2020-12-21 05:46:12 +00:00
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# Bootable image that:
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# - loads more sectors past the first boot sector (using BIOS primitives)
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# - switches to 32-bit mode (giving up access to BIOS primitives)
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2020-12-23 17:41:46 +00:00
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# - 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)
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2020-12-21 05:46:12 +00:00
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#
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# If the initial load fails, it prints 'D' to the top-left of the screen and
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# halts.
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#
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2020-12-21 08:04:38 +00:00
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# To convert to a disk image, first prepare a realistically sized disk image:
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# dd if=/dev/zero of=disk.img count=20160 # 512-byte sectors, so 10MB
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# Now fill in sectors:
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2020-12-21 05:46:12 +00:00
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# ./bootstrap run apps/hex < apps/boot.hex > boot.bin
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2020-12-21 08:04:38 +00:00
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# dd if=boot.bin of=disk.img conv=notrunc
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2020-12-21 05:46:12 +00:00
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# To run:
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2020-12-21 08:04:38 +00:00
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# qemu-system-i386 disk.img
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2020-12-21 05:46:12 +00:00
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# Or:
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2020-12-21 08:04:38 +00:00
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# bochs -f apps/boot.bochsrc # boot.bochsrc loads disk.img
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2020-12-21 05:46:12 +00:00
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#
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# Since we start out in 16-bit mode, we need instructions SubX doesn't
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# support.
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# This file contains just hex bytes and comments. Zero error-checking. Make
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# liberal use of:
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# - comments documenting expected offsets
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# - size checks on the emitted file (currently: 512 bytes)
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# - xxd to eyeball that offsets contain expected bytes
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## 16-bit entry point
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# Upon reset, the IBM PC
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# loads the first sector (512 bytes)
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# from some bootable image (see the boot sector marker at the end of this file)
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# to the address range [0x7c00, 0x7e00)
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2020-12-23 18:02:15 +00:00
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# offset 00 (address 0x7c00):
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2020-12-23 17:41:46 +00:00
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fa # cli # TODO: don't forget to reenable interrupts in a real program
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2020-12-23 00:55:19 +00:00
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# initialize segment registers
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# this isn't always needed, but is considered safe not to assume
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b8 00 00 # ax <- 0
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8e d8 # ds <- ax
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8e d0 # ss <- ax
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8e c0 # es <- ax
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8e e0 # fs <- ax
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8e e8 # gs <- ax
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# We don't read or write the stack before we get to 32-bit mode. No function
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# calls, so we don't need to initialize the stack.
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# 0d:
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# load more sectors from disk
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2020-12-21 05:46:12 +00:00
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b4 02 # ah <- 2 # read sectors from disk
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2020-12-23 01:31:35 +00:00
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# dl comes conveniently initialized at boot time with the index of the device being booted
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2020-12-21 05:46:12 +00:00
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b5 00 # ch <- 0 # cylinder 0
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b6 00 # dh <- 0 # track 0
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2020-12-21 08:04:38 +00:00
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b1 02 # cl <- 2 # second sector, 1-based
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2020-12-21 05:46:12 +00:00
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b0 01 # al <- 1 # number of sectors to read
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# address to write sectors to = es:bx = 0x7e00, contiguous with boot segment
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bb 00 00 # bx <- 0
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8e c3 # es <- bx
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bb 00 7e # bx <- 0x7e00
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cd 13 # int 13h, BIOS disk service
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0f 82 76 00 # jump-if-carry disk-error
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2020-12-23 01:31:35 +00:00
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# 26:
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2020-12-23 17:57:40 +00:00
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# undo the A20 hack: https://en.wikipedia.org/wiki/A20_line
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# this is from https://github.com/mit-pdos/xv6-public/blob/master/bootasm.S
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# seta20.1:
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e4 64 # al <- port 0x64
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a8 02 # set zf if bit 1 (second-least) is not set
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75 fa # if zf not set, goto seta20.1 (-6)
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b0 d1 # al <- 0xd1
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e6 64 # port 0x64 <- al
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# 30:
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# seta20.2:
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e4 64 # al <- port 0x64
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a8 02 # set zf if bit 1 (second-least) is not set
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75 fa # if zf not set, goto seta20.2 (-6)
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b0 df # al <- 0xdf
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e6 64 # port 0x64 <- al
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# 3a:
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2020-12-23 00:55:19 +00:00
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# switch to 32-bit mode
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2020-12-21 05:46:12 +00:00
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0f 01 16 # lgdt 00/mod/indirect 010/subop 110/rm32/TODO
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80 7c # *gdt_descriptor
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2020-12-23 17:57:40 +00:00
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# 3f:
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2020-12-21 05:46:12 +00:00
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0f 20 c0 # eax <- cr0
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66 83 c8 01 # eax <- or 0x1
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0f 22 c0 # cr0 <- eax
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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)
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# padding
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2020-12-23 17:57:40 +00:00
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# 4e:
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00 00
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2020-12-21 05:46:12 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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## GDT: 3 records of 8 bytes each
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# 60:
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# gdt_start:
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# gdt_null: mandatory null descriptor
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00 00 00 00 00 00 00 00
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# gdt_code: (offset 8 from gdt_start)
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ff ff # limit[0:16]
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00 00 00 # base[0:24]
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9a # 1/present 00/privilege 1/descriptor type = 1001b
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# 1/code 0/conforming 1/readable 0/accessed = 1010b
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cf # 1/granularity 1/32-bit 0/64-bit-segment 0/AVL = 1100b
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# limit[16:20] = 1111b
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00 # base[24:32]
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# gdt_data: (offset 16 from gdt_start)
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ff ff # limit[0:16]
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00 00 00 # base[0:24]
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92 # 1/present 00/privilege 1/descriptor type = 1001b
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# 0/data 0/conforming 1/readable 0/accessed = 0010b
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cf # same as gdt_code
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00 # base[24:32]
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# gdt_end:
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# padding
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# 78:
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00 00 00 00 00 00 00 00
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# 80:
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# gdt_descriptor:
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17 00 # final index of gdt = gdt_end - gdt_start - 1
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60 7c 00 00 # start = gdt_start
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# padding
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# 85:
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00 00 00 00 00 00 00 00 00 00
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# 90:
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# disk_error:
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# print 'D' to top-left of screen to indicate disk error
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# *0xb8000 <- 0x0f44
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# bx <- 0xb800
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bb 00 b8
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# ds <- bx
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8e db # 11b/mod 011b/reg/ds 011b/rm/bx
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# al <- 'D'
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b0 44
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# ah <- 0x0f # white on black
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b4 0f
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# bx <- 0
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bb 00 00
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# *ds:bx <- ax
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89 07 # 00b/mod/indirect 000b/reg/ax 111b/rm/bx
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e9 fb ff # loop forever
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# padding
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# a1:
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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## 32-bit code from this point (still some instructions not in SubX)
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# c0:
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# initialize_32bit_mode:
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66 b8 10 00 # ax <- offset 16 from gdt_start
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8e d8 # ds <- ax
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8e d0 # ss <- ax
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8e c0 # es <- ax
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8e e0 # fs <- ax
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8e e8 # gs <- ax
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2020-12-23 18:02:15 +00:00
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e9 2d 00 00 00 # jump to 0x7d00
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2020-12-21 05:46:12 +00:00
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# padding
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2020-12-23 17:41:46 +00:00
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# d3:
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00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-23 08:25:33 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-23 18:02:15 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-21 05:46:12 +00:00
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## 'application' SubX code: print one character to top-left of screen
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2020-12-23 18:02:15 +00:00
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# offset 100 (address 0x7d00):
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2020-12-21 05:46:12 +00:00
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# Entry:
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2020-12-23 17:41:46 +00:00
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# eax <- *0x7e04 # random address in second segment containing 'H'
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2020-12-21 08:19:50 +00:00
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8b # copy rm32 to r32
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05 # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
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# disp32
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2020-12-23 17:41:46 +00:00
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04 7e 00 00
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2020-12-21 08:19:50 +00:00
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# *0xb8000 <- eax
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89 # copy r32 to rm32
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05 # 00/mod/indirect 000/r32/eax 101/rm32/use-disp32
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2020-12-21 05:46:12 +00:00
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# disp32
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00 80 0b 00
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e9 fb ff ff ff # loop forever
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2020-12-23 18:06:04 +00:00
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# padding
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2020-12-23 18:02:15 +00:00
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# 111:
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2020-12-21 08:19:50 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-23 17:41:46 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-21 05:46:12 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-23 18:06:04 +00:00
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# final 2 bytes of boot sector
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55 aa
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2020-12-21 05:46:12 +00:00
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2020-12-21 08:19:50 +00:00
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## sector 2
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# not loaded on boot; loaded by load_disk
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2020-12-23 18:02:15 +00:00
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# offset 200 (address 0x7e00):
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2020-12-23 17:41:46 +00:00
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00 00 00 00 48 0f 00 00 00 00 00 00 00 00 00 00 # spot the 'H' with attributes
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-21 08:19:50 +00:00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
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2020-12-21 08:22:12 +00:00
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# vim:ft=conf
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