# Code for the first 2 disk sectors, that all programs in this directory need: # - load sectors past the first (using BIOS primitives) since only the first is available by default # - if this fails, print 'D' at top-left of screen and halt # - initialize a minimal graphics mode # - switch to 32-bit mode (giving up access to BIOS primitives) # - set up a handler for keyboard events # - jump to start of program # # To convert to a disk image, first prepare a realistically sized disk image: # dd if=/dev/zero of=disk.img count=20160 # 512-byte sectors, so 10MB # Create initial sectors from this file: # ./bootstrap run apps/hex < baremetal/boot.hex > boot.bin # Translate other sectors into a file called a.img # Load all sectors into the disk image: # cat boot.bin a.img > disk.bin # dd if=disk.bin of=disk.img conv=notrunc # To run: # qemu-system-i386 disk.img # Or: # bochs -f apps/boot.bochsrc # boot.bochsrc loads disk.img # # Since we start out in 16-bit mode, we need instructions SubX doesn't # support. # This file contains just lowercase hex bytes and comments. Zero # error-checking. Make liberal use of: # - comments documenting expected offsets # - size checks on the emitted file (currently: 512 bytes) # - xxd to eyeball that offsets contain expected bytes # # Programs using this initialization: # - can't use any syscalls # - can't print text to video memory (past these boot sectors) # - must only print raw pixels (256 colors) to video memory (resolution 1280x1024) # - must store their entry-point at address 0x8000 ## 16-bit entry point # Upon reset, the IBM PC: # - loads the first sector (512 bytes) # from some bootable image (see the boot sector marker at the end of this file) # to the address range [0x7c00, 0x7e00) # - starts executing code at address 0x7c00 # offset 00 (address 0x7c00): # disable interrupts for this initialization fa # cli # initialize segment registers # this isn't always needed, but the recommendation is to not make assumptions b8 00 00 # ax <- 0 8e d8 # ds <- ax 8e d0 # ss <- ax 8e c0 # es <- ax 8e e0 # fs <- ax 8e e8 # gs <- ax # We don't read or write the stack before we get to 32-bit mode. No function # calls, so we don't need to initialize the stack. # 0e: # load second sector from disk b4 02 # ah <- 2 # read sectors from disk # dl comes conveniently initialized at boot time with the index of the device being booted b5 00 # ch <- 0 # cylinder 0 b6 00 # dh <- 0 # track 0 b1 02 # cl <- 2 # second sector, 1-based b0 04 # al <- 4 # number of sectors to read; all sectors must be in a single track # address to write sectors to = es:bx = 0x7e00, contiguous with boot segment bb 00 00 # bx <- 0 8e c3 # es <- bx bb 00 7e # bx <- 0x7e00 cd 13 # int 13h, BIOS disk service 0f 82 76 00 # jump-if-carry disk-error # 26: # undo the A20 hack: https://en.wikipedia.org/wiki/A20_line # this is from https://github.com/mit-pdos/xv6-public/blob/master/bootasm.S # seta20.1: e4 64 # al <- port 0x64 a8 02 # set zf if bit 1 (second-least significant) is not set 75 fa # if zf not set, goto seta20.1 (-6) b0 d1 # al <- 0xd1 e6 64 # port 0x64 <- al # 30: # seta20.2: e4 64 # al <- port 0x64 a8 02 # set zf if bit 1 (second-least significant) is not set 75 fa # if zf not set, goto seta20.2 (-6) b0 df # al <- 0xdf e6 64 # port 0x64 <- al # 3a: # adjust video mode b4 4f # ah <- 4f (VBE) b0 02 # al <- 02 (set video mode) bb 07 01 # bx <- 0x0107 (graphics 1280x1024x256) # fallback: 0x0101 (640x480x256) # for other choices see http://www.ctyme.com/intr/rb-0069.htm cd 10 # int 10h, Vesa BIOS extensions # 43: # switch to 32-bit mode 0f 01 16 # lgdt 00/mod/indirect 010/subop 110/rm/use-disp16 80 7c # *gdt_descriptor 0f 20 c0 # eax <- cr0 66 83 c8 01 # eax <- or 0x1 0f 22 c0 # cr0 <- eax ea c0 7c 08 00 # far jump to initialize_32bit_mode after setting cs to the record at offset 8 in the gdt (gdt_code) # padding # 57: 00 00 00 00 00 00 00 00 00 ## GDT: 3 records of 8 bytes each # 60: # gdt_start: # gdt_null: mandatory null descriptor 00 00 00 00 00 00 00 00 # gdt_code: (offset 8 from gdt_start) ff ff # limit[0:16] 00 00 00 # base[0:24] 9a # 1/present 00/privilege 1/descriptor type = 1001b # 1/code 0/conforming 1/readable 0/accessed = 1010b cf # 1/granularity 1/32-bit 0/64-bit-segment 0/AVL = 1100b # limit[16:20] = 1111b 00 # base[24:32] # gdt_data: (offset 16 from gdt_start) ff ff # limit[0:16] 00 00 00 # base[0:24] 92 # 1/present 00/privilege 1/descriptor type = 1001b # 0/data 0/conforming 1/readable 0/accessed = 0010b cf # same as gdt_code 00 # base[24:32] # gdt_end: # padding # 78: 00 00 00 00 00 00 00 00 # 80: # gdt_descriptor: 17 00 # final index of gdt = gdt_end - gdt_start - 1 60 7c 00 00 # start = gdt_start # padding # 85: 00 00 00 00 00 00 00 00 00 00 # 90: # disk_error: # print 'D' to top-left of screen to indicate disk error # *0xb8000 <- 0x0f44 # bx <- 0xb800 bb 00 b8 # ds <- bx 8e db # 11b/mod 011b/reg/ds 011b/rm/bx # al <- 'D' b0 44 # ah <- 0x0f # white on black b4 0f # bx <- 0 bb 00 00 # *ds:bx <- ax 89 07 # 00b/mod/indirect 000b/reg/ax 111b/rm/bx e9 fb ff # loop forever # padding # a1: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ## 32-bit code from this point (still some instructions not in SubX) # c0: # initialize_32bit_mode: 66 b8 10 00 # ax <- offset 16 from gdt_start 8e d8 # ds <- ax 8e d0 # ss <- ax 8e c0 # es <- ax 8e e0 # fs <- ax 8e e8 # gs <- ax # load interrupt handlers 0f 01 1d # lidt 00/mod/indirect 011/subop 101/rm32/use-disp32 00 7f 00 00 # *idt_descriptor # enable keyboard IRQ b0 fd # al <- 0xfd # enable just IRQ1 e6 21 # port 0x21 <- al # initialization is done; enable interrupts fb e9 21 03 00 00 # jump to 0x8000 # padding # df: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 # 100: # null interrupt handler: cf # iret # padding # 101: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 # 110: # keyboard interrupt handler: # prologue fa # disable interrupts 60 # push all registers to stack # acknowledge interrupt b0 20 # al <- 0x20 e6 20 # port 0x20 <- al # TODO: perhaps we should check keyboard status # read keycode into eax 31 c0 # eax <- xor eax; 11/direct 000/r32/eax 000/rm32/eax e4 60 # al <- port 0x60 # map key '1' to ascii; if eax == 2, eax = 0x31 3d 02 00 00 00 # compare eax with 0x02 75 0b # if not equal, goto epilogue b8 31 0f 00 00 # eax <- 0x0f31 # print eax to top-left of screen (*0xb8000) 89 # copy r32 to rm32 05 # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32 # disp32 00 80 0b 00 # epilogue 61 # pop all registers fb # enable interrupts cf # iret # padding # 12f 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 # final 2 bytes of boot sector 55 aa ## sector 2 # loaded by load_disk, not automatically on boot # offset 200 (address 0x7e00): interrupt descriptor table # 32 entries * 8 bytes each = 256 bytes (0x100) # idt_start: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 # entry 8: clock 00 7d # target[0:16] = null interrupt handler 08 00 # segment selector (gdt_code) 00 # unused 8e # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate 00 00 # target[16:32] # entry 9: keyboard 10 7d # target[0:16] = keyboard interrupt handler 08 00 # segment selector (gdt_code) 00 # unused 8e # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate 00 00 # target[16:32] 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 # idt_end: # offset 300 (address 0x7f00): # idt_descriptor: ff 00 # idt_end - idt_start - 1 00 7e 00 00 # start = idt_start # padding 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 # offset 400 (address 0x8000) # vim:ft=subx