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# 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)
# - as an example program, prints a letter from the second sector to the top-left of the screen (by writing to memory-mapped VGA memory)
#
# 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 < apps/boot.hex > boot.bin
# dd if=boot.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 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)
#
# We don't read or write the stack before we get to 32-bit mode. No function
# calls.
# 00:
# load_disk:
b4 02 # ah <- 2 # read sectors from disk
b2 80 # dl <- 80 # hdd 0
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
# 1a:
fa # cli # TODO: don't forget to reenable interrupts in a real program
0f 01 16 # lgdt 00/mod/indirect 010/subop 110/rm32/TODO
80 7c # *gdt_descriptor
# 20:
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
# 2f:
00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 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
e9 1d 00 00 00 # jump to 0x7cf0
# padding
# d3:
00 00 00 00 00 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
# f0:
# Entry:
# eax <- *0x7e04 # random address in second segment containing 'H'
8b # copy rm32 to r32
05 # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
# disp32
04 7e 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 to 512 bytes
# 101:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00
55 aa # final 2 bytes: boot sector marker
## sector 2
# not loaded on boot; loaded by load_disk
# 200:
00 00 00 00 48 0f 00 00 00 00 00 00 00 00 00 00 # spot the 'H' with attributes
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# vim:ft=subx
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