PushPop

# PUSH and POP instructions:
----------------------------
Pushes or pops register values to or from the stack. A x86 CPU has five pair of push and pop instructins:

#PUSH:
------
     Pushes a value to the stack from a designated 16/32/64-bit register or memory variable.
      
     Syntax: push <reg>		; Pushes the value stored in register <reg> to stack.
	     push <mem-var>	; Pushes the value in variable <mem-var> to stack.
#POP:
-----
     Fills the designated 16/32/64-bit register/memory variable by popping the same sized value off the stack.

     syntax: pop <reg>		; Pop from stack to register <reg>.
             pop <mem-var>	; Pop from stack to variable <mem-var>.


These two are general purpose single step push and pop instructions. They push from or pop to any specified register/variable. The other four
pairs of push/pop instuction are specifialized - two pairs of instruction to push/pop only the flag register "eflags" and other two pairs to
push/pop all the registers at once:

#PUSHF								#POPF
      Pushes the 16 bit eflags register to stack.		     The complementary opcode of PUSHF. Pops last 16-bit value off the stack
								     to the 16-bit eflags register as current flag status.	

#PUSHFD								#POPFD
      Pushed the 32 bit eflags register to stack.		     Like POPF, its the complementary of PUSHFD. Pops the 32-bit value off the
								     top of stack to the 32-bit eflags register as current flag status.

#PUSHA								#POPA
      Pushes values from the lower 16-bit of all 16		      Pops 16-bit values and fills the lower 16-bit of all the general purpose
      general purpose register to stack. The order is:		      register except rsp, as rsp keeps the current stack address. The popping
      rax, rbx, rcx, rdx, rsi, rdi, rbp, rbp, r8 to r15.	      order is complementary to the pusha.

#PUSHAD								#POPAD
      Pushes values from the lower 32-bit of all 16		      Pops 16-bit values and fills the lower 32-bit of all the general purpose
      general purpose register to stack in the order		      register except rsp, as rsp keeps the current stack address. The popping
      described in PUSHA.					      order is complementary to that described in pusha.

In x86 architecture, stack ends at the higher memory location and grows towards lower memory location. So when ever something pus-
hed to the stack, the 64-bit stack pointer RSP is decremented by the size of the data. For example "push ecx" decrements ESP by 4
bytes and copies the 32-bit(4 bytes) ECX value to the 4 bytes of space freed up.

		  - - -                         	  - - -    			  - - -   		
		  - - -	   				  - - -    	             	  - - -	  
		|________|                              |________|                     	|________|
		|________|                              |________|                     	|________|
		|________|    ECX = 0x35c29549          |________|                     	|________|
		|________|	  push ECX       ESP--> |________|        	  	|________|
		|________|     -------------->          |00110101|       pop ECX 	|________|
		|________|                              |11000010|   -------------->   	|________|
		|________|                              |10010101|                    	|________|
     	 ESP-->	|________|                              |01001001|               ESP--> |________|
							----------

POP operates exactly the opposite way. RSP is decremented equal to the operand size and the value is takes away from the stack to the destina-
tion register/variable, just as shown in the figure. Similar thing happens for all other four pairs of instruction. For example PUSHAD decre-
ments ESP by (32X16)= 512/8 = 64-bytes then transfers register values to the stack. Similarly POPAD increments ESP 64-bytes, discards the
value coresponding to previous ESP value and restores all other 15 registers to respective values saved in stack.