hello world bootloader, boots properly on x86

boot.asm

+;--------------- PRELIMINARY SETUP ---------------;
+	[BITS 16]	; 16 bit real mode
+	[ORG 0x7C00]	; MBR is loaded at 0x7C00 
+;---------------- BOOTLOADER CODE ----------------;
+ls:	MOV SI, str1	; pointer to string in SI
+	CALL PrintStr	; print string
+	;MOV ebx, 1000		;start at 10000
+	;SUB ebx, 1		;down 1 each time
+	;NOP			;waste time
+	;JZ 6			;for loop
+	;JMP ls			; 
+	HLT
+;---------------- SCREEN FUNCTIONS ---------------;
+PrintStr:	; print string at SI
+nextchar:
+	MOV AL, [SI]	; Grab next char
+	OR AL, AL	; if char is null term
+	JZ exit		; then return
+	CALL PrintChar	; else print char
+	INC SI		; inc pointer to next char
+	JMP nextchar	; loop
+	exit:
+	RET
+
+PrintChar:	; print char at AL
+	MOV AH, 0x0E	; Teletype Mode
+	MOV BH, 0x00	; Page zero
+	MOV BL, 0x07	; Light Gray
+	INT 0x10	; Print Character
+	RET
+;------------------ DATA BLOCK ------------------;
+
+str1 db 'Hello World', 0		;stuff
+;-------------- PADDING / SIGNATURE -------------;
+	; $ is current line, $$ is first line, db 0 is a 00000000 byte
+	; So, pad the code with 0s until you reach 510 bytes
+TIMES 510 - ($ - $$) DB 0
+; Fill last two bytes (a word) with the MBR signature 0xAA55
+	DW 0xAA55
+
+Main:	;main section