What is a stack pointer used for in microprocessors

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I am preparing for a microprocessor exam  If the use of a program counter is to hold the address of the next instruction  what is use of stack pointer

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For 8085  Stack pointer is a special purpose 16-bit register in the Microprocessor  which holds the address of the top of the stack   The stack pointer register in a computer is made available for general purpose use by programs executing at lower privilege levels than interrupt handlers  A set of instructions in such programs  excluding stack operations  stores data other than the stack pointer  such as operands  and the like  in the stack pointer register  When switching execution to an interrupt handler on an interrupt  return address data for the currently executing program is pushed onto a stack at the interrupt handler s privilege level  Thus  storing other data in the stack pointer register does not result in stack corruption  Also  these instructions can store data in a scratch portion of a stack segment beyond the current stack pointer   Read this one for more info   General purpose use of a stack pointer register

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On some CPUs  there is a dedicated set of registers for the stack   When a call instruction is executed  one register is loaded with the program counter at the same time as a second register is loaded with the contents of the first  a third register is be loaded with the second  and a fourth with the third  etc   When a return instruction is executed  the program counter is latched with the contents of the first stack register and the same time as that register is latched from the second  that second register is loaded from a third  etc   Note that such hardware stacks tend to be rather small  many the smaller PIC series micros  for example  have a two-level stack    While a hardware stack does have some advantages  push and pop don t add any time to a call return  for example  having registers which can be loaded with two sources adds cost   If the stack gets very big  it will be cheaper to replace the push-pull registers with an addressable memory   Even if a small dedicated memory is used for this  it s cheaper to have 32 addressable registers and a 5-bit pointer register with increment decrement logic  than it is to have 32 registers each with two inputs   If an application might need more stack than would easily fit on the CPU  it s possible to use a stack pointer along with logic to store fetch stack data from main RAM

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The stack pointer holds the address to the top of the stack  A stack allows functions to pass arguments stored on the stack to each other  and to create scoped variables  Scope in this context means that the variable is popped of the stack when the stack frame is gone  and or when the function returns  Without a stack  you would need to use explicit memory addresses for everything  That would make it impossible  or at least severely difficult  to design high-level programming languages for the architecture  Also  each CPU mode usually have its own banked stack pointer  So when exceptions occur  interrupts for example   the exception handler routine can use its own stack without corrupting the user process

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A stack is a LIFO  last in  first out - the last entry you push on to the stack is the first one you get back when you pop  data structure that is typically used to hold stack frames  bits of the stack that belong to the current function    This includes  but is not limited to    the return address  a place for a return value  passed parameters  local variables    You push items onto the stack and pop them off  In a microprocessor  the stack can be used for both user data  such as local variables and passed parameters  and CPU data  such as return addresses when calling subroutines    The actual implementation of a stack depends on the microprocessor architecture  It can grow up or down in memory and can move either before or after the push pop operations   Operation which typically affect the stack are    subroutine calls and returns  interrupt calls and returns  code explicitly pushing and popping entries  direct manipulation of the SP register    Consider the following program in my  fictional  assembly language   Addr  Opcodes   Instructions      Comments ----  --------  --------------  ----------                                   1  pc lt -0000  sp lt -8000 0000  01 00 07  load r0 7         2  pc lt -0003  r0 lt -7 0003  02 00     push r0           3  pc lt -0005  sp lt -7ffe   sp 7ffe  lt -0007 0005  03 00 00  call 000b         4  pc lt -000b  sp lt -7ffc   sp 7ffc  lt -0008 0008  04 00     pop r0            7  pc lt -000a  r0 lt - sp 7ffe 0007    sp lt -8000 000a  05        halt              8  pc lt -000a 000b  06 01 02  load r1  sp 2     5  pc lt -000e  r1 lt - sp 2 7ffe 0007   000e  07        ret               6  pc lt - sp 7ffc 0008    sp lt -7ffe   Now let s follow the execution  describing the steps shown in the comments above    This is the starting condition where the program counter is zero and the stack pointer is 8000  all these numbers are hexadecimal   This simply loads register r0 with the immediate value 7 and moves to the next step  I ll assume that you understand the default behavior will be to move to the next step unless otherwise specified   This pushes r0 onto the stack by reducing the stack pointer by two then storing the value of the register to that location  This calls a subroutine  What would have been the program counter is pushed on to the stack in a similar fashion to r0 in the previous step and then the program counter is set to its new value  This is no different to a user-level push other than the fact it s done more as a system-level thing  This loads r1 from a memory location calculated from the stack pointer - it shows a way to pass parameters to functions  The return statement extracts the value from where the stack pointer points and loads it into the program counter  adjusting the stack pointer up at the same time  This is like a system-level pop  see next step   Popping r0 off the stack involves extracting the value from where the stack pointer points then adjusting that stack pointer up  Halt instruction simply leaves program counter where it is  an infinite loop of sorts    Hopefully from that description  it will become clear  Bottom line is  a stack is useful for storing state in a LIFO way and this is generally ideal for the way most microprocessors do subroutine calls   Unless you re a SPARC of course  in which case you use a circular buffer for your stack  -   Update  Just to clarify the steps taken when pushing and popping values in the above example  whether explicitly or by call return   see the following examples   LOAD R0 7 PUSH R0                      Adjust sp       Store val sp- gt   --------        --------        --------          xxxx     sp- gt    xxxx     sp- gt    0007                                                                                                                                                          --------        --------        --------   POP R0                      Get value       Adjust sp       --------        --------   sp- gt  --------  sp- gt     0007     sp- gt    0007            0007                                                                                                                                                          --------        --------        --------

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Should you ever crave deeper understanding  I heartily recommend Patterson and Hennessy as an intro and Hennessy and Patterson as an intermediate to advanced text  They re pricey  but truly non-pareil  I just wish either or both were available when I got my Masters  degree and entered the workforce designing chips  systems  and parts of system software for them  but  alas   that was WAY too long ago -   Stack pointers are so crucial  and the distinction between a microprocessor and any other kind of CPU so utterly meaningful in this context    or  for that matter  in ANY other context  in the last few decades    -  that I doubt anything but a couple of thorough from-the-ground-up refreshers can help -

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A stack pointer is a small register that stores the address of the top of stack  It is used for the purpose of pointing address of the top of the stack

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The Stack is an area of memory for keeping temporary data  Stack is used by the CALL instruction to keep the return address for procedures The return RET instruction gets this value from the stack and returns to that offset  The same thing happens when an INT instruction calls an interrupt  It stores in the Stack the flag register  code segment and offset  The IRET instruction is used to return from interrupt call    The Stack is a Last In First Out  LIFO  memory  Data is placed onto the Stack with a PUSH instruction and removed with a POP instruction  The Stack memory is maintained by two registers  the Stack Pointer  SP  and the Stack Segment  SS  register  When a word of data is PUSHED onto the stack the the High order 8-bit Byte is placed in location SP-1 and the Low 8-bit Byte is placed in location SP-2  The SP is then decremented by 2  The SP addds to the  SS x 10H  register  to form the physical stack memory address  The reverse sequence occurs when data is POPPED from the Stack  When a word of data is POPPED from the stack the the High order 8-bit Byte is obtained in location SP-1 and the Low 8-bit Byte is obtained in location SP-2  The SP is then incremented by 2

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The stack pointer stores the address of the most recent entry that was pushed onto the stack   To push a value onto the stack  the stack pointer is incremented to point to the next physical memory address  and the new value is copied to that address in memory   To pop a value from the stack  the value is copied from the address of the stack pointer  and the stack pointer is decremented  pointing it to the next available item in the stack   The most typical use of a hardware stack is to store the return address of a subroutine call   When the subroutine is finished executing  the return address is popped off the top of the stack and placed in the Program Counter register  causing the processor to resume execution at the next instruction following the call to the subroutine   http   en wikipedia org wiki Stack  28data structure 29 Hardware stacks

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You got more preparing  for the exam  to do  -   The Stack Pointer is a register which holds the address of the next available spot on the stack   The stack is a area in memory which is reserved to store a stack  that is a LIFO  Last In First Out  type of container  where we store the local variables and return address  allowing a simple management of the nesting of function calls in a typical program   See this Wikipedia article for a basic explanation of the stack management

[stack] What is a stack pointer used for in microprocessors?

Examples related to stack

Examples related to microprocessors