A stack is a LIFO data structure
 Can only add and remove from ONE end (TOP)
 PUSH
 POP

Managed by the CPU, using SP (stack pointer)
     note: esp is 32 bit pointer 

   
Emu stack documentation
*** We only use 16 bit pops and pushes with emu8086 ***


   PUSH syntax:
      PUSH r/m16     

   POP syntax:
      POP r/m16      

!! NOTE: can only push 16 bit values


Save and restore registers when they contain important values. Note that 
the PUSH and POP instructions are in the opposite order:

   push si ; push registers
   push cx
   push bx
   
   ..... code
   
   pop bx  ; opposite order
   pop cx
   pop si


Nested Loop
===========
Remember the nested loop we created ? It's easy to push the outer loop 
counter before entering the inner loop:

   mov cx, 100       ; set outer loop count
   L1:                ; begin the outer loop
       push cx       ; save outer loop count
   
       mov cx,20     ; set inner loop count
       L2:            ; begin the inner loop
        ;
        ;
       loop L2        ; repeat the inner loop
   
       pop cx        ; restore outer loop count
   loop L1            ; repeat the outer loop


Related Instructions:
====================
   PUSHF and POPF
      push and pop the FLAGS register

   PUSHA pushes the 32-bit general-purpose registers on the stack 
      order: AX, CX, DX, BX, SP, BP, SI, DI
   POPA pops the same registers off the stack in reverse order

      (PUSHAD and POPAD do the same for 32-bit registers)

; this sample shows how the stack works. 
; click 'stack' button in emulator to see the contents of the stack.
; stack is important element in computer architecture.
; this code does nothing useful, except printing "Hi" in the end.
name "stack"
org     100h   ; create tiny com file.

mov     ax, 1234h
push    ax

mov     dx, 5678h
push    dx

pop     bx
pop     cx

; function call pushes ip value of the next instruction:
call    tfunc
mov     ax, 7890h
push    ax
pop     bx

; interrupts are like funtions,
; but in addition they push code segment into the stack
mov     ax, 3
int     10h    ; set standard video mode.

; a typical use of stack is to set segment registers.
; set ds to video memory segment: can't set ds directly
mov     ax, 0b800h
push    ax
pop     ds

; print "hi", use colors
mov     [170h], 'H'
mov     [172h], 'i'
mov     [171h], 11001110b ; color attribute for 'h'
mov     [173h], 10011110b ; color attribute for 'i'

; wait for any key press....
mov     ah, 0
int     16h      

; here we "pop" the ip value, and return control to the OS
ret

; the test procedure:
tfunc   proc
        xor     bx, bx
        xor     cx, cx
        ; here we "pop" the ip value,and return control to the main program:
        ret
endp
end