path: root/boot.subx

# Code for the first few 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

# Code in this file needs to be more deliberate about the SubX facilities it
# uses:
#   - sigils only support 32-bit general-purpose registers, so don't work with segment registers or 16-bit or 8-bit registers
#   - metadata like rm32 and r32 can sometimes misleadingly refer to only the bottom 16 bits of the register; pay attention to the register name

# Memory map of a Mu computer:
#   code: 4 tracks of disk to [0x00007c00, 0x00027400)
#   stack grows down from 0x00070000
#     see below
#   heap: [0x01000000, 0x02000000)
#     see 120allocate.subx
# Consult https://wiki.osdev.org/Memory_Map_(x86) before modifying any of this.

== code

## 16-bit entry point: 0x7c00

# Upon reset, the IBM PC:
#   - loads the first sector (512 bytes)
#     from some bootable image (look for the boot-sector-marker further down this file)
#     to the address range [0x7c00, 0x7e00)
#   - starts executing code at address 0x7c00

  fa/disable-interrupts

  # initialize segment registers
  b8/copy-to-ax 0/imm16
  8e/->seg 3/mod/direct 0/rm32/ax 3/r32/ds
  8e/->seg 3/mod/direct 0/rm32/ax 0/r32/es
  8e/->seg 3/mod/direct 0/rm32/ax 4/r32/fs
  8e/->seg 3/mod/direct 0/rm32/ax 5/r32/gs

  # initialize stack to 0x00070000
  # We don't read or write the stack before we get to 32-bit mode, but BIOS
  # calls do. We need to move the stack in case BIOS initializes it to some
  # low address that we want to write code into.
  b8/copy-to-ax 0x7000/imm16
  8e/->seg 3/mod/direct 0/rm32/ax 2/r32/ss
  bc/copy-to-esp 0/imm16

  # 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
  {
    e4/read-port-into-al 0x64/imm8
    a8/test-bits-in-al 0x02/imm8  # set zf if bit 1 (second-least significant) is not set
    75/jump-if-!zero loop/imm8
    b0/copy-to-al 0xd1/imm8
    e6/write-al-into-port 0x64/imm8
  }
  {
    e4/read-port-into-al 0x64/imm8
    a8/test-bits-in-al 0x02/imm8  # set zf if bit 1 (second-least significant) is not set
    75/jump-if-!zero loop/imm8
    b0/copy-to-al 0xdf/imm8
    e6/write-al-into-port 0x64/imm8
  }

  # load remaining sectors from first two tracks of disk into addresses [0x7e00, 0x17800)
  b4/copy-to-ah 2/imm8/read-drive
  # dl comes conveniently initialized at boot time with the index of the device being booted
  b5/copy-to-ch 0/imm8/cylinder
  b6/copy-to-dh 0/imm8/head
  b1/copy-to-cl 2/imm8/sector  # 1-based
  b0/copy-to-al 0x7d/imm8/num-sectors  # 2*63 - 1 = 125
  # address to write sectors to = es:bx = 0x7e00, contiguous with boot segment
  bb/copy-to-bx 0/imm16
  8e/->seg 3/mod/direct 3/rm32/bx 0/r32/es
  bb/copy-to-bx 0x7e00/imm16
  cd/syscall 0x13/imm8/bios-disk-services
  0f 82/jump-if-carry disk_error/disp16

  # load two more tracks of disk into addresses [0x17800, 0x27400)
  b4/copy-to-ah 2/imm8/read-drive
  # dl comes conveniently initialized at boot time with the index of the device being booted
  b5/copy-to-ch 0/imm8/cylinder
  b6/copy-to-dh 2/imm8/head
  b1/copy-to-cl 1/imm8/sector  # 1-based
  b0/copy-to-al 0x7e/imm8/num-sectors  # 2*63 = 126
  # address to write sectors to = es:bx = 0x17800, contiguous with boot segment
  bb/copy-to-bx 0x1780/imm16
  8e/->seg 3/mod/direct 3/rm32/bx 0/r32/es
  bb/copy-to-bx 0/imm16
  cd/syscall 0x13/imm8/bios-disk-services
  0f 82/jump-if-carry disk_error/disp16

  # load two more tracks of disk into addresses [0x27400, 0x37000)
  b4/copy-to-ah 2/imm8/read-drive
  # dl comes conveniently initialized at boot time with the index of the device being booted
  b5/copy-to-ch 0/imm8/cylinder
  b6/copy-to-dh 4/imm8/head
  b1/copy-to-cl 1/imm8/sector  # 1-based
  b0/copy-to-al 0x7e/imm8/num-sectors  # 2*63 = 126
  # address to write sectors to = es:bx = 0x27400, contiguous with boot segment
  bb/copy-to-bx 0x2740/imm16
  8e/->seg 3/mod/direct 3/rm32/bx 0/r32/es
  bb/copy-to-bx 0/imm16
  cd/syscall 0x13/imm8/bios-disk-services
  0f 82/jump-if-carry disk_error/disp16

  # reset es
  bb/copy-to-bx 0/imm16
  8e/->seg 3/mod/direct 3/rm32/bx 0/r32/es

  # adjust video mode
  b4/copy-to-ah 0x4f/imm8  # VBE commands
  b0/copy-to-al 2/imm8  # set video mode
  bb/copy-to-bx 0x4105/imm16  # 0x0105 | 0x4000
                              # 0x0105 = graphics mode 1024x768x256
                              #  (alternative candidate: 0x0101 for 640x480x256)
                              # 0x4000 bit = configure linear frame buffer in Bochs emulator; hopefully this doesn't hurt anything when running natively
  cd/syscall 0x10/imm8/bios-video-services

  # load information for the (hopefully) current video mode
  # mostly just for the address to the linear frame buffer
  b4/copy-to-ah 0x4f/imm8  # VBE commands
  b0/copy-to-al 1/imm8  # get video mode info
  b9/copy-to-cx 0x0105/imm16  # mode we requested
  bf/copy-to-di Video-mode-info/imm16
  cd/syscall 0x10/imm8/bios-video-services

  ## switch to 32-bit mode
  # load global descriptor table
  # We can't refer to the label directly because SubX doesn't do the right
  # thing for lgdt, so rather than make errors worse in most places we instead
  # pin gdt_descriptor below.
  0f 01 2/subop/lgdt 0/mod/indirect 6/rm32/use-disp16 0x7cf8/disp16/gdt_descriptor
  # enable paging
  0f 20/<-cr 3/mod/direct 0/rm32/eax 0/r32/cr0
  66 83 1/subop/or 3/mod/direct 0/rm32/eax 1/imm8  # eax <- or 0x1
  0f 22/->cr 3/mod/direct 0/rm32/eax 0/r32/cr0
  # far jump to initialize_32bit_mode that sets cs to offset 8 in the gdt in the process
  # We can't refer to the label directly because SubX doesn't have syntax for
  # segment selectors. So we instead pin initialize_32bit_mode below.
  ea/jump-far-absolute 0x00087d00/disp32  # address 0x7d00 in offset 8 of the gdt

disk_error:
  # print 'D' to top-left of screen to indicate disk error
  # *0xb8000 <- 0x0f44
  bb/copy-to-bx 0xb800/imm16
  8e/->seg 3/mod/direct 3/rm32/bx 3/r32/ds
  b0/copy-to-al 0x44/imm8/D
  b4/copy-to-ah 0x0f/imm8/white-on-black
  bb/copy-to-bx 0/imm16
  89/<- 0/mod/indirect 7/rm32/bx 0/r32/ax  # *ds:bx <- ax
  # loop forever
  {
    eb/jump loop/disp8
  }

## GDT: 3 records of 8 bytes each
== data

gdt_start:
# offset 0: gdt_null:  mandatory null descriptor
  00 00 00 00 00 00 00 00
# offset 8: gdt_code
  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]
# offset 16: gdt_data
  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:

== data 0x7cf8
gdt_descriptor:
  0x17/imm16  # final index of gdt = size of gdt - 1
  gdt_start/imm32/start

## 32-bit code from this point

== code 0x7d00
initialize_32bit_mode:
  66 b8/copy-to-ax 0x10/imm16  # offset 16 from gdt_start
  8e/->seg 3/mod/direct 0/rm32/ax 3/r32/ds
  8e/->seg 3/mod/direct 0/rm32/ax 2/r32/ss
  8e/->seg 3/mod/direct 0/rm32/ax 0/r32/es
  8e/->seg 3/mod/direct 0/rm32/ax 4/r32/fs
  8e/->seg 3/mod/direct 0/rm32/ax 5/r32/gs

  bc/copy-to-esp 0x00070000/imm32

  ## load interrupt handlers
  # We can't refer to the label directly because SubX doesn't do the right
  # thing for lidt, so rather than make errors worse in most places we instead
  # pin idt_descriptor below.
  0f 01 3/subop/lidt 0/mod/indirect 5/rm32/use-disp32 0x8000/disp32/idt_descriptor

  # For now, not bothering reprogramming the IRQ to not conflict with software
  # exceptions.
  #   https://wiki.osdev.org/index.php?title=8259_PIC&oldid=24650#Protected_Mode
  #
  # Interrupt 1 (keyboard) conflicts with debugger faults. We don't use a
  # debugger.
  # Reference:
  #   https://wiki.osdev.org/Exceptions

  # enable keyboard IRQ (1)
  b0/copy-to-al 0xfd/imm8  # disable mask for IRQ1
  e6/write-al-into-port 0x21/imm8

  fb/enable-interrupts

  ## enable floating point
  db/floating-point-coprocessor e3/initialize
  # eax <- cr4
  0f 20/<-cr 3/mod/direct 0/rm32/eax 4/r32/cr4
  # eax <- or bit 9
  0f ba/bit-test 5/subop/bit-test-and-set 3/mod/direct 0/rm32/eax 9/imm8
  # cr4 <- eax
  0f 22/->cr 3/mod/direct 0/rm32/eax 4/r32/cr4

  e9/jump Entry/disp32

== boot-sector-marker 0x7dfe
# final 2 bytes of boot sector
55 aa

## sector 2 onwards loaded by load_disk, not automatically on boot
== code

null-interrupt-handler:
  cf/return-from-interrupt

keyboard-interrupt-handler:
  # prologue
  fa/disable-interrupts
  60/push-all-registers
  # acknowledge interrupt
  b0/copy-to-al 0x20/imm8
  e6/write-al-into-port 0x20/imm8
  31/xor %eax 0/r32/eax
  # 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/read-port-into-al 0x64/imm8
  a8/test-bits-in-al 0x01/imm8  # set zf if bit 0 (least significant) is not set
  74/jump-if-not-set $keyboard-interrupt-handler:epilogue/disp8
  # - if keyboard buffer is full, return
  # var dest-addr/ecx: (addr byte) = (keyboard-buffer + *keyboard-buffer:write)
  31/xor %ecx 1/r32/ecx
  8a/byte-> *Keyboard-buffer:write 1/r32/cl
  81 0/subop/add %ecx Keyboard-buffer:data/imm32
  # al = *dest-addr
  8a/byte-> *ecx 0/r32/al
  # if (al != 0) return
  3c/compare-al-and 0/imm8
  75/jump-if-!= $keyboard-interrupt-handler:epilogue/disp8
  # - read keycode
  e4/read-port-into-al 0x60/imm8
  # - key released
  # if (al == 0xaa) shift = false  # left shift is being lifted
  {
    3c/compare-al-and 0xaa/imm8
    75/jump-if-!= break/disp8
    # *shift = 0
    c7 0/subop/copy *Keyboard-shift-pressed? 0/imm32
  }
  # if (al == 0xb6) shift = false  # right shift is being lifted
  {
    3c/compare-al-and 0xb6/imm8
    75/jump-if-!= break/disp8
    # *shift = 0
    c7 0/subop/copy *Keyboard-shift-pressed? 0/imm32
  }
  # if (al == 0x9d) ctrl = false  # ctrl is being lifted
  {
    3c/compare-al-and 0x9d/imm8
    75/jump-if-!= break/disp8
    # *ctrl = 0
    c7 0/subop/copy *Keyboard-ctrl-pressed? 0/imm32
  }
  # if (al & 0x80) a key is being lifted; return
  50/push-eax
  24/and-al-with 0x80/imm8
  3c/compare-al-and 0/imm8
  58/pop-to-eax
  75/jump-if-!= $keyboard-interrupt-handler:epilogue/disp8
  # - key pressed
  # if (al == 0x2a) shift = true, return  # left shift pressed
  {
    3c/compare-al-and 0x2a/imm8
    75/jump-if-!= break/disp8
    # *shift = 1
    c7 0/subop/copy *Keyboard-shift-pressed? 1/imm32
    # return
    eb/jump $keyboard-interrupt-handler:epilogue/disp8
  }
  # if (al == 0x36) shift = true, return  # right shift pressed
  {
    3c/compare-al-and 0x36/imm8
    75/jump-if-!= break/disp8
    # *shift = 1
    c7 0/subop/copy *Keyboard-shift-pressed? 1/imm32
    # return
    eb/jump $keyboard-interrupt-handler:epilogue/disp8
  }
  # if (al == 0x1d) ctrl = true, return
  {
    3c/compare-al-and 0x1d/imm8
    75/jump-if-!= break/disp8
    # *ctrl = 1
    c7 0/subop/copy *Keyboard-ctrl-pressed? 1/imm32
    # return
    eb/jump $keyboard-interrupt-handler:epilogue/disp8
  }
  # - convert key to character
  # if (shift) use keyboard shift map
  {
    81 7/subop/compare *Keyboard-shift-pressed? 0/imm32
    74/jump-if-= break/disp8
    # sigils don't currently support labels inside *(eax+label)
    05/add-to-eax Keyboard-shift-map/imm32
    8a/byte-> *eax 0/r32/al
    eb/jump $keyboard-interrupt-handler:select-map-done/disp8
  }
  # if (ctrl) al = *(ctrl map + al)
  {
    81 7/subop/compare *Keyboard-ctrl-pressed? 0/imm32
    74/jump-if-= break/disp8
    05/add-to-eax Keyboard-ctrl-map/imm32
    8a/byte-> *eax 0/r32/al
    eb/jump $keyboard-interrupt-handler:select-map-done/disp8
  }
  # otherwise al = *(normal map + al)
  05/add-to-eax Keyboard-normal-map/imm32
  8a/byte-> *eax 0/r32/al
$keyboard-interrupt-handler:select-map-done:
  # - if there's no character mapping, return
  {
    3c/compare-al-and 0/imm8
    74/jump-if-= break/disp8
    # - store al in keyboard buffer
    88/<- *ecx 0/r32/al
    # increment index
    fe/increment-byte *Keyboard-buffer:write
    # clear top nibble of index (keyboard buffer is circular)
    80 4/subop/and-byte *Keyboard-buffer:write 0x0f/imm8
  }
$keyboard-interrupt-handler:epilogue:
  # epilogue
  61/pop-all-registers
  fb/enable-interrupts
  cf/return-from-interrupt

## the rest of this file is all data

== data 0x8000
idt_descriptor:
  ff 03  # final index of idt = size of idt - 1
  idt_start/imm32/start

Keyboard-shift-pressed?:  # boolean
  0/imm32

Keyboard-ctrl-pressed?:  # boolean
  0/imm32

# var keyboard circular buffer
Keyboard-buffer:write:  # nibble
  0/imm32
Keyboard-buffer:read:  # nibble
  0/imm32
Keyboard-buffer:data:  # byte[16]
  00 00 00 00
  00 00 00 00
  00 00 00 00
  00 00 00 00

== data 0x8100
Video-mode-info:
# video mode info {{{
  0/imm16  # attributes
  00  # winA
  00  # winB
# 04
  0/imm16  # granularity
  0/imm16  # winsize
# 08
  0/imm16  # segmentA
  0/imm16  # segmentB
# 0c
  0/imm32  # realFctPtr (who knows)
# 10
  0/imm16  # pitch
  0/imm16  # Xres
  0/imm16  # Yres
  0/imm16  # Wchar Ychar
# 18
  00  # planes
  00  # bpp
  00  # banks
  00  # memory_model
# 1c
  00  # bank_size
  00  # image_pages
  00  # reserved
# 1f
  0/imm16  # red_mask red_position
  0/imm16  # green_mask green_position
  0/imm16  # blue_mask blue_position
  0/imm16  # rsv_mask rsv_position
  00  # directcolor_attributes
# 28
Video-memory-addr:
  0/imm32  # physbase

# 2c
# reserved for video mode info
                                    00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# }}}

== data 0x8200
# interrupt descriptor table {{{
# 128 entries * 8 bytes each = 1024 bytes (0x400)
idt_start:

# entry 0
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# By default, BIOS maps IRQ0-7 to interrupt vectors 8-15.
# https://wiki.osdev.org/index.php?title=Interrupts&oldid=25102#Default_PC_Interrupt_Vector_Assignment

# entry 8: clock
  null-interrupt-handler/imm16  # target[0:16]
  8/imm16  # segment selector (gdt_code)
  00  # unused
  8e  # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate
  0/imm16  # target[16:32] -- null-interrupt-handler must be within address 0x10000

# entry 9: keyboard
  keyboard-interrupt-handler/imm16  # target[0:16]
  8/imm16  # segment selector (gdt_code)
  00  # unused
  8e  # 1/p 00/dpl 0 1110/type/32-bit-interrupt-gate
  0/imm16  # target[16:32] -- keyboard-interrupt-handler must be within address 0x10000

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# 500:
# entry 0x20
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# 600:
# entry 0x40
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00

# 700:
# entry 0x60
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
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 0x70
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
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:
# }}}

== data 0x8600
# translating keys to ASCII {{{
Keyboard-normal-map:
00
#  es
   1b
#     |<--- digits -------------->| -  =  backspace
      31 32 33 34 35 36 37 38 39 30 2d 3d 08
# 0f
# tab q  w  e  r  t  y  u  i  o  p  [  ]
   09 71 77 65 72 74 79 75 69 6f 70 5b 5d
# 1c
#                                         enter (newline)
                                          0a 00
# 1e
#     a  s  d  f  g  h  j  k  l  ;  '  `     \
      61 73 64 66 67 68 6a 6b 6c 3b 27 60 00 5c
                                        # ^ left shift
# 2c
#     z  x  c  v  b  n  m  ,  .  /     *
      7a 78 63 76 62 6e 6d 2c 2e 2f 00 2a
                                  # ^ right shift
# 38
#                          space
                        00 20
# 3a
                              00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# numeric keypad would start here, but isn't implemented

== data 0x8700
Keyboard-shift-map:
00
#  es
   1b
#     !  @  #  $  %  ^  &  *  (  )  _  +  backspace
      21 40 23 24 25 53 26 2a 28 29 5f 2b 08
# 0f
# tab Q  W  E  R  T  Y  U  I  O  P  {  }
   09 51 57 45 52 54 59 55 49 5f 50 7b 7d
# 1c
#                                         enter (newline)
                                          0a 00
# 1e
#     A  S  D  F  G  H  J  K  L  :  "  ~     |
      41 53 44 46 47 48 4a 4b 4c 3a 22 7e 00 7c
# 2c
#     Z  X  C  V  B  N  M  <  >  ?     *
      5a 58 43 56 42 4e 4d 3c 3e 3f 00 2a
# 38
#                          space
                        00 20
# 3a
                              00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# numeric keypad would start here, but isn't implemented

== data 0x8800
Keyboard-ctrl-map:
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# 10
#     ^q ^w ^e ^r ^t ^y ^u tb ^o ^p
      11 17 05 12 14 19 15 09 1f 10 00 00
# 1c
#                                         carriage-return
                                          0d 00
# 1e
#     ^a ^s ^d ^f ^g ^h ^j ^j ^l             ^\
      01 13 04 06 07 08 0a 0b 0c 00 00 00 00 1c
# 2c
#     ^z ^x ^c ^v ^b ^n ^m       ^/
      1a 18 03 16 02 0e 0d 00 00 1f 00 00
# }}}

== data 0x8c00
Font:
# Bitmaps for some ASCII characters (soon Unicode) {{{
# Part of GNU Unifont
# 8px wide, 16px tall
# Based on http://unifoundry.com/pub/unifont/unifont-13.0.05/font-builds/unifont-13.0.05.hex.gz
# See https://en.wikipedia.org/wiki/GNU_Unifont#The_.hex_font_format
# Website: http://unifoundry.com/unifont/index.html
# License: http://unifoundry.com/LICENSE.txt (GPL v2)
# Each line below is a bitmap for a single character.
#   Each byte is a bitmap for a single row of 8 pixels.

# some unprintable ASCII chars
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# 0x20 = space
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# !
  00 00 00 00 08 08 08 08 08 08 08 00 08 08 00 00
# "
  00 00 22 22 22 22 00 00 00 00 00 00 00 00 00 00
# 0x23 = '#'
  00 00 00 00 12 12 12 7e 24 24 7e 48 48 48 00 00
# $
  00 00 00 00 08 3e 49 48 38 0e 09 49 3e 08 00 00
# %
  00 00 00 00 31 4a 4a 34 08 08 16 29 29 46 00 00
# &
  00 00 00 00 1c 22 22 14 18 29 45 42 46 39 00 00
# '
  00 00 08 08 08 08 00 00 00 00 00 00 00 00 00 00
# (
  00 00 00 04 08 08 10 10 10 10 10 10 08 08 04 00
# )
  00 00 00 20 10 10 08 08 08 08 08 08 10 10 20 00
# *
  00 00 00 00 00 00 08 49 2a 1c 2a 49 08 00 00 00
# +
  00 00 00 00 00 00 08 08 08 7f 08 08 08 00 00 00
# ,
  00 00 00 00 00 00 00 00 00 00 00 00 18 08 08 10
# -
  00 00 00 00 00 00 00 00 00 3c 00 00 00 00 00 00
# .
  00 00 00 00 00 00 00 00 00 00 00 00 18 18 00 00
# /
  00 00 00 00 02 02 04 08 08 10 10 20 40 40 00 00
# 0x30 = '0'
  00 00 00 00 18 24 42 46 4a 52 62 42 24 18 00 00
# 1
  00 00 00 00 08 18 28 08 08 08 08 08 08 3e 00 00
# 2
  00 00 00 00 3c 42 42 02 0c 10 20 40 40 7e 00 00
# 3
  00 00 00 00 3c 42 42 02 1c 02 02 42 42 3c 00 00
# 4
  00 00 00 00 04 0c 14 24 44 44 7e 04 04 04 00 00
# 5
  00 00 00 00 7e 40 40 40 7c 02 02 02 42 3c 00 00
# 6
  00 00 00 00 1c 20 40 40 7c 42 42 42 42 3c 00 00
# 7
  00 00 00 00 7e 02 02 04 04 04 08 08 08 08 00 00
# 8
  00 00 00 00 3c 42 42 42 3c 42 42 42 42 3c 00 00
# 9
  00 00 00 00 3c 42 42 42 3e 02 02 02 04 38 00 00
# :
  00 00 00 00 00 00 18 18 00 00 00 18 18 00 00 00
# ;
  00 00 00 00 00 00 18 18 00 00 00 18 08 08 10 00
# <
  00 00 00 00 00 02 04 08 10 20 10 08 04 02 00 00
# =
  00 00 00 00 00 00 00 7e 00 00 00 7e 00 00 00 00
# >
  00 00 00 00 00 40 20 10 08 04 08 10 20 40 00 00
# ?
  00 00 00 00 3c 42 42 02 04 08 08 00 08 08 00 00
# 0x40 = @
  00 00 00 00 1c 22 4a 56 52 52 52 4e 20 1e 00 00
# A
  00 00 00 00 18 24 24 42 42 7e 42 42 42 42 00 00
# B
  00 00 00 00 7c 42 42 42 7c 42 42 42 42 7c 00 00
# C
  00 00 00 00 3c 42 42 40 40 40 40 42 42 3c 00 00
# D
  00 00 00 00 78 44 42 42 42 42 42 42 44 78 00 00
# E
  00 00 00 00 7e 40 40 40 7c 40 40 40 40 7e 00 00
# F
  00 00 00 00 7e 40 40 40 7c 40 40 40 40 40 00 00
# G
  00 00 00 00 3c 42 42 40 40 4e 42 42 46 3a 00 00
# H
  00 00 00 00 42 42 42 42 7e 42 42 42 42 42 00 00
# I
  00 00 00 00 3e 08 08 08 08 08 08 08 08 3e 00 00
# J
  00 00 00 00 1f 04 04 04 04 04 04 44 44 38 00 00
# K
  00 00 00 00 42 44 48 50 60 60 50 48 44 42 00 00
# L
  00 00 00 00 40 40 40 40 40 40 40 40 40 7e 00 00
# M
  00 00 00 00 42 42 66 66 5a 5a 42 42 42 42 00 00
# N
  00 00 00 00 42 62 62 52 52 4a 4a 46 46 42 00 00
# O
  00 00 00 00 3c 42 42 42 42 42 42 42 42 3c 00 00
# 0x50 = P
  00 00 00 00 7c 42 42 42 7c 40 40 40 40 40 00 00
# Q
  00 00 00 00 3c 42 42 42 42 42 42 5a 66 3c 03 00
# R
  00 00 00 00 7c 42 42 42 7c 48 44 44 42 42 00 00
# S
  00 00 00 00 3c 42 42 40 30 0c 02 42 42 3c 00 00
# T
  00 00 00 00 7f 08 08 08 08 08 08 08 08 08 00 00
# U
  00 00 00 00 42 42 42 42 42 42 42 42 42 3c 00 00
# V
  00 00 00 00 41 41 41 22 22 22 14 14 08 08 00 00
# W
  00 00 00 00 42 42 42 42 5a 5a 66 66 42 42 00 00
# X
  00 00 00 00 42 42 24 24 18 18 24 24 42 42 00 00
# Y
  00 00 00 00 41 41 22 22 14 08 08 08 08 08 00 00
# Z
  00 00 00 00 7e 02 02 04 08 10 20 40 40 7e 00 00
# [
  00 00 00 0e 08 08 08 08 08 08 08 08 08 08 0e 00
# \
  00 00 00 00 40 40 20 10 10 08 08 04 02 02 00 00
# ]
  00 00 00 70 10 10 10 10 10 10 10 10 10 10 70 00
# ^
  00 00 18 24 42 00 00 00 00 00 00 00 00 00 00 00
# _
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 7f 00
# 0x60 = backtick
  00 20 10 08 00 00 00 00 00 00 00 00 00 00 00 00
# a
  00 00 00 00 00 00 3c 42 02 3e 42 42 46 3a 00 00
# b
  00 00 00 40 40 40 5c 62 42 42 42 42 62 5c 00 00
# c
  00 00 00 00 00 00 3c 42 40 40 40 40 42 3c 00 00
# d
  00 00 00 02 02 02 3a 46 42 42 42 42 46 3a 00 00
# e
  00 00 00 00 00 00 3c 42 42 7e 40 40 42 3c 00 00
# f
  00 00 00 0c 10 10 10 7c 10 10 10 10 10 10 00 00
# g
  00 00 00 00 00 02 3a 44 44 44 38 20 3c 42 42 3c
# h
  00 00 00 40 40 40 5c 62 42 42 42 42 42 42 00 00
# i
  00 00 00 08 08 00 18 08 08 08 08 08 08 3e 00 00
# j
  00 00 00 04 04 00 0c 04 04 04 04 04 04 04 48 30
# k
  00 00 00 40 40 40 44 48 50 60 50 48 44 42 00 00
# l
  00 00 00 18 08 08 08 08 08 08 08 08 08 3e 00 00
# m
  00 00 00 00 00 00 76 49 49 49 49 49 49 49 00 00
# n
  00 00 00 00 00 00 5c 62 42 42 42 42 42 42 00 00
# o
  00 00 00 00 00 00 3c 42 42 42 42 42 42 3c 00 00
# 0x70 = p
  00 00 00 00 00 00 5c 62 42 42 42 42 62 5c 40 40
# q
  00 00 00 00 00 00 3a 46 42 42 42 42 46 3a 02 02
# r
  00 00 00 00 00 00 5c 62 42 40 40 40 40 40 00 00
# s
  00 00 00 00 00 00 3c 42 40 30 0c 02 42 3c 00 00
# t
  00 00 00 00 10 10 10 7c 10 10 10 10 10 0c 00 00
# u
  00 00 00 00 00 00 42 42 42 42 42 42 46 3a 00 00
# v
  00 00 00 00 00 00 42 42 42 24 24 24 18 18 00 00
# w
  00 00 00 00 00 00 41 49 49 49 49 49 49 36 00 00
# x
  00 00 00 00 00 00 42 42 24 18 18 24 42 42 00 00
# y
  00 00 00 00 00 00 42 42 42 42 42 26 1a 02 02 3c
# z
  00 00 00 00 00 00 7e 02 04 08 10 20 40 7e 00 00
# {
  00 00 00 0c 10 10 08 08 10 20 10 08 08 10 10 0c
# |
  00 00 08 08 08 08 08 08 08 08 08 08 08 08 08 08
# }
  00 00 00 30 08 08 10 10 08 04 08 10 10 08 08 30
# ~
  00 00 00 31 49 46 00 00 00 00 00 00 00 00 00 00
# 0x7f = del (unused)
  00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
# }}}

# offset 1800 (address 0x9400)
== code 0x9400

# Use 28-bit PIO mode to read the first sector (512 bytes) from an IDE (ATA)
# disk drive.
# Inspired by https://colorforth.github.io/ide.html
#
# Resources:
#   https://wiki.osdev.org/ATA_PIO_Mode
#   https://forum.osdev.org/viewtopic.php?f=1&p=167798
#   read-sector, according to https://www.scs.stanford.edu/11wi-cs140/pintos/specs/ata-3-std.pdf
load-first-sector-from-primary-bus-secondary-drive:  # out: (addr stream byte)
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  51/push-ecx
  52/push-edx
  # check for drive
  (secondary-drive-exists?)  # => eax
  3d/compare-eax-and 0/imm32/false
  0f 84/jump-if-= $load-first-sector-from-primary-bus-secondary-drive:end/disp32
  # kick off read
  (ata-drive-select 0xf0)  # primary bus, secondary drive; 4 LSBs contain 4 upper bits of LBA (here 0)
  (clear-ata-error)
  (ata-sector-count 1)
  (ata-lba 0 0 0)  # lower 24 bits of LBA, all 0
  (ata-command 0x20)  # read sectors with retries
  # poll for results
  (poll-ata-regular-status-word)
  # emit results
  ba/copy-to-edx 0x1f0/imm32
  b9/copy-to-ecx 0x200/imm32  # 512 bytes per sector
  {
    81 7/subop/compare %ecx 0/imm32
    74/jump-if-= break/disp8
    ed/read-port-dx-into-eax
    # write 4 bytes to stream one at a time
    (append-byte *(ebp+8) %eax)
    49/decrement-ecx
    c1/shift 5/subop/right-padding-zeroes %eax 8/imm8
    (append-byte *(ebp+8) %eax)
    49/decrement-ecx
    c1/shift 5/subop/right-padding-zeroes %eax 8/imm8
    (append-byte *(ebp+8) %eax)
    49/decrement-ecx
    c1/shift 5/subop/right-padding-zeroes %eax 8/imm8
    (append-byte *(ebp+8) %eax)
    49/decrement-ecx
    eb/jump loop/disp8
  }
$load-first-sector-from-primary-bus-secondary-drive:end:
  # . restore registers
  5a/pop-to-edx
  59/pop-to-ecx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

secondary-drive-exists?:  # -> _/eax: boolean
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  52/push-edx
  # check for floating bus
  {
    31/xor %eax 0/r32/eax
    ba/copy-to-edx 0x1f7/imm32
    ec/read-port-dx-into-al
    3d/compare-eax-and 0xff/imm32
    # if eax is 0xff, primary bus has no drives
    b8/copy-to-eax 0/imm32/false
    74/jump-if-= $secondary-drive-exists?:end/disp8
  }
  # identify
  (ata-drive-select 0xb0)  # primary bus, secondary drive
  (ata-sector-count 0)
  (ata-lba 0 0 0)
  (ata-command 0xec)  # identify
  # read status register
  # TODO: might need to spin here for 400ns: https://wiki.osdev.org/index.php?title=ATA_PIO_Mode&oldid=25664#400ns_delays
  31/xor %eax 0/r32/eax
  ba/copy-to-edx 0x1f7/imm32
  ec/read-port-dx-into-al
  # if eax is 0, secondary drive does not exist
  3d/compare-eax-and 0/imm32
  {
    74/jump-if-= break/disp8
    b8/copy-to-eax 1/imm32/true
    eb/jump $secondary-drive-exists?:complete-identify/disp8
  }
  # TODO: might need to perform remaining steps at https://wiki.osdev.org/index.php?title=ATA_PIO_Mode&oldid=25664#IDENTIFY_command
  b8/copy-to-eax 0/imm32/false
$secondary-drive-exists?:complete-identify:
  50/push-eax
  # clear FIFO from the drive
  ba/copy-to-edx 0x1f0/imm32
  b9/copy-to-ecx 0x200/imm32
  {
    81 7/subop/compare %ecx 0/imm32
    74/jump-if-= break/disp8
    # read 4 bytes
    ed/read-port-dx-into-eax
    49/decrement-ecx
    49/decrement-ecx
    49/decrement-ecx
    49/decrement-ecx
    eb/jump loop/disp8
  }
  58/pop-to-eax
$secondary-drive-exists?:end:
  # . restore registers
  5a/pop-to-edx
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

ata-drive-select:  # n: byte
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  52/push-edx
  #
  8b/-> *(ebp+8) 0/r32/eax
  ba/copy-to-edx 0x1f6/imm32
  ee/write-al-into-port-dx
$ata-drive-select:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

clear-ata-error:
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  52/push-edx
  #
  b8/copy-to-eax 0/imm32
  ba/copy-to-edx 0x1f1/imm32
  ee/write-al-into-port-dx
$ata-error:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

ata-sector-count:  # n: byte
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  52/push-edx
  #
  8b/-> *(ebp+8) 0/r32/eax
  ba/copy-to-edx 0x1f2/imm32
  ee/write-al-into-port-dx
$ata-sector-count:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

ata-lba:  # lo: byte, mid: byte, hi: byte
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  52/push-edx
  # lo
  8b/-> *(ebp+8) 0/r32/eax
  ba/copy-to-edx 0x1f3/imm32
  ee/write-al-into-port-dx
  # mid
  8b/-> *(ebp+0xc) 0/r32/eax
  ba/copy-to-edx 0x1f4/imm32
  ee/write-al-into-port-dx
  # hi
  8b/-> *(ebp+0x10) 0/r32/eax
  ba/copy-to-edx 0x1f5/imm32
  ee/write-al-into-port-dx
$ata-lba:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

# sector: [1, 63]
# cylinder: [0, 1023]
ata-cyl-sector:  # sector: byte, cyl-lo: byte, cyl-hi: byte
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  52/push-edx
  # sector
  8b/-> *(ebp+8) 0/r32/eax
  ba/copy-to-edx 0x1f3/imm32
  ee/write-al-into-port-dx
  # cyl-lo
  8b/-> *(ebp+0xc) 0/r32/eax
  ba/copy-to-edx 0x1f4/imm32
  ee/write-al-into-port-dx
  # cyl-hi
  8b/-> *(ebp+0x10) 0/r32/eax
  ba/copy-to-edx 0x1f5/imm32
  ee/write-al-into-port-dx
$ata-lba:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

ata-command:  # cmd: byte
  # . prologue
  55/push-ebp
  89/<- %ebp 4/r32/esp
  # . save registers
  50/push-eax
  52/push-edx
  #
  8b/-> *(ebp+8) 0/r32/eax
  ba/copy-to-edx 0x1f7/imm32
  ee/write-al-into-port-dx
$ata-command:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  89/<- %esp 5/r32/ebp
  5d/pop-to-ebp
  c3/return

poll-ata-regular-status-word:
  # . save registers
  50/push-eax
  52/push-edx
  #
  ba/copy-to-edx 0x1f7/imm32
  {
    ec/read-port-dx-into-al
    a8/test-bits-in-al 0x80/imm8/bsy  # set zf if bit 7 (most significant) is not set
    75/jump-if-zf-not-set-and-bit-7-set loop/disp8
    a8/test-bits-in-al 8/imm8/drq  # set zf if bit 3 is not set
    74/jump-if-zf-set-and-bit-3-not-set loop/disp8
  }
$poll-ata-regular-status-word:end:
  # . restore registers
  5a/pop-to-edx
  58/pop-to-eax
  # . epilogue
  c3/return

# vim:ft=subx