8086 Stack, Procedures

Courtesy
Md. Shahidul Salim
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Farhan Sadaf
Stack
• The 8086 microprocessor has a dedicated area in memory called the stack, which
is used for temporary storage of data during program execution.
• It operates on a Last-In-First-Out (LIFO) principle, meaning that the most recently
stored data is the first to be retrieved.
• The stack pointer (SP) is a 16-bit register that points to the current top of the
stack. It contains the offset address of the memory location in the stack segment.
• Stack Segment (SS) register contains the base address of the stack segment in the
memory.
• The stack segment, like any other segment, may have a memory block of a
maximum of 64 Kbytes locations, and thus may overlap with any other segments.

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Stack (Cont.)
• The Stack Segment register (SS) and Stack pointer register (SP) together address
the stack-top.

• For a selected value of SS, the maximum value of SP=FFFFH and the segment can
have maximum of 64K locations.
• If the SP starts with an initial value of FFFFH, it will be decremented by two
whenever a 16-bit data is pushed onto the stack.
• After successive push operations, when the stack pointer contains 0000H, any
attempt to further push the data to the stack will result in stack overflow.

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Stack (Cont.)
• Stack is used by CALL instruction to keep return address for procedure, RET
instruction gets this value from the stack and returns to that offset.
• Quite the same thing happens when INT instruction calls an interrupt, it stores in
stack flag register, code segment and offset.
• IRET instruction is used to return from interrupt call.

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Stack Operations
1. PUSH
• Stores a 16 bit value in the stack. Stack pointer (SP) is decremented by 2, for
every PUSH operation.
• E.g. PUSH AX means SP=SP-2 and AX->[SP].
Syntax for PUSH instruction:

PUSH REG
PUSH SREG
PUSH memory
PUSH immediate (Only works on 80186 CPU and later)

REG: AX, BX, CX, DX, DI, SI, BP, SP.

SREG: DS, ES, SS, CS.
memory: [BX], [BX+SI+7], 16 bit variable, etc...
immediate: 5, -24, 3Fh, 10001101b, etc...
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Stack Operations (Cont.)
2. POP
• Gets 16 bit value from the stack. Stack pointer (SP) is incremented by 2, for
every POP operation.
• E.g. POP AX means [SP]->AX and SP=SP+2.
Syntax for POP instruction:

POP REG
POP SREG
POP memory

REG: AX, BX, CX, DX, DI, SI, BP, SP.

SREG: DS, ES, SS, (except CS).
memory: [BX], [BX+SI+7], 16 bit variable, etc...

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Stack Operations (Cont.)
3. PUSHF and POPF
• These instructions are used to push and pop the flags register (FLAGS) onto
and from the stack.
• They are often used when preserving and restoring the processor's status
during subroutine calls or context switches.

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Stack Operations (Cont.)
• If we push these values one by one into the stack: 1, 2, 3, 4, 5
the first value that we will get on pop will be 5, then 4, 3, 2, and only then 1.

• It is very important to do equal number of PUSHs and POPs, otherwise the stack
maybe corrupted and it will be impossible to return to operating system.

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Stack Examples
• PUSH and POP instruction are especially useful because we don't have too much
registers to operate with, so here is a trick:
• Store original value of the register in stack (using PUSH).
• Use the register for any purpose.
• Restore the original value of the register from stack (using POP).
ORG 100h

MOV AX, 1234h

PUSH AX ; store value of AX in stack.

MOV AX, 5678h ; modify the AX value.

POP AX ; restore the original value of AX.

RET
END
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Stack Examples (Cont.)
• Another use of the stack is for exchanging the values.
ORG 100h

MOV AX, 1212h ; store 1212h in AX.

MOV BX, 3434h ; store 3434h in BX

PUSH AX ; store value of AX in stack.

PUSH BX ; store value of BX in stack.

POP AX ; set AX to original value of BX.

POP BX ; set BX to original value of AX.

RET
END

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Procedures
• In a program, we very frequently face situations where there is a need to perform
the same set of task again and again. So, for that instead of writing the same
sequence of instructions, again and again, they are written separately in a
subprogram. This subprogram is called a procedure.

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Procedure Types
1. Near Call or Intra-segment call
• A near call refers a procedure which is in the same code segment.
• Only Instruction Pointer (IP) contents will be changed in NEAR procedure.
• Near calls are more efficient in terms of execution time because they do not
involve changing the code segment register.
SP <- SP-2
IP -> stores onto stack

IP <- starting address of a procedure

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Procedure Types (Cont.)
2. Far Call or Inter-segment call
• A Far call refers a procedure which is in different code segment.
• In this case both Instruction Pointer (IP) and the Code Segment (CS) register
content will be changed.

SP <- sp-2
cs contents -> stored on stack

SP <- sp-2
IP contents -> stored on stack

CS <- Base address of segment having procedure

IP <- address of first instruction in procedure

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Procedure Syntax
• PROC is a keyword to define that the set of instructions enclosed by the given
name is a procedure.
• The ENDP keyword defines that the body of the procedure has been ended.
procedure_name PROC [NEAR / FAR]
Instruction 1
Instruction 2
- - - - - - - - - - -
- - - - - - - - - - -
Instruction n
procedure_name ENDP

• The procedure will be executed whenever a CALL to the procedure is made.

CALL procedure_name

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Procedure Examples

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Procedure Examples (Cont.)
• Read 100 samples of data at 1-ms interval

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Procedure Examples (Cont.)
• Read 100 samples of data at 1-ms interval (Cont.)

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References
• https://www.brainkart.com/article/memory-Stacks-in-8086-Microprocessor_7851/
• https://faculty.kfupm.edu.sa/COE/shazli/coe205/Help/asm_tutorial_09.html#:~:text=there%20ar
e%20two%20instructions%20that,bit%20value%20from%20the%20stack.&text=REG%3A%20AX%
2C%20BX%2C%20CX,%2C%20SI%2C%20BP%2C%20SP
• https://www.includehelp.com/embedded-system/procedures-in-the-8086-microprocessor.aspx
• https://www.snjb.org/polytechnic/up-images/downloads/chapter%206-MAPupFile_058d4fa990a
baa.pdf

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