Stack in microprocessor 8085(presantation)

Stack & Subroutines
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The stack
 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.

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The stack
 The stack normally grows backwards into
memory.
 The stack can be initialized anywhere in the
user memory map , but stack is initialized at the
highest memory location so that there will not
be any interface with the program.
 In 8085 microprocessor system the beginning of
the stack is defined in the program by using the
instruction
 LXI SP,16 bit.
 The LXI SP,a 16 bit state that load the 16 bit
address into the stack pointer register.
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Information is stored and retrieved
from the stack
•The 8085 provide two instruction PUSH & POP for storing infor
mation on the stack and retrieving it back.

•Information in the register pairs stored on the stack in reverse
order by using the instruction PUSH.

• Information retrieved from the stack by using the instruction
POP.

•PUSH & POP both instruction works with register pairs only.
•The storage and retrieval of the content of registers on the
stack fallows the LIFO(Last-In-First-Out) sequence.

•Information in the stack location may not be destroyed until
new information is stored in that memory location
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The PUSH Instruction

2000 LXI SP,2099H Load the stack pointer register with the addre
ss 2099.
2003 LXI H ,42F2H Loads data in the HL register pair.

2006 PUSH H The content of the HL register pair pushed int
o stack.
2007 DELAY COUNTER
200F
2010 POP H Saved data in stack pointer register to HL regi
ster pair.

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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. 8085 Register
F
A
 The stack pointer register is again B C
E
D
decremented by one to 2097H,and H 42 F2 L
SP 2097
the contents of the L register are Memory
copied to memory location 2097H.
 The contents of the register pair F2 2097
42 2098
HL are not destroyed ; however X 2099

HL is made available for delay
counter. Contents on the stack &in the register
after the PUSH instruction
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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. 8085 Register

 The contents of the top of the stack A B
F
C
(now it is 2098H) are copied in the D E
H 42 F2 L
H register,and the stack pointer is SP 2099

incremented by one. MEMORY
 The contents of memory location
F2 2097
2097H and 2098 are not destroyed 42 2098
until some other data bytes are X 2099

stored in these location. Contentsafterthe stack and in the registers
on
the POP instruction
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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.
 During poping, 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’’.

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PUSH PSW Register Pair
 PUSH PSW (1 Byte Instruction) A Flag
 Decrement SP
 Copy the contents of
12 80
register A to the memory
location pointed to by SP
 Decrement SP
 Copy the contents of
Flag register to the memory FFFB
FFFC
location pointed to by SP
FFFD 80
FFFE 12
FFFF

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Pop PSW Register Pair
 POP PSW (1 Byte Instruction) A FLAG
 Copy the contents of the 12 80
memory location pointed to
by the SP to Flag register
 Increment SP
 Copy the contents of the
FFFB
memory location pointed to FFFC
by the SP to register A FFFD 80
FFFE 12
 Increment SP FFFF

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Subroutines
 A subroutine is group of instruction written sepa
rately from the main program to perform a func
tion that occurs repeatedly in the main program
.
 When a main program calls a subroutine the pro
gram execution is transferred to the subroutine
after the completion of the subroutine ,the prog
ram execution returns to the main program.
 The microprocessor uses the stack to store the r
eturn address of the subroutine.

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Subroutines
 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 t
he main program at the end of the subro
utine .

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The CALL instruction
 CALL ,16 bit
 Call subroutine in conditionally 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.

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The RET instruction
 Return unconditionally from the subrouti
ne.
 This instruction locates the return addre
ss on the top of the stack and transfers t
he program execution back to the calling
program.

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General characteristics of CALL
& RTE instruction
1. The CALL instructions are 3-byte instruc
tion; the second byte specifies the low
order byte ,and the third byte specifies
the high order byte of the subroutine
address.
2. The return instruction are 1-byte instruc
tions.
3. A CALL instruction must be used in
conjunction with a return instruction in
the subroutine .
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Necessary steps to implement
a subroutine

 The stack pointer register must be initiali
zed ,preferably at the highest memory lo
cation of the R/W memory.
 The call instruction should be used in the
main program accompanied by the RET i
nstruction in the subroutine.

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Conditional CALL and RTE Instructions
 The 8085 supports conditional CALL and co
nditional RTE instructions.
– The same conditions used with conditional
JUMP instructions can be used.
– CC, call subroutine if Carry flag is set.
– CNC, call subroutine if Carry flag is not set
– RC, return from subroutine if Carry flag is
set
– RNC, return from subroutine if Carry flag
is not set.
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Stack in microprocessor 8085(presantation)

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