# # Copyright (c) 1999 Robert Nordier # All rights reserved. # # Redistribution and use in source and binary forms are freely # permitted provided that the above copyright notice and this # paragraph and the following disclaimer are duplicated in all # such forms. # # This software is provided "AS IS" and without any express or # implied warranties, including, without limitation, the implied # warranties of merchantability and fitness for a particular # purpose. # # $Id: mbr.s,v 1.1.1.1 1999/05/24 22:36:24 rnordier Exp $ # Master boot record .set LOAD,0x7c00 # Load address .set EXEC,0x600 # Execution address .set PT_OFF,0x1be # Partition table .set MAGIC,0xaa55 # Magic: bootable .set NDRIVE,0x8 # Drives to support .globl start # Entry point start: cld # String ops inc xorl %eax,%eax # Zero movl %eax,%es # Address movl %eax,%ds # data cli # Disable interrupts movl %eax,%ss # Set up movwir(LOAD,_sp) # stack sti # Enable interrupts movwir(main-EXEC+LOAD,_si) # Source movwir(main,_di) # Destination movwir(0x200-(main-start),_cx) # Byte count rep # Relocate movsb # code jmpnwi(main-LOAD+EXEC) # To relocated code main: xorl %esi,%esi # No active partition movwir(partbl,_bx) # Partition table movb $0x4,%cl # Number of entries main.1: cmpbr0(_ch,_bx_) # Null entry? je main.2 # Yes jg err_pt # If 0x1..0x7f testl %esi,%esi # Active already found? jnz err_pt # Yes movl %ebx,%esi # Point to active main.2: addb $0x10,%bl # Till loop main.1 # done testl %esi,%esi # Active found? jnz main.3 # Yes int $0x18 # BIOS: Diskless boot main.3: cmpb $0x80,%dl # Drive valid? jb main.4 # No cmpb $0x80+NDRIVE,%dl # Within range? jb main.5 # Yes main.4: movb0r(_si_,_dl) # Load drive main.5: movb1r(0x1,_si_,_dh) # Load head movw1r(0x2,_si_,_cx) # Load cylinder:sector movwir(LOAD,_bx) # Transfer buffer movwir(0x201,_ax) # BIOS: Read from int $0x13 # disk jc err_rd # If error cmpwi2(MAGIC,0x1fe,_bx_) # Bootable? jne err_os # No jmp *%ebx # Invoke bootstrap err_pt: movwir(msg_pt,_si) # "Invalid partition jmp putstr # table" err_rd: movwir(msg_rd,_si) # "Error loading jmp putstr # operating system" err_os: movwir(msg_os,_si) # "Missing operating jmp putstr # system" putstr.0: movwir(0x7,_bx) # Page:attribute movb $0xe,%ah # BIOS: Display int $0x10 # character putstr: lodsb # Get character testb %al,%al # End of string? jnz putstr.0 # No putstr.1: jmp putstr.1 # Await reset msg_pt: .asciz "Invalid partition table" msg_rd: .asciz "Error loading operating system" msg_os: .asciz "Missing operating system" .org PT_OFF partbl: .fill 0x10,0x4,0x0 # Partition table .word MAGIC # Magic number