lesson 3, boot sector with memory addressing

03-bootsector-memory/boot_sect_memory.asm

@@ -0,0 +1,47 @@
+mov ah, 0x0e
+
+; attempt 1
+; Fails because it tries to print the memory address (i.e. pointer)
+; not its actual contents
+mov al, "1"
+int 0x10
+mov al, the_secret
+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
+; so we need to add that padding beforehand. We'll do that in attempt 3
+mov al, "2"
+int 0x10
+mov al, [the_secret]
+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
+mov al, "3"
+int 0x10
+mov bx, the_secret
+add bx, 0x7c00
+mov al, [bx]
+int 0x10
+
+; attempt 4
+; We try a shortcut since we know that the X is stored at byte 0x2d in our binary
+mov al, "4"
+int 0x10
+mov al, [0x7c2d]
+int 0x10
+
+
+jmp $
+
+
+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)
+    db "X"
+
+times 510-($-$$) db 0
+dw 0xaa55