Computer Architecture: Stack Pointer and Instructions

Stack Pointer (SP)

• The Stack Pointer (SP) is a register used to point to the top of a stack.
• The stack is a data structure used to hold temporary data.
• During a POP operation, SP is incremented by 2 (SP=SP+2).
• During a PUSH operation, SP is decremented by 2 (SP=SP-2).

Instruction: PUSH

• The PUSH instruction copies data from a source location to the stack.
• The source can be a general-purpose register, a flag register, a segment register, or a memory location.
• The addressing mode for PUSH is register indirect.

Port Related Instructions: IN and OUT

• IN and OUT instructions are used for input and output operations respectively.
• They are used to interact with peripherals connected to the processor.

Conditional Transfer

• Conditional Transfer instructions allow for the transfer of program control based on the values of a condition code flag.

Arithmetic Instructions

• Arithmetic instructions perform mathematical operations like addition, subtraction, multiplication, and division.

String Related Instructions

• String instructions are designed for efficient manipulation of string data.
• String instructions can copy, compare, and scan strings of bytes or words.
• The direction flag (DF) determines the direction of string operations – forward or backward.

REP Prefix

• The REP prefix is used to repeat string operations multiple times.
• It uses the CX register as a counter.
• REP instruction repeats an operation while CX != 0.
• REPNE/REPNZ repeats an operation while CX != 0 and the previous operation resulted in a non-zero result.
• REPE/REPZ repeats an operation while CX != 0 and the previous operation resulted in a zero result.

LOOP Instruction

• The LOOP instruction repeats a block of code a specified number of times.
• The number of repetitions is stored in the CX register.
• LOOP decrements CX by 1 and jumps to a specified label if CX != 0.

Physical Address Calculation

• The Physical Address is calculated by adding the Segment Address and the Offset Address: Physical Address = Segment Address + Offset Address

Registers:

• IP (Instruction Pointer)
  • Stores the memory address of the next instruction to be executed.
• BP (Base Pointer)
  • Used to access data in the stack segment.
• SP (Stack Pointer)
  • Used to access data in the stack segment.
• SI (Source Index)
  • Used to store the offset of source data in the data segment.
• DI (Destination Index)
  • Used to store the offset of destination data in the data or extra segment.

Flag Register

• The Flag Register monitors the status of the processor and stores results of ALU operations.
• It is divided into two parts: Condition Code Flags and Machine Control Flags
• Condition Code Flags:
  • SF (Sign Flag): Set when the result is negative.
  • ZF (Zero Flag): Set when the result is zero.
  • PF (Parity Flag): Set when the result has even number of 1s.
  • CF (Carry Flag): Set when there is a carry out or a borrow.
• Machine Control Flags:
  • DF (Direction Flag): Controls the direction of string operations.
  • IF (Interrupt Flag): Enables or disables interrupts.
  • TF (Trap Flag): Activates single-step mode for debugging.

Interrupt Priority (IP) Register

• The IP Register assigns priorities to various interrupt sources.
• A bit value of 1 indicates high priority, and 0 indicates low priority.

Programming Timer Interrupts

• Timers in the processor are programmable and can be configured for different functions.
• When a timer expires, a timer interrupt signal is raised.

Programming External Hardware Interrupts

• The 8051 has two external hardware interrupts: INT0 and INT1.
• They can be triggered by a low-level signal (level-triggered mode) or by a change in the signal level (edge-triggered mode).
• The IE register controls enabling or disabling interrupts.

Internal RAM and ROM

• The 8051 has 128 bytes of internal RAM (00h to FFh), used for data storage and temporary variables.
• The 8051 also has internal ROM, used for program storage.

Interrupt Control

• Interrupts are events that can interrupt the normal flow of program execution.
• They allow the processor to handle events like timer expires, serial port data received, and external pin changes.
• The 8051 supports 5 sources of interrupts:
  • 2 external hardware interrupts (INT0 and INT1)
  • 3 internal interrupts (Serial Port, Timer 0, Timer 1)

Serial Data Communication

• The 8051 serial port uses the SBUF(Serial Port Data Buffer), SCON (Serial Control), and PCON (Power Control) registers for communication.
• Serial data communication involves sending and receiving bits one at a time.
• Data is transmitted via the TXD pin and received through the RXD pin.
• The 8051 supports four serial data communication modes: Mode 0, Mode 1, Mode 2, and Mode 3.

PSW (Program Status Word)

• The PSW register holds the status of the current program.
• It stores information about the program's state, including the carry flag, parity flag, auxiliary carry flag, and zero flag.

Stack Operations

• The stack is a data structure used for temporary data storage.
• PUSH and POP instructions are used to add data to and retrieve data from the stack.

Register Indirect Addressing Mode

• In this mode, the address of the data is stored in a register.
• The 8051 uses R0 and R1 registers for 8-bit addressing and DPTR for 16-bit addressing.

Indexed Addressing Mode

• In indexed addressing mode, the address of the data is calculated by adding the content of a register (like A, PC or DPTR) to a constant offset.
• The destination operand in indexed addressing mode is always register A.

This quiz covers the concepts of Stack Pointer (SP) and related instructions such as PUSH, IN, and OUT. Learn how data is manipulated in a stack and the role of conditional transfers and arithmetic instructions in computer programming. Test your understanding of these fundamental computer architecture concepts.