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100 lines
2.0 KiB
ArmAsm
100 lines
2.0 KiB
ArmAsm
; MEMORY LAYOUT
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; 0x0000 - 0x7000: stack
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; 0x7000 - 0x7c00: globals
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; 0x7000 - 0x7001: boot drive number
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; 0x7001 - 0x7002: number of hard disk drives
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; 0x7002 - 0x7003:
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; 0x7c00 - 0x7e00: boot sector
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; BIOS puts our boot sector at 0000:7c00
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org 0x7c00
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; We're (probably) in real mode
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bits 16
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; Disable interrupts
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cli
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xor ax, ax
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mov ds, ax
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mov es, ax
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; Put stack base at 0x7000. Stack grows high->low, so we'll grow away from our program text at
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; 0x7c00. We reserve the range [0x7000,0x7c00) for globals.
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mov ss, ax
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mov sp, 0x7000
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; Segment for VGA (0xb800 * 16 = 0xb8000)
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mov ax, 0xb800
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mov fs, ax
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mov di, 0x7000
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; Store boot drive number
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mov ds:[di], dl
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; Get drive geometry
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mov di, 0x00
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mov ah, 0x08
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int 0x13
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jb panic
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mov ds:[di + 1], dl
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; Set VGA mode
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; https://mendelson.org/wpdos/videomodes.txt
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mov ax, 0x0003
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int 0x10
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mov word fs:[0x0000], 0xc048
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mov word fs:[0x0002], 0xc069
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mov word fs:[0x0004], 0xc021
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hlt
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panic:
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mov ax, 0x0003
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int 0x10
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mov word fs:[0x0000], 0x4f46
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mov word fs:[0x0002], 0x4f41
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mov word fs:[0x0004], 0x4f49
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mov word fs:[0x0006], 0x4f4c
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hlt
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; TODO: enable A20
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; TODO: load a second stage
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; TODO: grab all the info we can from bios interrupts and deliver it to ths OS nicely
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; e.g. in a fixed memory location
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; TODO:
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; - Generate GPT in justfile
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; - Parse global parition table
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; - Load second stage from GPT partition with a particular UUID / name like GRUB does
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; (it's Hah!IdontNeedEFI in GRUB)
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; - https://en.wikipedia.org/wiki/BIOS_boot_partition
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; - Future work:
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; - Boot from UEFI
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; - Boot on non-GPT partitioned disk
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; MBR bootstrap field is 440 bytes long
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%if ($ - $$) > 440
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%error "exceeded mbr bootstrap field size"
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%endif
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; Pad to the end of the MBR bootstrap field
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times 440 - ($ - $$) db 0
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; Unique disk ID
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dd 0x00000000
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; Reserved
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dw 0x0000
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; Zero the partition fields
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; TODO: fill in the field pointing to the GPT table
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times 510 - ($ - $$) db 0
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; Boot signature
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db 0x55
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db 0xaa
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