diff options
Diffstat (limited to 'boot0.hex')
| -rw-r--r-- | boot0.hex | 344 |
1 files changed, 344 insertions, 0 deletions
diff --git a/boot0.hex b/boot0.hex new file mode 100644 index 00000000..adb320d1 --- /dev/null +++ b/boot0.hex @@ -0,0 +1,344 @@ +# A minimal bootable image that: +# - loads more sectors past the first boot sector (using BIOS primitives) +# - switches to 32-bit mode (giving up access to BIOS primitives) +# - sets up a keyboard handler to print '1' at the top-left of screen when '1' is typed +# +# When it's ready to accept keys, it prints 'H' to the top-left of the screen. +# +# If the initial load fails, it prints 'D' to the top-left of the screen and +# halts. +# +# 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 +# Now fill in sectors: +# ./bootstrap run apps/hex < baremetal/boot0.hex > boot.bin +# dd if=boot.bin of=disk.img conv=notrunc +# To run: +# qemu-system-i386 disk.img +# Or: +# bochs -f baremetal/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 + +## 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) + +# 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 more sectors 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 01 # al <- 1 # number of sectors to read + # 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: + # switch to 32-bit mode + 0f 01 16 # lgdt 00/mod/indirect 010/subop 110/rm/use-disp16 + 80 7c # *gdt_descriptor +# 3f: + 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 +# 4e: + 00 00 +00 00 00 00 00 00 00 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 00 00 00 # jump to 0x7d00 + +# 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 + +## 'application' SubX code: print one character to top-left of screen + +# offset 100 (address 0x7d00): +# Entry: + # eax <- *0x7ff4 # random address in second segment containing 'H' + 8b # copy rm32 to r32 + 05 # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32 + # disp32 + f4 7f 00 00 + # *0xb8000 <- eax + 89 # copy r32 to rm32 + 05 # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32 + # disp32 + 00 80 0b 00 + +e9 fb ff ff ff # loop forever + +# padding +# 111: + 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + +# 120: +# null interrupt handler: + cf # iret + +# padding +# 121: + 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 + +# 130: +# keyboard interrupt handler: + # prologue + fa # disable interrupts + 60 # push all registers to stack + # acknowledge interrupt + b0 20 # al <- 0x20 + e6 20 # port 0x20 <- al + # check output buffer of 8042 keyboard controller (https://web.archive.org/web/20040604041507/http://panda.cs.ndsu.nodak.edu/~achapwes/PICmicro/keyboard/atkeyboard.html) + e4 64 # al <- port 0x64 + a8 01 # set zf if bit 0 (least significant) is not set + 74 11 # if bit 0 is not set, skip to epilogue + # 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 +# 155 + 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 +00 00 00 00 00 00 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 + 20 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 + 30 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 48 0f 00 00 00 00 00 00 00 00 00 00 # spot the 'H' with attributes +# offset 400 (address 0x8000) + +# vim:ft=conf |