lesson 3, boot sector with memory addressing

cfenollosa · cfenollosa · commit bdca5e65da76 · 2014-10-05T11:11:03.000+02:00
diff --git a/03-bootsector-memory/.boot_sect_memory.asm.swp b/03-bootsector-memory/.boot_sect_memory.asm.swp
diff --git a/03-bootsector-memory/README.md b/03-bootsector-memory/README.md
@@ -1,3 +1,38 @@
-TBD
+*Concepts you may want to Google beforehand: memory offsets, pointers*
 
-Our new boot sector will refer to memory addresses and labels
+The only goal of this lesson is to learn where the boot sector is stored
+
+Please open page 14 [of this document](
+http://www.cs.bham.ac.uk/~exr/lectures/opsys/10_11/lectures/os-dev.pdf)<sup>*</sup>
+and look at the figure with the memory layout.
+
+I could just go ahead and tell you that it starts at `0x7C00`, but it's
+better with an example.
+
+We want to print an X on screen. We will try 4 different strategies
+and see which ones work and why.
+
+First, we will define the X as data, with a label:
+```nasm
+the_secret:
+    db "X"
+```
+
+Then we will try to access `the_secret` in many different ways:
+
+1. `mov al, the_secret`
+2. `mov al, [the_secret]`
+3. `mov al, the_secret + 0x7C00`
+4. `mov al, 2d + 0x7C00`, where `2d` is the actual position of the X in the binary
+
+Take a look at the code and read the comments.
+
+Compile and run the code. You should see a string similar to `1[2¢3X4X`, where
+the bytes following 1 and 2 are just random garbage.
+
+If you add or remove instructions, remember to compute the new offset of the X
+by counting the bytes, and replace `0x2d` with the new one.
+
+~~~~~
+This whole tutorial is heavily inspired on that document. Please read the
+root-level README for more information on that.
diff --git a/03-bootsector-memory/boot_sect_memory.asm b/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
