/bootloader/stage1.asm
Assembly | 243 lines | 199 code | 28 blank | 16 comment | 0 complexity | 8932d093f4233a442b387fbc1a649b54 MD5 | raw file
1; ------------------------------------------------------------- 2; Pokelover and Necro's Hobby OS Bootloader - Stage 1 3; 4; Based on the MikeOS bootloader, which is based on the free 5; bootloader by E Dehling. It looks for the STAGE2.BIN file on 6; a FAT12 filesystem (specifically a floppy). Takes up exactly 7; one sector and MUST end in 0xAA55, which is the bootloader 8; signature. 9; ------------------------------------------------------------- 10 11[BITS 16] ; Let NASM know we want 16 bit mode, even though 12 ; it should be able to detect it on it's own 13jmp short BootStart ; Jump over disk info. 14nop 15 16OEMLabel db "PNOSBOOT" ; Disk label 17BytesPerSector dw 512 ; Bytes per sector 18SectorsPerCluster db 1 ; Sectors per cluster 19ReservedForBoot dw 1 ; Reserved sectors for boot record 20NumberOfFats db 2 ; Number of copies of the FAT 21RootDirEntries dw 224 ; Number of entries in root dir 22 ; (224 * 32 = 7168 = 14 sectors to read) 23LogicalSectors dw 2880 ; Number of logical sectors 24MediumByte db 0F0h ; Medium descriptor byte 25SectorsPerFat dw 9 ; Sectors per FAT 26SectorsPerTrack dw 18 ; Sectors per track (36/cylinder) 27Sides dw 2 ; Number of sides/heads 28HiddenSectors dd 0 ; Number of hidden sectors 29LargeSectors dd 0 ; Number of LBA sectors 30DriveNo dw 0 ; Drive No: 0 31Signature db 41 ; Drive signature: 41 for floppy 32VolumeID dd 00000000h ; Volume ID: any number 33VolumeLabel db "PNOS "; Volume Label: any 11 chars 34FileSystem db "FAT12 " ; File system type: don't change! 35 36BootStart: 37 mov ax, 0x07C0 ; This is our entry point, we're going to set up 38 ; a 4K stack space above this 39 add ax, 544 ; And first we need a 544 paragraph buffer so we don't 40 ; interfere with this or the disk buffer! 41 cli ; Disable interrupts while changing stack 42 mov ss, ax 43 mov sp, 4096 44 sti ; Restore interrupts 45 mov ax, 0x07C0 ; Set data segment to where we've loaded to make things easier 46 mov ds, ax 47 mov byte [BootDev], dl ; Save boot device number 48 mov eax, 0 ; Older BIOSes may need this 49 50FloppyOk: ; Ready to read first block of data 51 mov ax, 19 ; Root dir starts at logical sector 19 52 call l2hts 53 mov si, Buffer ; Set ES:BX to our buffer 54 mov bx, ds 55 mov es, bx 56 mov bx, si 57 mov ah, 2 ; int 0x13 params: Read floppy sectors 58 mov al, 14 ; 14 floppy sectors to read 59 pusha ; Push registers to stack to prepare for loop 60 61ReadRootDir: 62 popa ; Just in case int 0x13 modified anything 63 pusha 64 stc ; Not all BIOSes set properly on error 65 int 0x13 ; Read sectors 66 jnc SearchDir ; If read was OK, skip ahead 67 call ResetFloppy ; If not, reset floppy and try again 68 jnc ReadRootDir ; Did the floppy reset fine? 69 jmp Reboot ; If it didn't, double error, restart 70 71SearchDir: 72 popa 73 mov ax, ds ; Root dir is now in [Buffer] 74 mov es, ax ; Give this info to DI 75 mov di, Buffer 76 mov cx, word [RootDirEntries] ; Search all entries 77 mov ax, 0 ; Start at 0 78 79NextRootEntry: 80 xchg cx, dx ; We use cx in the inner loop 81 mov si, BootFileName ; Start searching for stage2 filename 82 mov cx, 11 83 rep cmpsb 84 je FoundFileToLoad ; Pointer DI will be at offset 11 85 add ax, 32 ; Bump by one entry (32 bytes per entry) 86 mov di, Buffer ; Point to next entry 87 add di, ax 88 xchg dx, cx ; Get cx back 89 loop NextRootEntry 90 mov si, NoFile 91 call PrintString 92 jmp Reboot 93 94FoundFileToLoad: ; Let's load the FAT into RAM 95 mov ax, word[es:di+0x0F] ; Offset 11+15=26 contains our first cluster 96 mov word[Cluster], ax 97 mov ax, 1 ; Sector 1 = first sector of FAT 98 call l2hts 99 mov di, Buffer 100 mov bx, di ; ES:BX points to our buffer 101 mov ah, 2 ; int 0x13 params: read FAT sectors 102 mov al, 9 ; All 9 sectors of first FAT 103 pusha ; Push registers to stack to prepare for loop 104 105ReadFat: 106 popa ; In case int 0x13 modified anything 107 pusha 108 stc 109 int 0x13 ; Read sectors using BIOS 110 jnc ReadFatOkay ; If read was okay, skip ahead 111 call ResetFloppy ; Otherwise, reset floppy and try again 112 jnc ReadFat ; Floppy reset OK? 113 mov si, DiskError 114 call PrintString ; If not, print an error and reboot 115 jmp Reboot 116 117ReadFatOkay: 118 popa 119 mov ax, 0x2000 ; Segment where we're loading kernel 120 mov es, ax 121 mov bx, 0 122 mov ah, 2 ; int 0x13 read params 123 mov al, 1 124 push ax ; Save in case it gets lost 125 126; We're gonna load the FAT from the disk now 127 128LoadFileSector: 129 mov ax, word [Cluster] ; Convert sector to logical 130 add ax, 31 131 call l2hts 132 mov ax, 0x2000 ; Set buffer past what we've read 133 mov es, ax 134 mov bx, word [Pointer] 135 pop ax ; Save in case it gets lost 136 push ax 137 stc 138 int 0x13 139 jnc CalculateNextCluster ; If there's no error 140 call ResetFloppy ; If there is, reset floppy 141 jmp LoadFileSector 142 143CalculateNextCluster: 144 mov ax, [Cluster] 145 mov dx, 0 146 mov bx, 3 147 mul bx 148 mov bx, 2 149 div bx ; DX = [Cluster] mod 2 150 mov si, Buffer 151 add si, ax ; AX = word in FAT for 12 bit entry 152 mov ax, word [ds:si] 153 or dx, dx ; If DX = 0 [Cluster] is even; if 1, [Cluster] is odd 154 jz even ; If [Cluster] is even, drop last 4 bits of word with 155 ; next clusters; if odd, drop first 4 bits 156 157odd: 158 shr ax, 4 159 jmp short NextClusterCont 160 161even: 162 and ax, 0x0FFF ; Mask out final four bits 163 164NextClusterCont: 165 mov word [Cluster], ax ; Store cluster 166 cmp ax, 0x0FF8 ; 0xFF8 = end of file in FAT12 167 jae end 168 add word [Pointer], 512 ; Increase buffer point 1 sector 169 jmp LoadFileSector 170 171end: ; Hooray, we've reached the end and got the file to load! 172 pop ax ; Clean up stack, as we pushed AX earlier 173 mov dl, byte[BootDev] ; Provide Stage2/Kernel with boot device info 174 mov si, NoFile 175 call PrintString 176 jmp 0x2000:0x0 ; Jump to entry point of stage2! 177 178; Subroutines below 179 180Reboot: 181 mov ax, 0 182 int 0x16 ; Wait for keystroke 183 mov ax, 0 184 int 0x19 ; Reboot 185 186PrintString: 187 pusha 188 mov ax, 0x0E 189 190.repeat: 191 lodsb 192 cmp al, 0 193 je .done ; If char is 0, we've reached the end of the string 194 int 0x10 ; Otherwise, we print it 195 jmp short .repeat 196 197.done: 198 popa 199 ret 200 201ResetFloppy: ; Carry set on error 202 push ax 203 push dx 204 mov ax, 0 205 mov dl, byte [BootDev] 206 stc 207 int 0x13 208 pop dx 209 pop ax 210 ret 211 212l2hts: ; Calculate head, track and sector for int 0x13 213 push bx 214 push ax 215 mov bx, ax 216 mov dx, 0 217 div word [SectorsPerTrack] 218 add dl, 0x01 219 mov cl, dl 220 mov ax, bx 221 mov dx, 0 222 div word [SectorsPerTrack] 223 mov dx, 0 224 div word [Sides] 225 mov dh, dl 226 mov ch, al 227 pop ax 228 pop bx 229 mov dl, byte [BootDev] 230 ret 231 232 233BootFileName db "STAGE2 BIN" ; Stage2 filename 234DiskError db "Disk error! Press any key to reboot...", 0 ; Something went wrong! 235NoFile db "No STAGE2.BIN", 0 ; Someone forgot to add something... 236BootDev db 0 237Cluster dw 0 238Pointer dw 0 239 240times 510-($-$$) db 0 241dw 0AA55h 242 243Buffer: