bx, SECTORS_PER_TRACK*2 ; divisor
262 div bx ; do the division
263 mov [track], ax ; track is quotient
278 ; Now, try to actually read the sector from the floppy,
279 ; retrying up to 3 times.
281 mov [num_retries], byte 0
284 mov ax, [bp+6] ; dest segment...
285 mov es, ax ; goes in es
286 mov ax, (0x02 << 8) | 1 ; function = 02h in ah,
288 mov bx, [track] ; track number...
289 mov ch, bl ; goes in ch
290 mov bx, [sec] ; sector number...
291 mov cl, bl ; goes in cl...
292 inc cl ; but it must be 1-based, not 0-based
293 mov bx, [head] ; head number...
294 mov dh, bl ; goes in dh
295 xor dl, dl ; hard code drive=0
296 mov bx, [bp+4] ; offset goes in bx
297 ; (es:bx points to buffer)
298 ; Call the BIOS Read Diskette Sectors service
301 ; If the carry flag is NOT set, then there was no error
305 ; Error - code stored in ah
308 inc byte [num_retries]
309 cmp byte [num_retries], 3
312 ; If we got here, we failed thrice, so we give up
318 popa ; restore all regisiters
319 pop bp ; leave stack frame
322 ; Include utility routines:
324 ; REPLACED WITH FOLLOWING WHICH MUST BE COMPILED 16 FOR USE IN THIS CODE
327 mov cx, 4 ; 4 hex digits
329 rol dx, 4 ; rotate so that lowest 4 bits are used
330 mov ax, 0E0Fh ; ah = request, al = mask for nybble
332 add al, 90h ; convert al to ascii hex (four instructions)
333 daa ; I've spent 1 hour to understand how it works..
342 PrintNL: ; print CR and NL
353 ; ----------------------------------------------------------------------
355 ; ----------------------------------------------------------------------
357 ; These are used by ReadSector
363 ; Used for loops reading sectors from floppy
376 db 0,0x20,0x08 ; ! base = 0x082000
378 dw 0 ; ! limit16,base24 =0
382 db 0,0,0x10 ; ! base = 0x100000
384 dw 0 ; ! limit16,base24 =0
385 dw 0,0,0,0 ; ! BIOS CS
386 dw 0,0,0,0 ; ! BIOS DS
391 ; Padding to make the PFAT Boot Record sit just before the BIOS signature.
392 ;Pad_From_Symbol PFAT_BOOT_RECORD_OFFSET, BeginText
395 ; Describes how to load the setup program and kernel.
396 ; The default values are appropriate for creating a boot
397 ; floppy by concatenating the boot sector, setup program,
398 ; and kernel image. The buildFat program will change
399 ; these values if the boot floppy is formatted as a PFAT
416 ;; part of pfat boot record
418 dw 1 ; by default, setup is at first sector
420 ;; part of pfat boot record
422 dw NUM_SETUP_SECTORS ; number of sectors in setup
424 ;; part of pfat boot record
426 dw 1+NUM_SETUP_SECTORS ; default start sector for kernel
428 ;; part of pfat boot record
435 ; Finish by writing the BIOS signature to mark this as
436 ; a valid boot sector.
437 Pad_From_Symbol BIOS_SIGNATURE_OFFSET, BeginText
438 Signature dw 0xAA55 ; BIOS controls this to ensure this is a boot sector