345 lines
9.8 KiB
Plaintext
345 lines
9.8 KiB
Plaintext
# 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=code.img count=20160 # 512-byte sectors, so 10MB
|
|
# Now fill in sectors:
|
|
# bootstrap/bootstrap run hex < boot0.hex > boot.bin
|
|
# dd if=boot.bin of=code.img conv=notrunc
|
|
# To run:
|
|
# qemu-system-i386 code.img
|
|
# Or:
|
|
# bochs -f bochsrc # bochsrc loads code.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
|