From dde61c13e7fe20cb218834a07888578dcc6cc0aa Mon Sep 17 00:00:00 2001
From: Carlos Fenollosa <carlos.fenollosa@gmail.com>
Date: Sun, 5 Oct 2014 12:03:44 +0200
Subject: [PATCH] lesson 4, manipulating the stack

---
 04-bootsector-stack/README.md           | 13 ++++++++
 04-bootsector-stack/boot_sect_stack.asm | 42 +++++++++++++++++++++++++
 2 files changed, 55 insertions(+)
 create mode 100644 04-bootsector-stack/README.md
 create mode 100644 04-bootsector-stack/boot_sect_stack.asm

diff --git a/04-bootsector-stack/README.md b/04-bootsector-stack/README.md
new file mode 100644
index 0000000..8d02f90
--- /dev/null
+++ b/04-bootsector-stack/README.md
@@ -0,0 +1,13 @@
+*Concepts you may want to Google beforehand:stack*
+
+The usage of the stack is important, so we'll write yet another boot sector
+with an example.
+
+Remember that the `bp` register stores the base address (i.e. bottom) of the stack,
+and `sp` stores the top, and that the stack grows downwards from `bp` (i.e. `sp` gets
+decremented)
+
+This lesson is quite straightforward, so jump ahead to the code.
+
+I suggest that you try accessing in-stack memory addresses by yourself, 
+at different points in the code, and see what happens.
diff --git a/04-bootsector-stack/boot_sect_stack.asm b/04-bootsector-stack/boot_sect_stack.asm
new file mode 100644
index 0000000..37399f0
--- /dev/null
+++ b/04-bootsector-stack/boot_sect_stack.asm
@@ -0,0 +1,42 @@
+mov ah, 0x0e
+
+mov bp, 0x8000 ; this is an address far away from 0x7c00 so that we don't get overwritten
+mov sp, bp
+
+push 'A'
+push 'B'
+push 'C'
+
+; to show how the stack grows downwards
+mov al, [0x7ffe] ; 0x8000 - 2
+int 0x10
+
+; however, don't try to access [0x8000] now, because it won't work
+; you can only access the stack top so, at this point, only 0x7ffe (look above)
+mov al, [0x8000]
+int 0x10
+
+
+; recover our characters using the standard procedure: 'pop'
+; We can only pop full words so we need an auxiliary register to manipulate
+; the lower byte
+pop bx
+mov al, bl
+int 0x10 ; prints C
+
+pop bx
+mov al, bl
+int 0x10 ; prints B
+
+pop bx
+mov al, bl
+int 0x10 ; prints A
+
+; data that has been pop'd from the stack is garbage now
+mov al, [0x8000]
+int 0x10
+
+
+jmp $
+times 510-($-$$) db 0
+dw 0xaa55