87 lines
2.6 KiB
NASM
87 lines
2.6 KiB
NASM
[BITS 16]
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[ORG 0x7c00]
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db `#!/usr/bin/env sh\n`
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db `: <<'EOF'\n`
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db `GOTO msdos\n`
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;;; The #!… above is interpreted as … jnz short 0x7c78 in x86 assembly.
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times 0x7c78-0x7c00-($-$$) db 0
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;;; Switch to 320x200x256 VGA mode
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mov ax, 0x0013
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int 10h
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;;; Framebuffer address is 0xa000, store it into the fs register (the segment base, in multiples of 16)
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push 0xa000
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pop fs
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;;; Set pixel value (0, then increase at each step below)
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xor bl, bl
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;;; set register di to 0
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xor di,di
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;;; Store pixels, display something flashy.
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loop:
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mov byte [fs:di], bl
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inc bl
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inc di
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cmp bl, 255
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je endline
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jmp loop
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endline:
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add di, 65
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xor bl, bl
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mov ax, di
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cmp ax, (320*200)
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je end
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jmp loop
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;;; Infinite loop
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end:
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jmp end
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;;; Fill the remaining bytes with 0 and write a partition table
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times 0x1b8-($-$$) db 0
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db "ExOSxx" ;; 0x1b8 unique disk ID (4-6 bytes? Can be any value)
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;;; Partition table entries follow this format:
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;;; 1 byte Bootable flag (0x00 = no, 0x80 = yes)
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;;; 3 bytes Start Head Sector Cylinder (8 bits + 6 bits + 10 bits)
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;;; 1 byte Partition type (0x01 = FAT12)
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;;; 3 bytes End Head Sector Cylinder (8 bits + 6 bits + 10 bits)
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;;; 4 bytes LBA offset
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;;; 4 bytes LBA length
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;;;0x1be 0x1c1 0x1c2 0x1c5 0x1c6 0x1c9 0x1ca 0x1cd
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;;; This is filled with dummy values, and later patched with fdisk.
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db 0x80, 0x00, 0x00, 0x00, 0x01, 0x00, 0x00, 0x00, 0xa0, 0x05, 0x00, 0x00, 0x9b, 0x05, 0x00, 0x00 ;; 0x1be p1
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db 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ;; 0x1ce p2
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db 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ;; 0x1de p3
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db 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00 ;; 0x1ee p4
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db 0x55, 0xaa ;; 0x1fe End the bootsector with 55 AA, which is the MBR signature.
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;;; This is the end of the first 512 bytes (the bootsector).
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;;; After the bootsector, close the sh here-document skipped via : <<'EOF'
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db `\n`
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db `EOF\n`
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db `echo Hello world by the OS, from sh!\n`
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db `while sleep 10; do :; done\n`
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db `exit\n`
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;;; for good measure: go into an infinite loop if the exit did not happen.
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db `while :; do sleep 1; done\n`
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;;; end of the SH section, everything until this point is skipped by MS-DOS batch due to the GOTO'
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db `:msdos\n`
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db `@cls\n`
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db `@echo Hello world by the OS, from MS-DOS!\n`
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db `command.com\n`
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db `exit\n`
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;;; for good measure: go into an infinite loop if the exit did not happen.
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db `:loop\n`
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db `GOTO loop\n`
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;;; Fill up to 32k with 0. This constitutes the first 32k of the ISO image.
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times (32*1024)-($-$$) db 0
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