├── .gdbinit ├── .gitignore ├── LICENSE ├── README.md ├── i386-32bit.xml ├── makefile ├── src ├── boot12.asm ├── boot16.asm └── demo.asm └── target.xml /.gdbinit: -------------------------------------------------------------------------------- 1 | # .gdbinit 2 | # 3 | # Copyright (c) 2017-2023, Joshua Riek 4 | # 5 | # This program is free software: you can redistribute it and/or modify 6 | # it under the terms of the GNU General Public License as published by 7 | # the Free Software Foundation, either version 3 of the License, or 8 | # (at your option) any later version. 9 | # 10 | # This program is distributed in the hope that it will be useful, 11 | # but WITHOUT ANY WARRANTY; without even the implied warranty of 12 | # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 13 | # GNU General Public License for more details. 14 | # 15 | # You should have received a copy of the GNU General Public License 16 | # along with this program. If not, see . 17 | # 18 | 19 | set tdesc filename target.xml 20 | 21 | target remote localhost:1234 22 | 23 | set confirm off 24 | set verbose off 25 | set prompt \033[31mgdb$ \033[0m 26 | 27 | set output-radix 0d10 28 | set input-radix 0d10 29 | 30 | # These make gdb never pause in its output 31 | set height 0 32 | set width 0 33 | set pagination off 34 | 35 | # Intel syntax 36 | set disassembly-flavor intel 37 | 38 | # Real mode 39 | set architecture i8086 40 | 41 | set $SHOW_CONTEXT = 1 42 | 43 | set $REAL_MODE = 1 44 | 45 | # By default A20 is present 46 | set $ADDRESS_MASK = 0x1FFFFF 47 | 48 | # nb of instructions to display 49 | set $CODE_SIZE = 10 50 | 51 | define enable-a20 52 | set $ADDRESS_MASK = 0x1FFFFF 53 | end 54 | define disable-a20 55 | set $ADDRESS_MASK = 0x0FFFFF 56 | end 57 | 58 | # get current register values 59 | define compute_regs 60 | set $rax = ((unsigned long)$eax & 0xFFFF) 61 | set $rbx = ((unsigned long)$ebx & 0xFFFF) 62 | set $rcx = ((unsigned long)$ecx & 0xFFFF) 63 | set $rdx = ((unsigned long)$edx & 0xFFFF) 64 | set $rsi = ((unsigned long)$esi & 0xFFFF) 65 | set $rdi = ((unsigned long)$edi & 0xFFFF) 66 | set $rbp = ((unsigned long)$ebp & 0xFFFF) 67 | set $rsp = ((unsigned long)$esp & 0xFFFF) 68 | set $rcs = ((unsigned long)$cs & 0xFFFF) 69 | set $rds = ((unsigned long)$ds & 0xFFFF) 70 | set $res = ((unsigned long)$es & 0xFFFF) 71 | set $rss = ((unsigned long)$ss & 0xFFFF) 72 | set $rip = ((((unsigned long)$cs & 0xFFFF) << 4) + ((unsigned long)$eip & 0xFFFF)) & $ADDRESS_MASK 73 | set $r_ss_sp = ((((unsigned long)$ss & 0xFFFF) << 4) + ((unsigned long)$esp & 0xFFFF)) & $ADDRESS_MASK 74 | set $r_ss_bp = ((((unsigned long)$ss & 0xFFFF) << 4) + ((unsigned long)$ebp & 0xFFFF)) & $ADDRESS_MASK 75 | end 76 | 77 | # print CPU registers 78 | define print_regs 79 | printf " AX: 0x%04X BX: 0x%04X ", $rax, $rbx 80 | printf " CX: 0x%04X DX: 0x%04X\n", $rcx, $rdx 81 | printf " SI: 0x%04X DI: 0x%04X ", $rsi, $rdi 82 | printf " SP: 0x%04X BP: 0x%04X\n", $rsp, $rbp 83 | printf " CS: 0x%04X DS: 0x%04X ", $rcs, $rds 84 | printf " ES: 0x%04X SS: 0x%04X\n", $res, $rss 85 | printf "\n" 86 | printf " IP: 0x%04X EIP: 0x%08X\n", ((unsigned short)$eip & 0xFFFF), $eip 87 | printf " CS:IP: %04X:%04X (0x%05X)\n", $rcs, ((unsigned short)$eip & 0xFFFF), $rip 88 | printf " SS:SP: %04X:%04X (0x%05X)\n", $rss, $rsp, $r_ss_sp 89 | printf " SS:BP: %04X:%04X (0x%05X)\n", $rss, $rbp, $r_ss_bp 90 | end 91 | 92 | # print eflags registers 93 | define print_eflags 94 | printf "\n OF <%d> DF <%d> IF <%d> TF <%d>",\ 95 | (($eflags >> 0xB) & 1), (($eflags >> 0xA) & 1), \ 96 | (($eflags >> 9) & 1), (($eflags >> 8) & 1) 97 | printf " SF <%d> ZF <%d> AF <%d> PF <%d> CF <%d>\n",\ 98 | (($eflags >> 7) & 1), (($eflags >> 6) & 1),\ 99 | (($eflags >> 4) & 1), (($eflags >> 2) & 1), ($eflags & 1) 100 | printf " ID <%d> VIP <%d> VIF <%d> AC <%d>",\ 101 | (($eflags >> 0x15) & 1), (($eflags >> 0x14) & 1), \ 102 | (($eflags >> 0x13) & 1), (($eflags >> 0x12) & 1) 103 | printf " VM <%d> RF <%d> NT <%d> IOPL <%d>\n",\ 104 | (($eflags >> 0x11) & 1), (($eflags >> 0x10) & 1),\ 105 | (($eflags >> 0xE) & 1), (($eflags >> 0xC) & 3) 106 | end 107 | 108 | # dump content of memory in words 109 | # arg0 : addr 110 | # arg1 : nb of words 111 | define _dump_memw 112 | if $argc < 2 113 | printf "Arguments: address number_of_words\n" 114 | else 115 | set $_nb = $arg1 116 | set $_i = 0 117 | set $_addr = $arg0 118 | while ($_i < $_nb) 119 | printf "%04X ", *((unsigned short*)$_addr + $_i) 120 | set $_i++ 121 | end 122 | end 123 | end 124 | 125 | # display data at given address 126 | define print_data 127 | if ($argc > 0) 128 | set $wil = 4 129 | if ($argc > 2) 130 | set $wil = $arg2 131 | end 132 | 133 | set $seg = $arg0 134 | set $off = $arg1 135 | 136 | set $raddr = ($arg0 << 16) + $arg1 137 | set $maddr = ($arg0 << 4) + $arg1 138 | 139 | set $w = 16 140 | set $i = (int)0 141 | while ($i < $wil) 142 | printf "%04X %04X: ", ($seg), ($off+ $i * $w) 143 | set $j = (int)0 144 | while ($j < $w) 145 | printf "%02X ", *(unsigned char*)($maddr + $i * $w + $j) 146 | set $j++ 147 | end 148 | printf " " 149 | set $j = (int)0 150 | while ($j < $w) 151 | set $c = *(unsigned char*)($maddr + $i * $w + $j) 152 | if ($c > 32) && ($c < 128) 153 | printf "%c", $c 154 | else 155 | printf "." 156 | end 157 | set $j++ 158 | end 159 | printf "\n" 160 | set $i++ 161 | end 162 | end 163 | end 164 | 165 | # print context window, i.e. regs, stack, ds:esi and disassemble cs:eip 166 | define context 167 | printf "--------------------------------------------------------------------------------[stack]\n" 168 | _dump_memw $r_ss_sp 8 169 | printf "\n" 170 | set $_a = $r_ss_sp + 16 171 | _dump_memw $_a 8 172 | printf "\n" 173 | printf "--------------------------------------------------------------------------------[ds:si]\n" 174 | print_data $ds $rsi 175 | printf "--------------------------------------------------------------------------------[es:di]\n" 176 | print_data $es $rdi 177 | printf "--------------------------------------------------------------------------------[regs]\n" 178 | print_regs 179 | print_eflags 180 | printf "--------------------------------------------------------------------------------[code]\n" 181 | set $_code_size = $CODE_SIZE 182 | if ($_code_size > 0) 183 | x /i $rip 184 | set $_code_size-- 185 | end 186 | while ($_code_size > 0) 187 | x /i 188 | set $_code_size-- 189 | end 190 | printf "--------------------------------------------------------------------------------------\n" 191 | end 192 | 193 | define hook-stop 194 | compute_regs 195 | if ($SHOW_CONTEXT > 0) 196 | context 197 | end 198 | end 199 | document hook-stop 200 | !!! 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No Surrender of Others' Freedom. 541 | 542 | If conditions are imposed on you (whether by court order, agreement or 543 | otherwise) that contradict the conditions of this License, they do not 544 | excuse you from the conditions of this License. If you cannot convey a 545 | covered work so as to satisfy simultaneously your obligations under this 546 | License and any other pertinent obligations, then as a consequence you may 547 | not convey it at all. For example, if you agree to terms that obligate you 548 | to collect a royalty for further conveying from those to whom you convey 549 | the Program, the only way you could satisfy both those terms and this 550 | License would be to refrain entirely from conveying the Program. 551 | 552 | 13. Use with the GNU Affero General Public License. 553 | 554 | Notwithstanding any other provision of this License, you have 555 | permission to link or combine any covered work with a work licensed 556 | under version 3 of the GNU Affero General Public License into a single 557 | combined work, and to convey the resulting work. The terms of this 558 | License will continue to apply to the part which is the covered work, 559 | but the special requirements of the GNU Affero General Public License, 560 | section 13, concerning interaction through a network will apply to the 561 | combination as such. 562 | 563 | 14. Revised Versions of this License. 564 | 565 | The Free Software Foundation may publish revised and/or new versions of 566 | the GNU General Public License from time to time. Such new versions will 567 | be similar in spirit to the present version, but may differ in detail to 568 | address new problems or concerns. 569 | 570 | Each version is given a distinguishing version number. If the 571 | Program specifies that a certain numbered version of the GNU General 572 | Public License "or any later version" applies to it, you have the 573 | option of following the terms and conditions either of that numbered 574 | version or of any later version published by the Free Software 575 | Foundation. If the Program does not specify a version number of the 576 | GNU General Public License, you may choose any version ever published 577 | by the Free Software Foundation. 578 | 579 | If the Program specifies that a proxy can decide which future 580 | versions of the GNU General Public License can be used, that proxy's 581 | public statement of acceptance of a version permanently authorizes you 582 | to choose that version for the Program. 583 | 584 | Later license versions may give you additional or different 585 | permissions. However, no additional obligations are imposed on any 586 | author or copyright holder as a result of your choosing to follow a 587 | later version. 588 | 589 | 15. Disclaimer of Warranty. 590 | 591 | THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY 592 | APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT 593 | HOLDERS AND/OR OTHER PARTIES PROVIDE THE PROGRAM "AS IS" WITHOUT WARRANTY 594 | OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, 595 | THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 596 | PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM 597 | IS WITH YOU. SHOULD THE PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF 598 | ALL NECESSARY SERVICING, REPAIR OR CORRECTION. 599 | 600 | 16. Limitation of Liability. 601 | 602 | IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING 603 | WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MODIFIES AND/OR CONVEYS 604 | THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY 605 | GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE 606 | USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED TO LOSS OF 607 | DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD 608 | PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER PROGRAMS), 609 | EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF 610 | SUCH DAMAGES. 611 | 612 | 17. Interpretation of Sections 15 and 16. 613 | 614 | If the disclaimer of warranty and limitation of liability provided 615 | above cannot be given local legal effect according to their terms, 616 | reviewing courts shall apply local law that most closely approximates 617 | an absolute waiver of all civil liability in connection with the 618 | Program, unless a warranty or assumption of liability accompanies a 619 | copy of the Program in return for a fee. 620 | 621 | END OF TERMS AND CONDITIONS 622 | 623 | How to Apply These Terms to Your New Programs 624 | 625 | If you develop a new program, and you want it to be of the greatest 626 | possible use to the public, the best way to achieve this is to make it 627 | free software which everyone can redistribute and change under these terms. 628 | 629 | To do so, attach the following notices to the program. It is safest 630 | to attach them to the start of each source file to most effectively 631 | state the exclusion of warranty; and each file should have at least 632 | the "copyright" line and a pointer to where the full notice is found. 633 | 634 | {one line to give the program's name and a brief idea of what it does.} 635 | Copyright (C) 2018 {name of author} 636 | 637 | This program is free software: you can redistribute it and/or modify 638 | it under the terms of the GNU General Public License as published by 639 | the Free Software Foundation, either version 3 of the License, or 640 | (at your option) any later version. 641 | 642 | This program is distributed in the hope that it will be useful, 643 | but WITHOUT ANY WARRANTY; without even the implied warranty of 644 | MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 645 | GNU General Public License for more details. 646 | 647 | You should have received a copy of the GNU General Public License 648 | along with this program. If not, see . 649 | 650 | Also add information on how to contact you by electronic and paper mail. 651 | 652 | If the program does terminal interaction, make it output a short 653 | notice like this when it starts in an interactive mode: 654 | 655 | NASM-Bootloader Copyright (C) 2018 Joshua Riek 656 | This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'. 657 | This is free software, and you are welcome to redistribute it 658 | under certain conditions; type `show c' for details. 659 | 660 | The hypothetical commands `show w' and `show c' should show the appropriate 661 | parts of the General Public License. Of course, your program's commands 662 | might be different; for a GUI interface, you would use an "about box". 663 | 664 | You should also get your employer (if you work as a programmer) or school, 665 | if any, to sign a "copyright disclaimer" for the program, if necessary. 666 | For more information on this, and how to apply and follow the GNU GPL, see 667 | . 668 | 669 | The GNU General Public License does not permit incorporating your program 670 | into proprietary programs. If your program is a subroutine library, you 671 | may consider it more useful to permit linking proprietary applications with 672 | the library. If this is what you want to do, use the GNU Lesser General 673 | Public License instead of this License. But first, please read 674 | . -------------------------------------------------------------------------------- /README.md: -------------------------------------------------------------------------------- 1 | ## Overview 2 | 3 | A collection of small 512 byte programs that are capable of finding, 4 | loading, then executing a program on a FAT12/16 formatted floppy or hard disk 5 | (including USB and CDs). Typically, this would be used as a boot sector 6 | for an operating system, second stage bootloader, or low level kernel. 7 | 8 | ## Limitations and Implementation Details 9 | 10 | Requires an i8086 or better CPU. 11 | 12 | This bootloader may allocate up to 128KB of RAM in order to load the entire 13 | File Allocation Table (FAT) into memory; therefore, leaving approximately 14 | 400KB for loading your program or kernel (assuming at least 640KB of available 15 | conventional memory). The bootloader will reallocate itself to the top of 16 | conventional memory, allocate space for the stack and FAT. Then finally search for and 17 | load the program *DEMO.BIN* at the physical address specified by *LOAD_ADDR*. 18 | 19 | ## Requirements 20 | 21 | Please install the packages below, or type: 22 | ``` 23 | sudo apt-get install gdb nasm qemu dosfstools mtools 24 | ``` 25 | > This project uses an optional i686-elf cross-compiler, you can click 26 | [here](https://wiki.osdev.org/GCC_Cross-Compiler) for more 27 | information on compiling it yourself, or use some precompiled binaries 28 | [here](https://github.com/lordmilko/i686-elf-tools/releases). 29 | 30 | ## Building 31 | 32 | To checkout the source and build: 33 | ``` 34 | git clone https://github.com/Joshua-Riek/x86-bootloader 35 | cd x86-bootloader 36 | make 37 | ``` 38 | 39 | ## Virtual Machine 40 | 41 | To run the bootloader in a virtual machine: 42 | ``` 43 | make run 44 | ``` 45 | 46 | ## Virtual Machine Debugging 47 | 48 | Start a virtual machine with a GDB stub: 49 | ``` 50 | make debug 51 | ``` 52 | 53 | Open another ternimal and connect to the virtual machine's GDB stub: 54 | ``` 55 | make gdb 56 | ``` 57 | > For debug symbols to be generated, you must compile with an i686-elf cross-compiller. 58 | -------------------------------------------------------------------------------- /i386-32bit.xml: -------------------------------------------------------------------------------- 1 | 2 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | 22 | 23 | 24 | 25 | 26 | 27 | 28 | 29 | 30 | 31 | 32 | 33 | 34 | 35 | 36 | 37 | 38 | 39 | 40 | 41 | 42 | 43 | 44 | 45 | 46 | 47 | 48 | 49 | 50 | 51 | 52 | 53 | 54 | 55 | 56 | 57 | 58 | 62 | 63 | 64 | 65 | 66 | 67 | 68 | 69 | 70 | 71 | 72 | 73 | 74 | 75 | 76 | 77 | 78 | 79 | 80 | 81 | 82 | 86 | 87 | 88 | 89 | 90 | 91 | 92 | 93 | 94 | 95 | 96 | 97 | 98 | 99 | 100 | 101 | 102 | 103 | 104 | 105 | 106 | 107 | 108 | 109 | 110 | 111 | 112 | 113 | 114 | 115 | 116 | 117 | 118 | 119 | 120 | 121 | 122 | 123 | 124 | 125 | 126 | 127 | 128 | 129 | 130 | 131 | 132 | 133 | 134 | 135 | 136 | 137 | 138 | 139 | 140 | 141 | 142 | 143 | 144 | 145 | 146 | 147 | 148 | 150 | 151 | 152 | 153 | 154 | 155 | 156 | 157 | 158 | 159 | 160 | 161 | 162 | 163 | 164 | 165 | 166 | 167 | 168 | 169 | 170 | 171 | 172 | 173 | 174 | 175 | 176 | 177 | 178 | 179 | 180 | 181 | 182 | 183 | 184 | 185 | 186 | 187 | 188 | 189 | 190 | 191 | 192 | 193 | -------------------------------------------------------------------------------- /makefile: -------------------------------------------------------------------------------- 1 | # makefile 2 | # 3 | # Copyright (c) 2017-2023, Joshua Riek 4 | # 5 | # This program is free software: you can redistribute it and/or modify 6 | # it under the terms of the GNU General Public License as published by 7 | # the Free Software Foundation, either version 3 of the License, or 8 | # (at your option) any later version. 9 | # 10 | # This program is distributed in the hope that it will be useful, 11 | # but WITHOUT ANY WARRANTY; without even the implied warranty of 12 | # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 13 | # GNU General Public License for more details. 14 | # 15 | # You should have received a copy of the GNU General Public License 16 | # along with this program. If not, see . 17 | # 18 | 19 | # Build tools 20 | CC := i686-elf-gcc 21 | LD := i686-elf-ld 22 | AR := i686-elf-ar 23 | OBJCOPY := i686-elf-objcopy 24 | NASM := nasm 25 | 26 | # Output directory 27 | SRCDIR = ./src 28 | OBJDIR = ./obj 29 | BINDIR = ./bin 30 | 31 | # Build flags 32 | CFLAGS += 33 | LDFLAGS += 34 | ARFLAGS += 35 | LDFLAGS += -m elf_i386 -Ttext=0x0000 36 | NASMFLAGS += -f elf -g3 -F dwarf 37 | OBJCOPYFLAGS += -O binary 38 | 39 | 40 | # Set phony targets 41 | .PHONY: all clean clobber demo bootloader image debug run gdb 42 | 43 | 44 | # Rule to make targets 45 | all: bootloader demo image 46 | 47 | 48 | # Makefile target for both bootloaders 49 | ifeq ($(and $(shell which $(LD)),$(shell which $(OBJCOPY))),) 50 | bootloader: $(BINDIR)/boot12.bin $(BINDIR)/boot16.bin 51 | 52 | $(BINDIR)/boot12.bin: $(SRCDIR)/boot12.asm | $(BINDIR) 53 | $(NASM) $^ -f bin -o $@ 54 | 55 | $(BINDIR)/boot16.bin: $(SRCDIR)/boot16.asm | $(BINDIR) 56 | $(NASM) $^ -f bin -o $@ 57 | else 58 | bootloader: $(BINDIR)/boot12.bin $(BINDIR)/boot16.bin 59 | 60 | $(BINDIR)/boot12.bin: $(BINDIR)/boot12.elf 61 | $(OBJCOPY) $^ $(OBJCOPYFLAGS) $@ 62 | 63 | $(BINDIR)/boot12.elf: $(OBJDIR)/boot12.o | $(BINDIR) 64 | $(LD) $^ $(LDFLAGS) -o $@ 65 | 66 | $(OBJDIR)/boot12.o: $(SRCDIR)/boot12.asm | $(OBJDIR) 67 | $(NASM) $^ $(NASMFLAGS) -o $@ 68 | 69 | $(BINDIR)/boot16.bin: $(BINDIR)/boot16.elf 70 | $(OBJCOPY) $^ $(OBJCOPYFLAGS) $@ 71 | 72 | $(BINDIR)/boot16.elf: $(OBJDIR)/boot16.o | $(BINDIR) 73 | $(LD) $^ $(LDFLAGS) -o $@ 74 | 75 | $(OBJDIR)/boot16.o: $(SRCDIR)/boot16.asm | $(OBJDIR) 76 | $(NASM) $^ $(NASMFLAGS) -o $@ 77 | endif 78 | 79 | 80 | # Makefile target for the demo file 81 | demo: $(BINDIR)/demo.bin 82 | 83 | $(BINDIR)/demo.bin: $(SRCDIR)/demo.asm | $(BINDIR) 84 | $(NASM) $^ -f bin -o $@ 85 | 86 | 87 | # Makefile target to create both disk images 88 | image: $(BINDIR)/boot12.img $(BINDIR)/boot16.img 89 | 90 | $(BINDIR)/boot12.img: $(BINDIR)/boot12.bin $(BINDIR)/demo.bin 91 | dd if=/dev/zero of=$@ bs=1024 count=1440 status=none 92 | mkfs.vfat -F12 $@ 1> /dev/null 93 | mcopy -n -i $@ $(BINDIR)/demo.bin :: 94 | dd if=$< of=$@ bs=1 skip=62 seek=62 conv=notrunc status=none 95 | 96 | $(BINDIR)/boot16.img: $(BINDIR)/boot16.bin $(BINDIR)/demo.bin 97 | dd if=/dev/zero of=$@ bs=1024 count=16384 status=none 98 | mkfs.vfat -F16 $@ 1> /dev/null 99 | mcopy -i $@ $(BINDIR)/demo.bin :: 100 | dd if=$< of=$@ bs=1 skip=62 seek=62 conv=notrunc status=none 101 | 102 | 103 | # Create the obj dir 104 | $(OBJDIR): 105 | @mkdir -p $@ 106 | 107 | # Create the bin dir 108 | $(BINDIR): 109 | @mkdir -p $@ 110 | 111 | 112 | # Clean produced files 113 | clean: 114 | rm -f $(OBJDIR)/* $(BINDIR)/* 115 | 116 | # Clean files from emacs 117 | clobber: clean 118 | rm -f $(SRCDIR)/*~ $(SRCDIR)\#*\# ./*~ 119 | 120 | 121 | # Makefile target to run or debug both disk images 122 | ifeq ($(FAT), 16) 123 | run: image 124 | qemu-system-i386 -serial stdio -rtc base=localtime -drive file=bin/boot16.img,format=raw 125 | 126 | debug: image 127 | qemu-system-i386 -serial stdio -rtc base=localtime -S -s -drive file=bin/boot16.img,format=raw 128 | 129 | gdb: image 130 | -gdb -q -ex "exec-file bin/boot16.elf" -ex "add-symbol-file bin/boot16.elf 0x9FA00 -readnow" -ex "break reallocatedEntry" 131 | else 132 | run: image 133 | qemu-system-i386 -serial stdio -rtc base=localtime -drive file=bin/boot12.img,format=raw,if=floppy 134 | 135 | debug: image 136 | qemu-system-i386 -serial stdio -rtc base=localtime -S -s -drive file=bin/boot12.img,format=raw,if=floppy 137 | 138 | gdb: image 139 | -gdb -q -ex "exec-file bin/boot12.elf" -ex "add-symbol-file bin/boot12.elf 0x9FA00 -readnow" -ex "break reallocatedEntry" 140 | endif -------------------------------------------------------------------------------- /src/boot12.asm: -------------------------------------------------------------------------------- 1 | ; boot12.asm 2 | ; 3 | ; This program is a FAT12 bootloader 4 | ; Copyright (c) 2017-2023, Joshua Riek 5 | ; 6 | ; This program is free software: you can redistribute it and/or modify 7 | ; it under the terms of the GNU General Public License as published by 8 | ; the Free Software Foundation, either version 3 of the License, or 9 | ; (at your option) any later version. 10 | ; 11 | ; This program is distributed in the hope that it will be useful, 12 | ; but WITHOUT ANY WARRANTY; without even the implied warranty of 13 | ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14 | ; GNU General Public License for more details. 15 | ; 16 | ; You should have received a copy of the GNU General Public License 17 | ; along with this program. If not, see . 18 | ; 19 | 20 | %define LOAD_ADDR 0x001000 ; Physical load address (where the program loads) 21 | 22 | %define LOAD_SEG (LOAD_ADDR >> 4) ; (LOAD_SEG << 4) + LOAD_OFF = LOAD_ADDR 23 | %define LOAD_OFF (LOAD_ADDR & 0xf) 24 | 25 | bits 16 ; Ensure 16-bit code 26 | cpu 8086 ; Assemble with the 8086 instruction set 27 | 28 | ;--------------------------------------------------- 29 | ; Disk description table 30 | ;--------------------------------------------------- 31 | 32 | jmp short _start ; Jump over OEM / BIOS param block 33 | nop 34 | 35 | %define OEMName bp+0x03 ; Disk label 36 | %define bytesPerSector bp+0x0b ; Bytes per sector 37 | %define sectorsPerCluster bp+0x0d ; Sectors per cluster 38 | %define reservedSectors bp+0x0e ; Reserved sectors 39 | %define fats bp+0x10 ; Number of fats 40 | %define rootDirEntries bp+0x11 ; Number of entries in root dir 41 | %define sectors bp+0x13 ; Logical sectors 42 | %define mediaType bp+0x15 ; Media descriptor byte 43 | %define fatSectors bp+0x16 ; Sectors per FAT 44 | %define sectorsPerTrack bp+0x18 ; Sectors per track 45 | %define heads bp+0x1a ; Number of sides/heads 46 | %define hiddenSectors bp+0x1c ; Hidden sectors 47 | %define hugeSectors bp+0x20 ; LBA sectors 48 | %define biosDriveNum bp+0x24 ; Drive number 49 | %define reserved bp+0x25 ; This is not used 50 | %define bootSignature bp+0x26 ; Drive signature 51 | %define volumeId bp+0x27 ; Volume ID 52 | %define volumeLabel bp+0x2b ; Volume Label 53 | %define fatTypeLabel bp+0x36 ; File system type 54 | 55 | times 0x3b db 0x00 56 | 57 | ;--------------------------------------------------- 58 | ; Start of the main bootloader code and entry point 59 | ;--------------------------------------------------- 60 | 61 | global _start 62 | _start: 63 | cld 64 | 65 | int 0x12 ; Get Conventional memory size in kb 66 | 67 | mov cl, 6 ; Shift bits left (ax*(2^6)) 68 | shl ax, cl ; Convert the memory to 16-byte paragraphs 69 | 70 | sub ax, 512 >> 4 ; Reserve 512 bytes for the boot sector 71 | mov es, ax ; Set the segment register to the new boot sector location 72 | xor di, di ; Now es:di points to the new bs 73 | 74 | mov ds, di ; Set the segment register to the current boot sector location 75 | mov si, 0x7c00 ; Now ds:si points to the current bs 76 | 77 | sub ax, 4096 >> 4 ; Reserve 4k bytes for the stack 78 | mov ss, ax ; Set segment register to the bottom of the stack 79 | mov sp, 4096 ; Now ss:sp points to the top of the 4k stack 80 | 81 | mov cx, 256 ; 256 words (512 bytes) 82 | rep movsw ; Copy cx times from ds:si to es:di 83 | 84 | mov ax, reallocatedEntry ; Now we want to jump to this label 85 | 86 | push es ; The new Code Segment 87 | push ax ; The new Instruction Pointer 88 | retf ; Far jump to the reallocated boot sector! 89 | 90 | ;--------------------------------------------------- 91 | ; Jump here after allocating the boot sector 92 | ;--------------------------------------------------- 93 | 94 | reallocatedEntry: 95 | push cs 96 | pop ds 97 | 98 | mov bp, sp ; Correct bp for the disk description table 99 | 100 | or dl, dl ; When booting from a hard drive, some of the 101 | jz allocDiskbuffer ; you need to call int 13h to fix some bpb entries 102 | 103 | mov byte [drive], dl ; Save boot device number 104 | 105 | xor di, di ; To guard against BIOS bugs 106 | mov es, di ; http://www.ctyme.com/intr/rb-0621.htm 107 | 108 | mov ah, 0x08 ; Get Drive Parameters func of int 13h 109 | int 0x13 ; Call int 13h (BIOS disk I/O) 110 | jc loadRoot 111 | 112 | and cx, 0x003f ; Maximum sector number is the high bits 6-7 of cl 113 | mov word [sectorsPerTrack], cx ; And whose low 8 bits are in ch 114 | 115 | mov dl, dh ; Convert the maximum head number to a word 116 | xor dh, dh 117 | inc dx ; Head numbers start at zero, so add one 118 | mov word [heads], dx ; Save the head number 119 | 120 | ;--------------------------------------------------- 121 | ; Reserve memory for the disk buffer and FAT (16kb max) 122 | ;--------------------------------------------------- 123 | 124 | allocDiskbuffer: 125 | xor ah, ah 126 | xor dx, dx ; Calculate the size of fat in sectors 127 | mov al, byte [fats] ; Take the number of fats 128 | mul word [fatSectors] ; And multiply that by the number of sectors per fat 129 | 130 | push ax ; Save the size of the fat in sectors for later 131 | push ax ; Save it again to calculate the starting sector of the root dir 132 | 133 | mov bx, word [bytesPerSector] ; Get the size of fat in 16-byte paragraphs 134 | mov cl, 4 ; Shift bits left (ax*(2^4)) 135 | shr bx, cl ; Align to 16-byte paragraphs 136 | mul bx 137 | 138 | mov di, ss ; Allocate space after the stack 139 | sub di, ax ; Reserve (ax*(2^4)) bytes for the disk buffer 140 | 141 | mov es, di ; Set the segment register to the disk buffer location 142 | xor di, di ; Now es:di points to the allocated disk buffer 143 | 144 | ;--------------------------------------------------- 145 | ; Load the root directory from the disk 146 | ;--------------------------------------------------- 147 | 148 | loadRoot: 149 | pop cx 150 | add cx, word [reservedSectors] ; Increase cx by the reserved sectors 151 | 152 | xor dx, dx 153 | mov ax, 32 ; Calculate the size of the root dir in sectors 154 | mul word [rootDirEntries] ; Multiply the number of root dir entries by 32 155 | div word [bytesPerSector] ; Divide by the number of bytes used per sector 156 | xchg cx, ax 157 | 158 | mov word [startOfData], ax ; Calculate the first data sector 159 | add word [startOfData], cx ; Just add the size and location of the root dir 160 | 161 | xor dx, dx 162 | call readSectors ; Load the root directory into the disk buffer 163 | 164 | ;--------------------------------------------------- 165 | ; Find the file to load from the loaded root dir 166 | ;--------------------------------------------------- 167 | 168 | findFile: 169 | mov dx, word [rootDirEntries] ; Search through all of the root dir entries for the kernel 170 | push di 171 | 172 | .searchRoot: 173 | push di 174 | mov si, filename ; Load the filename 175 | mov cx, 11 ; Compare first 11 bytes 176 | rep cmpsb ; Compare si and di cx times 177 | pop di 178 | je loadFat ; We found the file :) 179 | 180 | add di, 32 ; Point to the next entry 181 | dec dx ; Continue to search for the file 182 | jnz .searchRoot 183 | 184 | jmp error ; Could not find the file 185 | 186 | ;--------------------------------------------------- 187 | ; Load the fat from the found file 188 | ;-------------------------------------------------- 189 | 190 | loadFat: 191 | mov bx, word [es:di+26] ; Get the file cluster at offset 26 192 | 193 | pop di ; Offset into disk buffer 194 | pop cx ; Size of fat in sectors 195 | 196 | xor dx, dx 197 | mov ax, word [reservedSectors] ; Convert the first fat on the disk 198 | call readSectors ; load the fat sectors into the disk buffer 199 | 200 | ;--------------------------------------------------- 201 | ; Load the clusters of the file and jump to it 202 | ;--------------------------------------------------- 203 | 204 | loadFile: 205 | mov si, es 206 | mov ds, si ; Set ds:si to the loaded fat 207 | mov si, di 208 | 209 | mov di, LOAD_SEG 210 | mov es, di ; Set es:di to where the file will load 211 | mov di, LOAD_OFF 212 | 213 | push es 214 | push di ; Push the load addr to the stack 215 | 216 | mov ax, bx ; File cluster 217 | call readClusters ; Read clusters from the file 218 | 219 | mov dl, byte [cs:drive] ; Pass the boot drive into dl 220 | retf ; Far jump to the file loaded! 221 | 222 | hlt ; This should never be hit 223 | 224 | 225 | ;--------------------------------------------------- 226 | ; Bootloader routines below 227 | ;--------------------------------------------------- 228 | 229 | 230 | ;--------------------------------------------------- 231 | readClusters: 232 | ; 233 | ; Read file clusters, starting at the given cluster, 234 | ; expects FAT to be loaded into the disk buffer. 235 | ; 236 | ; Expects: DS:SI = Location of FAT 237 | ; ES:DI = Location to load clusters 238 | ; AX = Starting cluster 239 | ; 240 | ; Returns: None 241 | ; 242 | ;-------------------------------------------------- 243 | .clusterLoop: 244 | push ax 245 | 246 | dec ax 247 | dec ax 248 | xor dx, dx 249 | xor bh, bh ; Calculate the first sector of the given cluster in ax 250 | mov bl, byte [sectorsPerCluster] ; First subtract 2 from the cluster 251 | mul bx ; Multiply the cluster by the sectors per cluster 252 | add ax, word [cs:startOfData] ; Finally add the first data sector 253 | 254 | mov cx, bx ; Sectors to read 255 | call readSectors ; Read the sectors 256 | 257 | pop ax 258 | 259 | .loadNextCluster: 260 | push ds 261 | push si 262 | 263 | xor dx, dx ; Get the next cluster for fat 264 | mov bl, 3 ; We want to multiply by 1.5 so divide by 3/2 265 | mul bx ; Multiply the cluster by the numerator 266 | dec bx ; Return value in ax and remainder in dx 267 | div bx ; Divide the cluster by the denominator 268 | 269 | add si, ax ; Point to the next cluster in the fat entry 270 | mov ax, word [ds:si] ; Load ax to the next cluster in fat 271 | 272 | pop si 273 | pop ds 274 | 275 | or dx, dx ; Is the cluster caluclated even? 276 | jz .evenCluster 277 | 278 | .oddCluster: 279 | mov cl, 4 ; Drop the first 4 bits of the next cluster 280 | shr ax, cl 281 | jmp .nextClusterCalculated 282 | 283 | .evenCluster: 284 | and ax, 0x0fff ; Drop the last 4 bits of next cluster 285 | 286 | .nextClusterCalculated: 287 | cmp ax, 0x0ff8 ; Are we at the end of the file? 288 | jae .done 289 | 290 | xchg bx, ax 291 | xor dx, dx 292 | xor ah, ah ; Calculate the size in bytes per cluster 293 | mov al, byte [sectorsPerCluster] ; So, take the sectors per cluster 294 | mul word [bytesPerSector] ; And mul that by the bytes per sector 295 | xchg bx, ax ; Bytes per cluster in bx:dx 296 | 297 | mov cl, 4 298 | shl dx, cl ; Correct the segment offset based on the bytes per cluster 299 | mov cx, es ; Shift left by 4 bits 300 | add ch, dl ; Then add the lower half to the segment 301 | mov es, cx 302 | 303 | clc ; Add to the pointer offset 304 | add di, bx 305 | jnc .clusterLoop 306 | 307 | .fixBuffer: ; An error will occur if the buffer in memory 308 | mov dx, es ; overlaps a 64k page boundry, when di overflows 309 | add dh, 0x10 ; it will trigger the carry flag, so correct 310 | mov es, dx ; extra segment by 0x1000 311 | 312 | jmp .clusterLoop ; Load the next file cluster 313 | 314 | .done: 315 | ret 316 | 317 | ;--------------------------------------------------- 318 | readSectors: 319 | ; 320 | ; Read sectors starting at a given sector by 321 | ; the given times and load into a buffer. 322 | ; 323 | ; Expects: AX:DX = Starting sector/ lba 324 | ; ES:DI = Location to load sectors 325 | ; CX = Number of sectors to read 326 | ; 327 | ; Returns: None 328 | ; 329 | ;-------------------------------------------------- 330 | push ax 331 | push bx 332 | push cx 333 | push dx 334 | push di 335 | push es 336 | 337 | mov bx, di ; Convert es:di to es:bx for int 13h 338 | 339 | .sectorLoop: 340 | push ax 341 | push cx 342 | push dx 343 | 344 | div word [sectorsPerTrack] ; Divide lba by the sectors per track 345 | mov cx, dx ; Save the absolute sector value 346 | inc cx 347 | 348 | xor dx, dx ; Divide by the number of heads 349 | div word [heads] ; to get absolute head and track values 350 | mov dh, dl ; Move the absolute head into dh 351 | 352 | mov ch, al ; Low 8 bits of absolute track 353 | push cx 354 | mov cl, 6 ; High 2 bits of absolute track 355 | shl ah, cl 356 | pop cx 357 | or cl, ah ; Now cx is set with respective track and sector numbers 358 | 359 | mov dl, byte [cs:drive] ; Set correct drive for int 13h 360 | 361 | mov di, 5 ; Try five times to read the sector 362 | 363 | .attemptRead: 364 | mov ax, 0x0201 ; Read Sectors func of int 13h, read one sector 365 | int 0x13 ; Call int 13h (BIOS disk I/O) 366 | jnc .readOk ; If no carry set, the sector has been read 367 | 368 | xor ah, ah ; Reset Drive func of int 13h 369 | int 0x13 ; Call int 13h (BIOS disk I/O) 370 | 371 | dec di ; Decrease read attempt counter 372 | jnz .attemptRead ; Try to read the sector again 373 | 374 | push cs 375 | pop ds 376 | 377 | jmp error ; Error reading the disk 378 | 379 | .readOk: 380 | pop dx 381 | pop cx 382 | pop ax 383 | 384 | clc 385 | add ax, 1 ; Add one for the the next lba value 386 | adc dx, 0 ; Make sure to adjust dx for carry 387 | 388 | clc 389 | add bx, word [bytesPerSector] ; Add to the buffer address for the next sector 390 | jnc .nextSector 391 | 392 | .fixBuffer: 393 | push dx ; An error will occur if the buffer in memory 394 | mov dx, es ; overlaps a 64k page boundry, when di overflows 395 | add dh, 0x10 ; it will trigger the carry flag, so correct 396 | mov es, dx ; es segment by 0x1000 397 | pop dx 398 | 399 | .nextSector: 400 | loop .sectorLoop 401 | 402 | pop es 403 | pop di 404 | pop dx 405 | pop cx 406 | pop bx 407 | pop ax 408 | 409 | ret 410 | 411 | ;--------------------------------------------------- 412 | print: 413 | ; 414 | ; Print out a simple string. 415 | ; 416 | ; Expects: DS:SI = String to print 417 | ; 418 | ; Returns: None 419 | ; 420 | ;--------------------------------------------------- 421 | lodsb ; Load byte from ds:si to al 422 | or al, al ; If al is empty stop looping 423 | jz .done ; Done looping and return 424 | mov ah, 0x0e ; Teletype output 425 | int 0x10 ; Video interupt 426 | jmp print ; Loop untill string is null 427 | .done: 428 | ret 429 | 430 | 431 | ;--------------------------------------------------- 432 | error: 433 | ; 434 | ; Print out an error message and wait for input 435 | ; before rebooting. 436 | ; 437 | ; Expects: None 438 | ; 439 | ; Returns: None 440 | ; 441 | ;--------------------------------------------------- 442 | mov si, errorMsg 443 | call print 444 | 445 | xor ax, ax 446 | int 0x16 ; Get a single keypress 447 | 448 | mov ah, 0x0e ; Teletype output 449 | mov al, 0x0d ; Carriage return 450 | int 0x10 ; Video interupt 451 | mov al, 0x0a ; Line feed 452 | int 0x10 ; Video interupt 453 | int 0x10 ; Video interupt 454 | 455 | xor ax, ax 456 | int 0x19 ; Reboot the system 457 | 458 | hlt 459 | 460 | 461 | ;--------------------------------------------------- 462 | ; Bootloader varables below 463 | ;--------------------------------------------------- 464 | 465 | errorMsg db "Error!", 0 ; Error reading disk or file was not found 466 | 467 | startOfData dw 0 ; Start of the data sectors 468 | drive db 0 ; Boot drive number 469 | 470 | filename db "DEMO BIN" ; Filename 471 | 472 | times 510 - ($ - $$) db 0 ; Pad remainder of boot sector with zeros 473 | dw 0xaa55 ; Boot signature 474 | -------------------------------------------------------------------------------- /src/boot16.asm: -------------------------------------------------------------------------------- 1 | ; boot16.asm 2 | ; 3 | ; This program is a FAT16 bootloader 4 | ; Copyright (c) 2017-2023, Joshua Riek 5 | ; 6 | ; This program is free software: you can redistribute it and/or modify 7 | ; it under the terms of the GNU General Public License as published by 8 | ; the Free Software Foundation, either version 3 of the License, or 9 | ; (at your option) any later version. 10 | ; 11 | ; This program is distributed in the hope that it will be useful, 12 | ; but WITHOUT ANY WARRANTY; without even the implied warranty of 13 | ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14 | ; GNU General Public License for more details. 15 | ; 16 | ; You should have received a copy of the GNU General Public License 17 | ; along with this program. If not, see . 18 | ; 19 | 20 | %define LOAD_ADDR 0x001000 ; Physical load address (where the program loads) 21 | 22 | %define LOAD_SEG (LOAD_ADDR >> 4) ; (LOAD_SEG << 4) + LOAD_OFF = LOAD_ADDR 23 | %define LOAD_OFF (LOAD_ADDR & 0xf) 24 | 25 | bits 16 ; Ensure 16-bit code 26 | cpu 8086 ; Assemble with the 8086 instruction set 27 | 28 | ;--------------------------------------------------- 29 | ; Disk description table 30 | ;--------------------------------------------------- 31 | 32 | jmp short _start ; Jump over OEM / BIOS param block 33 | nop 34 | 35 | %define OEMName bp+0x03 ; Disk label 36 | %define bytesPerSector bp+0x0b ; Bytes per sector 37 | %define sectorsPerCluster bp+0x0d ; Sectors per cluster 38 | %define reservedSectors bp+0x0e ; Reserved sectors 39 | %define fats bp+0x10 ; Number of fats 40 | %define rootDirEntries bp+0x11 ; Number of entries in root dir 41 | %define sectors bp+0x13 ; Logical sectors 42 | %define mediaType bp+0x15 ; Media descriptor byte 43 | %define fatSectors bp+0x16 ; Sectors per FAT 44 | %define sectorsPerTrack bp+0x18 ; Sectors per track 45 | %define heads bp+0x1a ; Number of sides/heads 46 | %define hiddenSectors bp+0x1c ; Hidden sectors 47 | %define hugeSectors bp+0x20 ; LBA sectors 48 | %define biosDriveNum bp+0x24 ; Drive number 49 | %define reserved bp+0x25 ; This is not used 50 | %define bootSignature bp+0x26 ; Drive signature 51 | %define volumeId bp+0x27 ; Volume ID 52 | %define volumeLabel bp+0x2b ; Volume Label 53 | %define fatTypeLabel bp+0x36 ; File system type 54 | 55 | times 0x3b db 0x00 56 | 57 | ;--------------------------------------------------- 58 | ; Start of the main bootloader code and entry point 59 | ;--------------------------------------------------- 60 | 61 | global _start 62 | _start: 63 | cld 64 | 65 | int 0x12 ; Get Conventional memory size in kb 66 | 67 | mov cl, 6 ; Shift bits left (ax*(2^6)) 68 | shl ax, cl ; Convert the memory to 16-byte paragraphs 69 | 70 | sub ax, 512 >> 4 ; Reserve 512 bytes for the boot sector 71 | mov es, ax ; Set the segment register to the new boot sector location 72 | xor di, di ; Now es:di points to the new bs 73 | 74 | mov ds, di ; Set the segment register to the current boot sector location 75 | mov si, 0x7c00 ; Now ds:si points to the current bs 76 | 77 | sub ax, 4096 >> 4 ; Reserve 4k bytes for the stack 78 | mov ss, ax ; Set segment register to the bottom of the stack 79 | mov sp, 4096 ; Now ss:sp points to the top of the 4k stack 80 | 81 | mov cx, 256 ; 256 words (512 bytes) 82 | rep movsw ; Copy cx times from ds:si to es:di 83 | 84 | mov ax, reallocatedEntry ; Now we want to jump to this label 85 | 86 | push es ; The new Code Segment 87 | push ax ; The new Instruction Pointer 88 | retf ; Far jump to the reallocated boot sector! 89 | 90 | ;--------------------------------------------------- 91 | ; Jump here after allocating the boot sector 92 | ;--------------------------------------------------- 93 | 94 | reallocatedEntry: 95 | push cs 96 | pop ds 97 | 98 | mov bp, sp ; Correct bp for the disk description table 99 | 100 | mov byte [drive], dl ; Save boot device number 101 | 102 | xor di, di ; To guard against BIOS bugs 103 | mov es, di ; http://www.ctyme.com/intr/rb-0621.htm 104 | 105 | mov ah, 0x08 ; Get Drive Parameters func of int 13h 106 | int 0x13 ; Call int 13h (BIOS disk I/O) 107 | jc loadRoot 108 | 109 | and cx, 0x003f ; Maximum sector number is the high bits 6-7 of cl 110 | mov word [sectorsPerTrack], cx ; And whose low 8 bits are in ch 111 | 112 | mov dl, dh ; Convert the maximum head number to a word 113 | xor dh, dh 114 | inc dx ; Head numbers start at zero, so add one 115 | mov word [heads], dx ; Save the head number 116 | 117 | ;--------------------------------------------------- 118 | ; Reserve memory for the disk buffer and FAT (128kb max) 119 | ;--------------------------------------------------- 120 | 121 | allocDiskbuffer: 122 | xor ah, ah 123 | mov dx, dx ; Calculate the size of fat in sectors 124 | mov al, byte [fats] ; Take the number of fats 125 | mul word [fatSectors] ; And multiply that by the number of sectors per fat 126 | 127 | push ax ; Save the size of the fat in sectors for later 128 | push ax ; Save it again to calculate the starting sector of the root dir 129 | 130 | mov bx, word [bytesPerSector] ; Get the size of fat in 16-byte paragraphs 131 | mov cl, 4 ; Shift bits left (ax*(2^4)) 132 | shr bx, cl ; Align to 16-byte paragraphs 133 | mul bx 134 | 135 | mov di, ss ; Allocate space after the stack 136 | sub di, ax ; Reserve (ax*(2^4)) bytes for the disk buffer 137 | 138 | mov es, di ; Set the segment register to the disk buffer location 139 | xor di, di ; Now es:di points to the allocated disk buffer 140 | 141 | ;--------------------------------------------------- 142 | ; Load the root directory from the disk 143 | ;--------------------------------------------------- 144 | 145 | loadRoot: 146 | pop cx 147 | add cx, word [reservedSectors] ; Increase cx by the reserved sectors 148 | 149 | xor dx, dx 150 | mov ax, 32 ; Calculate the size of the root dir in sectors 151 | mul word [rootDirEntries] ; Multiply the number of root dir entries by 32 152 | div word [bytesPerSector] ; Divide by the number of bytes used per sector 153 | xchg cx, ax 154 | 155 | mov word [startOfData], ax ; Calculate the first data sector 156 | add word [startOfData], cx ; Just add the size and location of the root dir 157 | 158 | xor dx, dx 159 | call readSectors ; Load the root directory into the disk buffer 160 | 161 | ;--------------------------------------------------- 162 | ; Find the file to load from the loaded root dir 163 | ;--------------------------------------------------- 164 | 165 | findFile: 166 | mov dx, word [rootDirEntries] ; Search through all of the root dir entries for the kernel 167 | push di 168 | 169 | .searchRoot: 170 | push di 171 | mov si, filename ; Load the filename 172 | mov cx, 11 ; Compare first 11 bytes 173 | rep cmpsb ; Compare si and di cx times 174 | pop di 175 | je loadFat ; We found the file :) 176 | 177 | add di, 32 ; Point to the next entry 178 | dec dx ; Continue to search for the file 179 | jnz .searchRoot 180 | 181 | jmp error ; Could not find the file 182 | 183 | ;--------------------------------------------------- 184 | ; Load the fat from the found file 185 | ;-------------------------------------------------- 186 | 187 | loadFat: 188 | mov bx, word [es:di+26] ; Get the file cluster at offset 26 189 | 190 | pop di ; Offset into disk buffer 191 | pop cx ; Size of fat in sectors 192 | 193 | xor dx, dx 194 | mov ax, word [reservedSectors] ; Convert the first fat on the disk 195 | call readSectors ; load the fat sectors into the disk buffer 196 | 197 | ;--------------------------------------------------- 198 | ; Load the clusters of the file and jump to it 199 | ;--------------------------------------------------- 200 | 201 | loadFile: 202 | mov si, es 203 | mov ds, si ; Set ds:si to the loaded fat 204 | mov si, di 205 | 206 | mov di, LOAD_SEG 207 | mov es, di ; Set es:di to where the file will load 208 | mov di, LOAD_OFF 209 | 210 | push es 211 | push di ; Push the load addr to the stack 212 | 213 | mov ax, bx ; File cluster 214 | call readClusters ; Read clusters from the file 215 | 216 | mov dl, byte [cs:drive] ; Pass the boot drive into dl 217 | retf ; Far jump to the file loaded! 218 | 219 | hlt ; This should never be hit 220 | 221 | 222 | ;--------------------------------------------------- 223 | ; Bootloader routines below 224 | ;--------------------------------------------------- 225 | 226 | 227 | ;--------------------------------------------------- 228 | readClusters: 229 | ; 230 | ; Read file clusters, starting at the given cluster, 231 | ; expects FAT to be loaded into the disk buffer. 232 | ; 233 | ; Expects: DS:SI = Location of FAT 234 | ; ES:DI = Location to load clusters 235 | ; AX = Starting cluster 236 | ; 237 | ; Returns: None 238 | ; 239 | ;-------------------------------------------------- 240 | .clusterLoop: 241 | push ax 242 | 243 | dec ax 244 | dec ax 245 | xor dx, dx 246 | xor bh, bh ; Calculate the first sector of the given cluster in ax 247 | mov bl, byte [sectorsPerCluster] ; First subtract 2 from the cluster 248 | mul bx ; Multiply the cluster by the sectors per cluster 249 | add ax, word [cs:startOfData] ; Finally add the first data sector 250 | 251 | mov cx, bx ; Sectors to read 252 | call readSectors ; Read the sectors 253 | 254 | pop ax 255 | 256 | .loadNextCluster: 257 | push ds 258 | push si 259 | 260 | xor dx, dx 261 | mov cx, 2 ; Get the next cluster for fat 262 | mul cx ; Multiply the cluster by 2 263 | 264 | xor bx, bx 265 | add si, ax ; Add the offset into the fat table 266 | adc bx, dx ; Make sure to adjust for carry 267 | 268 | mov cl, 4 269 | shl bl, cl ; Correct the segment based on the offset into the fat table 270 | mov dx, ds ; Shift left by 4 bits 271 | add dh, bl ; Then add the higher half to the segment 272 | mov ds, dx 273 | 274 | mov ax, word [ds:si] ; Load ax to the next cluster in the fat table 275 | 276 | pop si 277 | pop ds 278 | 279 | .nextClusterCalculated: 280 | cmp ax, 0xfff8 ; Are we at the end of the file? 281 | jae .done 282 | 283 | xchg bx, ax 284 | xor dx, dx 285 | xor ah, ah ; Calculate the size in bytes per cluster 286 | mov al, byte [sectorsPerCluster] ; So, take the sectors per cluster 287 | mul word [bytesPerSector] ; And mul that by the bytes per sector 288 | xchg bx, ax ; Bytes per cluster in bx:dx 289 | 290 | mov cl, 4 291 | shl dx, cl ; Correct the segment offset based on the bytes per cluster 292 | mov cx, es ; Shift left by 4 bits 293 | add ch, dl ; Then add the lower half to the segment 294 | mov es, cx 295 | 296 | clc ; Add to the pointer offset 297 | add di, bx 298 | jnc .clusterLoop 299 | 300 | .fixBuffer: ; An error will occur if the buffer in memory 301 | mov dx, es ; overlaps a 64k page boundry, when di overflows 302 | add dh, 0x10 ; it will trigger the carry flag, so correct 303 | mov es, dx ; extra segment by 0x1000 304 | 305 | jmp .clusterLoop ; Load the next file cluster 306 | 307 | .done: 308 | ret 309 | 310 | ;--------------------------------------------------- 311 | readSectors: 312 | ; 313 | ; Read sectors starting at a given sector by 314 | ; the given times and load into a buffer. 315 | ; 316 | ; Expects: AX:DX = Starting sector/ lba 317 | ; ES:DI = Location to load sectors 318 | ; CX = Number of sectors to read 319 | ; 320 | ; Returns: None 321 | ; 322 | ;-------------------------------------------------- 323 | push ax 324 | push bx 325 | push cx 326 | push dx 327 | push di 328 | push es 329 | 330 | mov bx, di ; Convert es:di to es:bx for int 13h 331 | 332 | .sectorLoop: 333 | push ax 334 | push cx 335 | push dx 336 | 337 | div word [sectorsPerTrack] ; Divide lba by the sectors per track 338 | mov cx, dx ; Save the absolute sector value 339 | inc cx 340 | 341 | xor dx, dx ; Divide by the number of heads 342 | div word [heads] ; to get absolute head and track values 343 | mov dh, dl ; Move the absolute head into dh 344 | 345 | mov ch, al ; Low 8 bits of absolute track 346 | push cx 347 | mov cl, 6 ; High 2 bits of absolute track 348 | shl ah, cl 349 | pop cx 350 | or cl, ah ; Now cx is set with respective track and sector numbers 351 | 352 | mov dl, byte [cs:drive] ; Set correct drive for int 13h 353 | 354 | mov di, 5 ; Try five times to read the sector 355 | 356 | .attemptRead: 357 | mov ax, 0x0201 ; Read Sectors func of int 13h, read one sector 358 | int 0x13 ; Call int 13h (BIOS disk I/O) 359 | jnc .readOk ; If no carry set, the sector has been read 360 | 361 | xor ah, ah ; Reset Drive func of int 13h 362 | int 0x13 ; Call int 13h (BIOS disk I/O) 363 | 364 | dec di ; Decrease read attempt counter 365 | jnz .attemptRead ; Try to read the sector again 366 | 367 | push cs 368 | pop ds 369 | 370 | jmp error ; Error reading the disk 371 | 372 | .readOk: 373 | pop dx 374 | pop cx 375 | pop ax 376 | 377 | clc 378 | add ax, 1 ; Add one for the the next lba value 379 | adc dx, 0 ; Make sure to adjust dx for carry 380 | 381 | clc 382 | add bx, word [bytesPerSector] ; Add to the buffer address for the next sector 383 | jnc .nextSector 384 | 385 | .fixBuffer: 386 | push dx ; An error will occur if the buffer in memory 387 | mov dx, es ; overlaps a 64k page boundry, when di overflows 388 | add dh, 0x10 ; it will trigger the carry flag, so correct 389 | mov es, dx ; es segment by 0x1000 390 | pop dx 391 | 392 | .nextSector: 393 | loop .sectorLoop 394 | 395 | pop es 396 | pop di 397 | pop dx 398 | pop cx 399 | pop bx 400 | pop ax 401 | 402 | ret 403 | 404 | ;--------------------------------------------------- 405 | print: 406 | ; 407 | ; Print out a simple string. 408 | ; 409 | ; Expects: DS:SI = String to print 410 | ; 411 | ; Returns: None 412 | ; 413 | ;--------------------------------------------------- 414 | lodsb ; Load byte from ds:si to al 415 | or al, al ; If al is empty stop looping 416 | jz .done ; Done looping and return 417 | mov ah, 0x0e ; Teletype output 418 | int 0x10 ; Video interupt 419 | jmp print ; Loop untill string is null 420 | .done: 421 | ret 422 | 423 | 424 | ;--------------------------------------------------- 425 | error: 426 | ; 427 | ; Print out an error message and wait for input 428 | ; before rebooting. 429 | ; 430 | ; Expects: None 431 | ; 432 | ; Returns: None 433 | ; 434 | ;--------------------------------------------------- 435 | mov si, errorMsg 436 | call print 437 | 438 | xor ax, ax 439 | int 0x16 ; Get a single keypress 440 | 441 | mov ah, 0x0e ; Teletype output 442 | mov al, 0x0d ; Carriage return 443 | int 0x10 ; Video interupt 444 | mov al, 0x0a ; Line feed 445 | int 0x10 ; Video interupt 446 | int 0x10 ; Video interupt 447 | 448 | xor ax, ax 449 | int 0x19 ; Reboot the system 450 | 451 | hlt 452 | 453 | 454 | ;--------------------------------------------------- 455 | ; Bootloader varables below 456 | ;--------------------------------------------------- 457 | 458 | errorMsg db "Error!", 0 ; Error reading disk or file was not found 459 | 460 | startOfData dw 0 ; Start of the data sectors 461 | drive db 0 ; Boot drive number 462 | 463 | filename db "DEMO BIN" ; Filename 464 | 465 | times 510 - ($ - $$) db 0 ; Pad remainder of boot sector with zeros 466 | dw 0xaa55 ; Boot signature 467 | -------------------------------------------------------------------------------- /src/demo.asm: -------------------------------------------------------------------------------- 1 | ; demo.asm 2 | ; 3 | ; This is just a demo file for the bootloader 4 | ; Copyright (c) 2017-2023, Joshua Riek 5 | ; 6 | ; This program is free software: you can redistribute it and/or modify 7 | ; it under the terms of the GNU General Public License as published by 8 | ; the Free Software Foundation, either version 3 of the License, or 9 | ; (at your option) any later version. 10 | ; 11 | ; This program is distributed in the hope that it will be useful, 12 | ; but WITHOUT ANY WARRANTY; without even the implied warranty of 13 | ; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 14 | ; GNU General Public License for more details. 15 | ; 16 | ; You should have received a copy of the GNU General Public License 17 | ; along with this program. If not, see . 18 | ; 19 | 20 | bits 16 ; Ensure 16-bit code 21 | cpu 8086 ; Assemble with the 8086 instruction set 22 | 23 | ;--------------------------------------------------- 24 | ; Demo entry-point 25 | ;--------------------------------------------------- 26 | 27 | demo: 28 | mov ax, cs 29 | mov ds, ax ; Just set the segments equal to the code segment 30 | mov es, ax 31 | 32 | xor bx, bx 33 | mov ah, 0x0e ; Teletype output 34 | mov al, 0x4f 35 | int 0x10 ; Video interupt 36 | mov al, 0x77 37 | mov ah, 0x0e 38 | int 0x10 ; Video interupt 39 | mov al, 0x4f 40 | mov ah, 0x0e 41 | int 0x10 ; Video interupt 42 | 43 | xor ax, ax 44 | int 0x16 ; Get a single keypress 45 | 46 | xor bx, bx 47 | mov ah, 0x0e ; Teletype output 48 | mov al, 0x0d ; Carriage return 49 | int 0x10 ; Video interupt 50 | mov al, 0x0a ; Line feed 51 | int 0x10 ; Video interupt 52 | mov al, 0x0a ; Line feed 53 | int 0x10 ; Video interupt 54 | 55 | xor ax, ax 56 | int 0x19 ; Reboot the system 57 | 58 | hlt 59 | -------------------------------------------------------------------------------- /target.xml: -------------------------------------------------------------------------------- 1 | 2 | --------------------------------------------------------------------------------