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1.1 ! root 1: | ! 2: | bootsect.s (C) 1991 Linus Torvalds ! 3: | ! 4: | bootsect.s is loaded at 0x7c00 by the bios-startup routines, and moves ! 5: | iself out of the way to address 0x90000, and jumps there. ! 6: | ! 7: | It then loads 'setup' directly after itself (0x90200), and the system ! 8: | at 0x10000, using BIOS interrupts. ! 9: | ! 10: | NOTE! currently system is at most 8*65536 bytes long. This should be no ! 11: | problem, even in the future. I want to keep it simple. This 512 kB ! 12: | kernel size should be enough, especially as this doesn't contain the ! 13: | buffer cache as in minix ! 14: | ! 15: | The loader has been made as simple as possible, and continuos ! 16: | read errors will result in a unbreakable loop. Reboot by hand. It ! 17: | loads pretty fast by getting whole sectors at a time whenever possible. ! 18: ! 19: .globl begtext, begdata, begbss, endtext, enddata, endbss ! 20: .text ! 21: begtext: ! 22: .data ! 23: begdata: ! 24: .bss ! 25: begbss: ! 26: .text ! 27: ! 28: SETUPLEN = 4 | nr of setup-sectors ! 29: BOOTSEG = 0x07c0 | original address of boot-sector ! 30: INITSEG = 0x9000 | we move boot here - out of the way ! 31: SETUPSEG = 0x9020 | setup starts here ! 32: SYSSEG = 0x1000 | system loaded at 0x10000 (65536). ! 33: ENDSEG = SYSSEG + SYSSIZE | where to stop loading ! 34: ! 35: | ROOT_DEV: 0x000 - same type of floppy as boot. ! 36: | 0x301 - first partition on first drive etc ! 37: ROOT_DEV = 0 | 0x306 ! 38: ! 39: entry start ! 40: start: ! 41: mov ax,#BOOTSEG ! 42: mov ds,ax ! 43: mov ax,#INITSEG ! 44: mov es,ax ! 45: mov cx,#256 ! 46: sub si,si ! 47: sub di,di ! 48: rep ! 49: movw ! 50: jmpi go,INITSEG ! 51: go: mov ax,cs ! 52: mov ds,ax ! 53: mov es,ax ! 54: | put stack at 0x9ff00. ! 55: mov ss,ax ! 56: mov sp,#0xFF00 | arbitrary value >>512 ! 57: ! 58: | load the setup-sectors directly after the bootblock. ! 59: | Note that 'es' is already set up. ! 60: ! 61: load_setup: ! 62: mov dx,#0x0000 | drive 0, head 0 ! 63: mov cx,#0x0002 | sector 2, track 0 ! 64: mov bx,#0x0200 | address = 512, in INITSEG ! 65: mov ax,#0x0200+SETUPLEN | service 2, nr of sectors ! 66: int 0x13 | read it ! 67: jnc ok_load_setup | ok - continue ! 68: mov dx,#0x0000 ! 69: mov ax,#0x0000 | reset the diskette ! 70: int 0x13 ! 71: j load_setup ! 72: ! 73: ok_load_setup: ! 74: ! 75: | Get disk drive parameters, specifically nr of sectors/track ! 76: ! 77: mov dl,#0x00 ! 78: mov ax,#0x0800 | AH=8 is get drive parameters ! 79: int 0x13 ! 80: mov ch,#0x00 ! 81: seg cs ! 82: mov sectors,cx ! 83: mov ax,#INITSEG ! 84: mov es,ax ! 85: ! 86: | Print some inane message ! 87: ! 88: mov ah,#0x03 | read cursor pos ! 89: xor bh,bh ! 90: int 0x10 ! 91: ! 92: mov cx,#24 ! 93: mov bx,#0x0007 | page 0, attribute 7 (normal) ! 94: mov bp,#msg1 ! 95: mov ax,#0x1301 | write string, move cursor ! 96: int 0x10 ! 97: ! 98: | ok, we've written the message, now ! 99: | we want to load the system (at 0x10000) ! 100: ! 101: mov ax,#SYSSEG ! 102: mov es,ax | segment of 0x010000 ! 103: call read_it ! 104: call kill_motor ! 105: ! 106: | After that we check which root-device to use. If the device is ! 107: | defined (!= 0), nothing is done and the given device is used. ! 108: | Otherwise, either /dev/PS0 (2,28) or /dev/at0 (2,8), depending ! 109: | on the number of sectors that the BIOS reports currently. ! 110: ! 111: seg cs ! 112: mov ax,root_dev ! 113: cmp ax,#0 ! 114: jne root_defined ! 115: seg cs ! 116: mov bx,sectors ! 117: mov ax,#0x0208 | /dev/ps0 - 1.2Mb ! 118: cmp bx,#15 ! 119: je root_defined ! 120: mov ax,#0x021c | /dev/PS0 - 1.44Mb ! 121: cmp bx,#18 ! 122: je root_defined ! 123: undef_root: ! 124: jmp undef_root ! 125: root_defined: ! 126: seg cs ! 127: mov root_dev,ax ! 128: ! 129: | after that (everyting loaded), we jump to ! 130: | the setup-routine loaded directly after ! 131: | the bootblock: ! 132: ! 133: jmpi 0,SETUPSEG ! 134: ! 135: | This routine loads the system at address 0x10000, making sure ! 136: | no 64kB boundaries are crossed. We try to load it as fast as ! 137: | possible, loading whole tracks whenever we can. ! 138: | ! 139: | in: es - starting address segment (normally 0x1000) ! 140: | ! 141: sread: .word 1+SETUPLEN | sectors read of current track ! 142: head: .word 0 | current head ! 143: track: .word 0 | current track ! 144: ! 145: read_it: ! 146: mov ax,es ! 147: test ax,#0x0fff ! 148: die: jne die | es must be at 64kB boundary ! 149: xor bx,bx | bx is starting address within segment ! 150: rp_read: ! 151: mov ax,es ! 152: cmp ax,#ENDSEG | have we loaded all yet? ! 153: jb ok1_read ! 154: ret ! 155: ok1_read: ! 156: seg cs ! 157: mov ax,sectors ! 158: sub ax,sread ! 159: mov cx,ax ! 160: shl cx,#9 ! 161: add cx,bx ! 162: jnc ok2_read ! 163: je ok2_read ! 164: xor ax,ax ! 165: sub ax,bx ! 166: shr ax,#9 ! 167: ok2_read: ! 168: call read_track ! 169: mov cx,ax ! 170: add ax,sread ! 171: seg cs ! 172: cmp ax,sectors ! 173: jne ok3_read ! 174: mov ax,#1 ! 175: sub ax,head ! 176: jne ok4_read ! 177: inc track ! 178: ok4_read: ! 179: mov head,ax ! 180: xor ax,ax ! 181: ok3_read: ! 182: mov sread,ax ! 183: shl cx,#9 ! 184: add bx,cx ! 185: jnc rp_read ! 186: mov ax,es ! 187: add ax,#0x1000 ! 188: mov es,ax ! 189: xor bx,bx ! 190: jmp rp_read ! 191: ! 192: read_track: ! 193: push ax ! 194: push bx ! 195: push cx ! 196: push dx ! 197: mov dx,track ! 198: mov cx,sread ! 199: inc cx ! 200: mov ch,dl ! 201: mov dx,head ! 202: mov dh,dl ! 203: mov dl,#0 ! 204: and dx,#0x0100 ! 205: mov ah,#2 ! 206: int 0x13 ! 207: jc bad_rt ! 208: pop dx ! 209: pop cx ! 210: pop bx ! 211: pop ax ! 212: ret ! 213: bad_rt: mov ax,#0 ! 214: mov dx,#0 ! 215: int 0x13 ! 216: pop dx ! 217: pop cx ! 218: pop bx ! 219: pop ax ! 220: jmp read_track ! 221: ! 222: /* ! 223: * This procedure turns off the floppy drive motor, so ! 224: * that we enter the kernel in a known state, and ! 225: * don't have to worry about it later. ! 226: */ ! 227: kill_motor: ! 228: push dx ! 229: mov dx,#0x3f2 ! 230: mov al,#0 ! 231: outb ! 232: pop dx ! 233: ret ! 234: ! 235: sectors: ! 236: .word 0 ! 237: ! 238: msg1: ! 239: .byte 13,10 ! 240: .ascii "Loading system ..." ! 241: .byte 13,10,13,10 ! 242: ! 243: .org 508 ! 244: root_dev: ! 245: .word ROOT_DEV ! 246: boot_flag: ! 247: .word 0xAA55 ! 248: ! 249: .text ! 250: endtext: ! 251: .data ! 252: enddata: ! 253: .bss ! 254: endbss:
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