Making an OS (x86) - Part 2 challenge

boot.asm

mov ah, 0x0e		; initialize ah
mov al, 'a'		; first letter
label:			; enter loop
	int 0x10	; print char
	xor al, 32	; switch from uppercase to lowercase or vice-versa,
	inc al		; next letter

	mov bl, al	; This checks if the last 5 digits of al are 11010 
	and bl, 31	; meaning 26, without modifying it. This is the case 
	cmp bl, 26	; for 'Z' and 'z' but not for any other ascii letter. 
	jle label	; The code loops as long as the end of al is less than or equal to 26.

jmp $			; End of the program.

times 510-($-$$) db 0
db 0x55, 0xaa

Hey I also did the challenge and I don't quite understand what the jmp $ instruction in your code does.

Oh wait is that just an endless loop?

Oh wait is that just an endless loop?

Yes, exactly that, it jumps to itself, looping endlessly.

mov ah, 0x0e			; Switch to teletype mode
mov al, 'A'			; Initialize al with first letter
	
loop:
	int 0x10		; Output al to screen
	add al, 'a' - 'A'	; Move to lowercase ascii
	int 0x10		; Output al to screen
	sub al, 'a' - 'A' - 1	; Move to next UPPERCASE letter in ascii
	cmp al, 'Z' + 1		; Check if we've reached the end of the
	jne loop		; alphabet. If not, keep looping.

jmp $				; Infinite loop

Just skimmed it by one less line by making the switch between UPPER and lower also increment al. I'm sure you could implement it with your and command. Not that it really matters, I just enjoy seeing how little lines I can squish something into!

Oh wait is that just an endless loop?

Already been answered but for clarity $ in NASM Syntax is the current address/line the processor is executing. Therefore by telling the computer to jmp $ we're asking it "Jump to the beginning of this line again" and it'll continue jumping to itself infinitely. Just wanted to clear that up as it's used in the Bootloader Padding section of the code too ($ - $$).