# 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 b0/copy-to-al 0xfe/imm8 # disable mask for IRQ0 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 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 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 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