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54 lines
1.5 KiB
Markdown
54 lines
1.5 KiB
Markdown
*Concepts you may want to Google beforehand: interrupts, CPU
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registers*
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**Goal: Make our previously silent boot sector print some text**
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We will improve a bit on our infinite-loop boot sector and print
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something on the screen. We will raise an interrupt for this.
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On this example we are going to write each character of the "Hello"
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word into the register `al` (lower part of `ax`), the bytes `0x0e`
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into `ah` (the higher part of `ax`) and raise interrupt `0x10` which
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is a general interrupt for video services.
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`0x0e` on `ah` tells the video interrupt that the actual function
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we want to run is to 'write the contents of `al` in tty mode'.
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We will set tty mode only once though in the real world we
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cannot be sure that the contents of `ah` are constant. Some other
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process may run on the CPU while we are sleeping, not clean
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up properly and leave garbage data on `ah`.
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For this example we don't need to take care of that since we are
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the only thing running on the CPU.
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Our new boot sector looks like this:
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```nasm
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mov ah, 0x0e ; tty mode
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mov al, 'H'
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int 0x10
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mov al, 'e'
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int 0x10
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mov al, 'l'
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int 0x10
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int 0x10 ; 'l' is still on al, remember?
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mov al, 'o'
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int 0x10
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jmp $ ; jump to current address = infinite loop
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; padding and magic number
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times 510 - ($-$$) db 0
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dw 0xaa55
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```
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You can examine the binary data with `xxd file.bin`
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Anyway, you know the drill:
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`nasm -f bin boot_sect_hello.asm -o boot_sect_hello.bin`
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`qemu boot_sect_hello.bin`
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Your boot sector will say 'Hello' and hang on an infinite loop
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