8085

 

 

1.     What is Stack? Explain about stack in 8085.

·       The stack is a group of memory location in the R/W memory that is used for temporary storage of binary information during the execution of a program

·       The stack is a LIFO structure. – Last In First Out.

·       The starting location of the stack is defined by loading a 16 bit address into the stack pointer that spaced is reserved, usually at the top of the memory map

 

Stack in 8085

 

·       In 8085 microprocessor system the beginning of the stack is defined in the program by using the instruction  LXI SP,<16 bit>

·       The 8085 provide two instruction PUSH & POP for storing information on the stack and retrieving it back.

PUSH  <Reg Pair>    :   Stores content of register pair on to stack 

POP     <Reg Pair>    :   Retrieves content from top of the stack in to register pair

 

PUSH     H

 

·       The stack pointer is decremented by one to 2098 H , and the contents of the H register are copied to memory location 2098H.

·       The stack pointer register is again decremented by one to 2097H, and the contents of the L register are copied to memory location 2097H.

·       The contents of the register pair HL are not destroyed ; however HL is made available for delay counter.

POP    H

·       The contents of the top of the stack location shown by the stack pointer are copied in the L register and the stack pointer register is incremented by one to 2098 H.

·       The contents of the top of the stack  (now it is 2098H) are copied in the  H register, and the stack pointer is  incremented by one.

·       The contents of memory location  2097H and 2098 are not destroyed  until some other data bytes are  stored in these location.

Operation of the stack

·       During PUSH, the stack operates in a “decrement then store” style. The stack pointer is decremented first, then the information is placed on the stack.

·       During POP, the stack operates in a “use then increment” style. The information is retrieved from the top of the stack and then the pointer is incremented.

·       The SP pointer always points to “the top of the stack’’

 

1.     What is subroutine? Explain implementation of subroutines in 8085.

 

·       A subroutine is group of instruction written separately from the main program to perform a function that occurs repeatedly in the main program.

·       When a main program calls a subroutine the program execution is transferred to the subroutine after the completion of the subroutine ,the program execution returns to the main program.

·       The microprocessor uses the stack to store the return address of the subroutine.

Subroutines in 8085:

·       The 8085 has two instructions for dealing with subroutines.

·       The CALL instruction is used to redirect program execution to the subroutine.

·       The RET instruction is used to return to the main program at the end of the subroutine

 

TThe CALL instruction

CALL    <16-bit address of subroutine>

 

·       Call subroutine located at the memory address specified by the 16 bit operand.

·       This instruction places the address of the next instruction on the stack and transfer the

program execution to the subroutine address.

The RET instruction

 

RET

 

·       Return unconditionally from the subroutine.

·       This instruction locates the return address on the top of the stack and transfers the program execution back to the calling program.