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Stacks & subroutines 1
The document discusses stacks, subroutines, and the 8085 microprocessor. It provides the following key points: 1. The stack is an area of memory used for temporary storage of information in LIFO (last in first out) order, growing backwards into memory with the stack pointer register defining the bottom. 2. Subroutines allow groups of instructions to be called from different locations to avoid repetition. The 8085 uses CALL to redirect execution to a subroutine and RTE to return to the calling routine. 3. Data can be passed to subroutines through registers or memory locations. Proper subroutines only enter at the start and exit at the end, with a single entry point
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Stacks & subroutines 1
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- 2. o The stackis an area of memory identified by the programmer for temporary storage of information. o The stack is a LIFO (Last In First Out. ) structure. o The stack normally grows backwards into memory. o In other words, the programmer defines the bottom of the stack and the stack grows up into reducing address range. The Stack
- 3. o Given thatthe 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. o In the 8085, the stack is defined by setting the SP (Stack Pointer) register. LXI SP, FFFFH o This sets the Stack Pointer to location FFFFH (end of memory for the 8085). The Stack
- 4. o Information issaved on the stack by PUSHing it on. o It is retrieved from the stack by POPing it off. o The 8085 provides two instructions: PUSH and POP for storing information on the stack and retrieving it back. o Both PUSH and POP work with register pairs ONLY. Saving Information on the Stack
- 5. The PUSH Instruction ●PUSH B/D/H/PSW o Decrement SP o Copy the contents of register B to the memory location pointed to by SP o Decrement SP o Copy the contents of register C to the memory location pointed to by SP
- 6. The POP Instruction ●POP B/D/H/PSW o Copy the contents of the memory location pointed to by the SP to register E o Increment SP o Copy the contents of the memory location pointed to by the SP to register D o Increment SP
- 7. o During pushing,the stack operates in a “decrement then store” style. o The stack pointer is decremented first, then the information is placed on the stack. o During poping, the stack operates in a “use then increment” style. o The information is retrieved from the top of the the stack and then the pointer is incremented. o The SP pointer always points to “the top of the stack”. Operation of the Stack
- 8. ● The orderof PUSHs and POPs must be opposite of each other in order to retrieve information back into its original location. PUSH B PUSH D ... POP D POP B ● Reversing the order of the POP instructions will result in the exchange of the contents of BC and DE. LIFO
- 9. The PSW RegisterPair o The 8085 recognizes one additional register pair called the PSW (Program Status Word). o This register pair is made up of the Accumulator and the Flags registers. o It is possible to push the PSW onto the stack, do whatever operations are needed, then POP it off of the stack. o 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.
- 10. Cautions with PUSHand POP● PUSH and POP should be used in opposite order. ● 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. ● It is not advisable to place PUSH or POP inside a loop.
- 11. Program to Resetand display Flags oClear all Flags. oLoad 00H in the accumulator, and demonstrate that the zero flag is not affected by data transfer instruction. oLogically OR the accumulator with itself to set the Zero flag, and display the flag at PORT1 or store all flags on the stack.
- 12. Program to Resetand 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
- 13. Program to Resetand 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
- 14. o A subroutineis a group of instructions that will be used repeatedly in different locations of the program. o Rather than repeat the same instructions several times, they can be grouped into a subroutine that is called from the different locations. o In Assembly language, a subroutine can exist anywhere in the code. o However, it is customary to place subroutines separately from the main program. Subroutines
- 15. o The 8085has two instructions for dealing with subroutines. o The CALL instruction is used to redirect program execution to the subroutine. o The RTE instruction is used to return the execution to the calling routine. Subroutines
- 16. The CALL Instruction ●CALL 4000H o 3-byte instruction. o Push the address of the instruction immediately following the CALL onto the stack and decrement the stack pointer register by two. o Load the program counter with the 16-bit address supplied with the CALL instruction. o Jump Unconditionally to memory location.
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- 18. ● RTE o 1-byteinstruction o Retrieve the return address from the top of the stack and increments stack pointer register by two. o Load the program counter with the return address. o Unconditionally returns from a subroutine. The RTE Instruction
- 19. Illustrates the exchangeof information between stack and Program Counter
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- 21. CALL Execution ● Instructionrequires 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.)
- 22. RET Execution ● Programexecution 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.
- 23. Passing Data toa Subroutine o In Assembly Language data is passed to a subroutine through registers. o The data is stored in one of the registers by the calling program and the subroutine uses the value from the register. o The other possibility is to use agreed upon memory locations. o The calling program stores the data in the memory location and the subroutine retrieves the data from the location and uses it.
- 24. RESTART, CONDITIONAL CALL &RETURN INSTRUCTIONS RST Instruction
- 25. RESTART, CONDITIONAL CALL &RETURN INSTRUCTIONS Conditional CALL
- 26. RESTART, CONDITIONAL CALL &RETURN INSTRUCTIONS Conditional RETURN
- 27. A Proper Subroutine oAccording to Software Engineering practices, a proper subroutine: o Is only entered with a CALL and exited with an RTE o Has a single entry point o Do not use a CALL statement to jump into different points of the same subroutine.
- 28. Writing Subroutines Write aProgram 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
- 29. 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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