lesson 3, boot sector with memory addressing

03-bootsector-memory/boot_sect_memory.asm

@@ -10,7 +10,7 @@ int 0x10
 
 ; attempt 2
 ; It tries to print the memory address of 'the_secret' which is the correct approach.
-; However, BIOS starts loading at address 0x7c00
+; However, BIOS places our bootsector binary at address 0x7c00
 ; so we need to add that padding beforehand. We'll do that in attempt 3
 mov al, "2"
 int 0x10
@@ -19,7 +19,9 @@ int 0x10
 
 ; attempt 3
 ; Add the BIOS starting offset 0x7c00 to the memory address of the X
-; and then dereference the contents of that pointer
+; and then dereference the contents of that pointer.
+; We need the help of a different register 'bx' because 'mov al, [ax]' is illegal.
+; A register can't be used as source and destination for the same command.
 mov al, "3"
 int 0x10
 mov bx, the_secret
@@ -29,19 +31,21 @@ int 0x10
 
 ; attempt 4
 ; We try a shortcut since we know that the X is stored at byte 0x2d in our binary
+; That's smart but ineffective, we don't want to be recounting label offsets
+; every time we change the code
 mov al, "4"
 int 0x10
 mov al, [0x7c2d]
 int 0x10
 
 
-jmp $
-
+jmp $ ; infinite loop
 
 the_secret:
-    ; ASCII code 0x58 is stored just before the zero-padding
-    ; on this code that is at byte 0x2d (check it out using xdd)
+    ; ASCII code 0x58 ('X') is stored just before the zero-padding.
+    ; On this code that is at byte 0x2d (check it out using 'xdd file.bin')
     db "X"
 
+; zero padding and magic bios number
 times 510-($-$$) db 0
 dw 0xaa55