/MakingAnOS

Following the tutorial from Daedalus Community

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Following the tutorial from Daedalus Community. link

The following are my notes

Part 1: Bootloader

Pre-requisites - nasm, qemu - installed via package manager.

  • Code file: boot.asm
  • bootloader is exactly 512 bytes long
  • Ends with hexadecimal values 55 aa
  • $ is current memory address
  • times - repeats an action - something like a pre-processor macro
  • it pads out the length of the file to reach the required size
  • db is define byte
  • $$ - address of current section
  • Command: nasm -f bin boot.asm -o boot.bin

Command: nasm

  • nasm - the assembler
  • -f bin - set format to bin
  • boot.asm - input file to assemble
  • -o boot.bin - output file
  • This will create a boot.bin file

Booting the file

  • Command: qemu-system-x86-64 boot.bin
  • This will start the virtual machine and show "Booting from Hard Disk..." which means it was successful.
  • Changing the length or the file will result in the emulator not being able to boot
  • Screenshot: booting successful

Part 2:

  • cmp compares the values and stores a flag
  • jump instructions use that flag to determine to jump
  • List of jump instructions: 
     je	   ; A==B
 jne	   ; A!=B
 jg	   ; A >B
 jl	   ; A <B
 jge	   ; A>=B
 jle	   ; A<=B
  • jump instruction is followed by a label.

Part 2 homework:

  • Print alphabet with alternating capitalizations

Solution:

  • start with lowercase a and flip flop labels -- evens and odds
  • starting at a, Z is the last letter, in evens.
  • odds do not need to be compared, 90 happens in evens. then jump to exit