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Stacks & Subroutines�Slides-7

Dr. Ritika

Department of Computer Application

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The Stack

  • The stack is an area of memory identified by the programmer for temporary storage of information.
  • The stack is a LIFO (Last In First Out. ) structure.
  • The stack normally grows backwards into memory.
    • In other words, the programmer �defines the bottom of the stack �and the stack grows up into �reducing address range.

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The Stack

  • Given that the stack grows backwards into memory, it is customary to place the bottom of the stack at the end of memory to keep it as far away from user programs as possible.

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  • In the 8085, the stack is defined by setting the SP (Stack Pointer) register.

LXI SP, FFFFH

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  • This sets the Stack Pointer to location FFFFH (end of memory for the 8085).

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Saving Information on the Stack

  • Information is saved on the stack by PUSHing it on.
    • It is retrieved from the stack by POPing it off.

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  • The 8085 provides two instructions: PUSH and POP for storing information on the stack and retrieving it back.

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    • Both PUSH and POP work with register pairs ONLY.

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The PUSH Instruction

  • PUSH B/D/H/PSW
    • Decrement SP
    • Copy the contents of register B to the memory location pointed to by SP
    • Decrement SP
    • Copy the contents of register C to the memory location pointed to by SP

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The POP Instruction

  • POP B/D/H/PSW
    • Copy the contents of the memory location pointed to by the SP to register E
    • Increment SP
    • Copy the contents of the memory location pointed to by the SP to register D
    • Increment SP

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Operation of the Stack

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

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

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  • The SP pointer always points to “the top of the stack”.

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LIFO

  • The order of PUSHs and POPs must be opposite of each other in order to retrieve information back into its original location.

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PUSH B

PUSH D

...

POP D

POP B

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  • Reversing the order of the POP instructions will result in the exchange of the contents of BC and DE.

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The PSW Register Pair

  • The 8085 recognizes one additional register pair called the PSW (Program Status Word).

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    • This register pair is made up of the Accumulator and the Flags registers.

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  • It is possible to push the PSW onto the stack, do whatever operations are needed, then POP it off of the stack.

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    • The result is that the contents of the Accumulator and the status of the Flags are returned to what they were before the operations were executed.

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Cautions with PUSH and POP

  • PUSH and POP should be used in opposite order.

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  • There has to be as many POP’s as there are PUSH’s.
    • If not, the RET statement will pick up the wrong information from the top of the stack and the program will fail.

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  • It is not advisable to place PUSH or POP inside a loop.

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Program to Reset and display Flags

  • Clear all Flags.
  • Load 00H in the accumulator, and demonstrate that the zero flag is not affected by data transfer instruction.
  • Logically OR the accumulator with itself to set the Zero flag, and display the flag at PORT1 or store all flags on the stack.

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Program to Reset and display Flags

  • XX00 LXI SP, XX99H Initialize the stack
    • 03 MVI L, 00H Clear L
    • 05 PUSH H Place (L) on stack
    • 06 POP PSW Clear Flags
    • 07 MVI A, 00H Load 00H
    • 09 PUSH PSW Save Flags on stack
    • 0A POP H Retrieve flags in L
    • 0B MOV A, L
    • 0C OUT PORT0 Display Flags (00H)
    • 0E MVI A, 00H Load 00H Again

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Program to Reset and display Flags

  • XX10 ORA A Set Flags and reset CY, AC
    • 11 PUSH PSW Save Flags on Stack
    • 12 POP H Retrieve Flags in L
    • 13 MOV A, L
    • 14 ANI 40H Mask all Flags except Z
    • 16 OUT PORT1 Displays 40H
    • 18 HLT End of Program

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Subroutines

  • A subroutine is a group of instructions that will be used repeatedly in different locations of the program.

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    • Rather than repeat the same instructions several times, they can be grouped into a subroutine that is called from the different locations.

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  • In Assembly language, a subroutine can exist anywhere in the code.
    • However, it is customary to place subroutines separately from the main program.

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Subroutines

  • The 8085 has two instructions for dealing with subroutines.

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

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    • The RTE instruction is used to return the execution to the calling routine.

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The CALL Instruction

  • CALL 4000H
    • 3-byte instruction.
    • Push the address of the instruction immediately following the CALL onto the stack and decrement the stack pointer register by two.
    • Load the program counter with the 16-bit address supplied with the CALL instruction.
    • Jump Unconditionally to memory location.

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The CALL Instruction

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The RTE Instruction

  • RTE
    • 1-byte instruction
    • Retrieve the return address from the top of the stack and increments stack pointer register by two.
    • Load the program counter with the return address.
    • Unconditionally returns from a subroutine.

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Illustrates the exchange of information between stack and Program Counter

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Program Execution

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CALL Execution

  • Instruction requires five machine cycles and eighteen T-states: Call instruction is fetched, 16-bit address is read during M2 and M3 and stored temporarily in W/Z registers. In next two cycles content of program counter are stored on the stack (address from where microprocessor continue it execution of program after completion of the subroutine.)

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RET Execution

  • Program execution sequence is transferred to the memory location 2043H location.M1 is normal fetch cycle during M2 contents of stack pointer are placed on address bus so 43H data is fetched and stored on Z register and SP is upgraded. Similarly for M3. Program sequence is transfered to2043H by placing contents of W/Z on address bus.

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Passing Data to a Subroutine

  • In Assembly Language data is passed to a subroutine through registers.
    • The data is stored in one of the registers by the calling program and the subroutine uses the value from the register.

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  • The other possibility is to use agreed upon memory locations.
    • The calling program stores the data in the memory location and the subroutine retrieves the data from the location and uses it.

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RESTART, CONDITIONAL CALL & RETURN INSTRUCTIONS

RST Instruction

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RESTART, CONDITIONAL CALL & RETURN INSTRUCTIONS

Conditional CALL

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RESTART, CONDITIONAL CALL & RETURN INSTRUCTIONS

Conditional RETURN

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A Proper Subroutine

  • According to Software Engineering practices, a proper subroutine:
    • Is only entered with a CALL and exited with an RTE
    • Has a single entry point
      • Do not use a CALL statement to jump into different points of the same subroutine.

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Writing Subroutines

Write a Program that will display FF and 11 repeatedly on the seven segment display. Write a ‘delay’ subroutine and Call it as necessary.

C000: LXI SP, FFFF C003: MVI A, FF

C005: OUT 00 C007: CALL C014 C00A: MVI A, 11

C00C: OUT 00 C00E: CALL 1420 C011: JMP C003

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Writing Subroutines

DELAY: C014: MVIB, FF

C016: MVIC, FF

C018: DCR C

C019: JNZ C018

C01C: DCR B

C01D: JNZ C016

C020: RET

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Nesting Subroutines

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Problem Statement

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Problem Statement

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Problem Statement

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Problem Statement

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Problem Statement