path: root/timer.subx

# Example bootloader that handles just the timer interrupt.
#
# 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
#
# While most of Mu is thoroughly tested, this file is not. I don't yet
# understand hardware interfaces well enough to explain to others.

# Memory map of a Mu computer:
#   code: currently 4 tracks loaded from the primary disk to [0x00007c00, 0x00048600)
#   stack: grows down from 0x02000000 to 0x01000000
#   heap: [0x02000000, 0x08000000)
#     see 120allocate.subx; Qemu initializes with 128MB RAM by default
# Consult https://wiki.osdev.org/Memory_Map_(x86) before modifying any of
# this. And don't forget to keep *stack-debug.subx in sync.

== 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

  # Temporarily initialize stack to 0x00070000 in real mode.
  # 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/disp8
    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/disp8
    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

  # load two more tracks of disk into addresses [0x37000, 0x46c00)
  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 6/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 = 0x37000, contiguous with boot segment
  bb/copy-to-bx 0x3700/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 [0x46c00, 0x56800)
  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 8/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 = 0x46c00, contiguous with boot segment
  bb/copy-to-bx 0x46c0/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 [0x56800, 0x66400)
  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 0xa/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 = 0x56800, contiguous with boot segment
  bb/copy-to-bx 0x5680/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 0x7de0/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 0x00087e00/disp32  # address 0x7e00 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 0x7de0
gdt_descriptor:
  0x17/imm16  # final index of gdt = size of gdt - 1
  gdt_start/imm32/start

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:

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

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

## 32-bit code from this point

== code 0x7e00
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 0x02000000/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 0x7f00/disp32/idt_descriptor

  # enable timer IRQ0 and keyboard IRQ1
  b0/copy-to-al 0xfc/imm8  # disable mask for IRQ0 and IRQ1
  e6/write-al-into-port 0x21/imm8

  fb/enable-interrupts

  {
    eb/jump loop/disp8
  }

== data 0x7f00
idt_descriptor:
  ff 03  # final index of idt = size of idt - 1
  idt_start/imm32/start

# interrupt descriptor table {{{
# 32 entries of 8 bytes each
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: https://wiki.osdev.org/Programmable_Interval_Timer
  timer-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] -- timer-interrupt-handler must be within address 0x10000

# entry 9: keyboard
  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

00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00
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:
# }}}

== code

null-interrupt-handler:
  # prologue
  fa/disable-interrupts
  60/push-all-registers
  9c/push-flags
  # acknowledge interrupt
  b0/copy-to-al 0x20/imm8
  e6/write-al-into-port 0x20/imm8
  31/xor %eax 0/r32/eax
$null-interrupt-handler:epilogue:
  # epilogue
  9d/pop-flags
  61/pop-all-registers
  fb/enable-interrupts
  cf/return-from-interrupt

timer-interrupt-handler:
  # prologue
  fa/disable-interrupts
  60/push-all-registers
  9c/push-flags
  # acknowledge interrupt
  b0/copy-to-al 0x20/imm8
  e6/write-al-into-port 0x20/imm8
  31/xor %eax 0/r32/eax
  # ecx = *Current-color
  8b/-> *Current-color 1/r32/ecx
  # increment *Current-color
  81 0/subop/add *Current-color 0x01010101/imm32
  # eax = *Video-memory + 0x1200 (a few rows down from top, around middle of screen)
  8b/-> *Video-memory-addr 0/r32/eax
  05/add-to-eax 0x1200/imm32
  89/copy *eax 1/r32/ecx
  # eax += 0x400
  05/add-to-eax 0x400/imm32
  89/copy *eax 1/r32/ecx
  # eax += 0x400
  05/add-to-eax 0x400/imm32
  89/copy *eax 1/r32/ecx
  # eax += 0x400
  05/add-to-eax 0x400/imm32
  89/copy *eax 1/r32/ecx
$timer-interrupt-handler:epilogue:
  # epilogue
  9d/pop-flags
  61/pop-all-registers
  fb/enable-interrupts
  cf/return-from-interrupt

== data

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
# }}}

Current-color:
  0/imm32

# vim:ft=subx