Memory and Register Organization (part 2)
Control Registers
IP (Instruction Pointer Register)
The Instruction Pointer (IP) is a 16-bit register. The IP register specifically points to the memory address of the next instruction to be executed by the CPU. Together with the Code Segment (CS) register, the IP forms the effective address of the next instruction.
Pairing with Code Segment (CS):
The IP register works in conjunction with the Code Segment (CS) register to form the complete memory address of the next instruction.
The combination of CS:IP is used to specify the address of the next instruction to be fetched and executed.
Segmented Memory Model:
The 8086 architecture employs a segmented memory model where memory is divided into segments.
The CS register holds the segment base address, and the IP register holds the offset within that segment.
Program Counter Concept:
The IP register is often referred to as the Program Counter (PC) in other processor architectures.
Certain instructions, such as jumps (e.g., JMP), calls (e.g., CALL), and returns (e.g., RET), can modify the value of the IP register.
Interrupts:
When an interrupt occurs, the IP register is typically pushed onto the stack as part of the interrupt handling process. The IP value is later restored during the interrupt return process.
Flag Registry
The FLAGS register in the Intel 8086 microprocessor is a 16-bit register that contains various flags, each of which indicates a specific condition or status resulting from the execution of instructions. The FLAGS register plays a crucial role in program control and decision-making.
Carry Flag (CF) - bit 0
Purpose: Indicates whether an arithmetic operation resulted in a carry out of the most significant bit. Affected by: Arithmetic instructions like ADD, ADC, SUB, SBB, etc.
Parity Flag (PF) – bit 2
Purpose: Indicates whether the number of set bits in the result is even. Affected by: Generally unaffected by direct instructions; arithmetic and logical operations indirectly affect it.
Auxiliary Carry Flag (AF) - bit 4
Purpose: Indicates whether there was a carry out of the low nibble (4 bits) during an arithmetic operation. Affected by: Arithmetic instructions.
Zero Flag (ZF) – bit 6
Purpose: Set if the result of an operation is zero; cleared otherwise. Affected by: Arithmetic and logical operations, as well as comparison instructions.
Sign Flag (SF) – bit 7
Purpose: Reflects the sign (positive or negative) of the result of an operation. Affected by: Arithmetic operations.
Trap Flag (TF) – bit 8
Purpose: Used for single-step debugging; enables the CPU to execute one instruction at a time. Affected by: Debugging instructions.
Interrupt Flag (IF) – bit 9
Purpose: Enables or disables interrupts. Affected by: STI (Set Interrupt) and CLI (Clear Interrupt) instructions.
Direction Flag (DF) – bit 10
Purpose: Determines the direction of string operations; if set, the string is processed from high memory to low memory, and vice versa if cleared. Affected by: STD (Set Direction) and CLD (Clear Direction) instructions.
Overflow Flag (OF) – bit 11
Purpose: Indicates whether an arithmetic operation resulted in an overflow (result too large to be represented in the destination operand). Affected by: Arithmetic operations like ADD, SUB, etc.
Examples
Segment Registers
DS - Data Segment Points the segment where the data is stored
CS - Code Segment Points the start segment of the executable code
SS - Stack segment point to the base address of the segment that contains the program stack. The stack is a region of memory used for storing temporary data, return addresses, and local variables during subroutine calls.
EX - Extra Segment Used for extra storage
Extended Registers and Special Purpose Registers
SI (Source Index): String operations and data movement. One of the index registers used primarily in string operations. It is often paired with the Extra Segment (ES) register for source data.
DI (Destination Index): String operations and data movement.
SP (Stack Pointer): Managing the stack during function calls and returns. SP role is crucial for managing the program stack. It keeps track of the top of the stack, and various instructions use it for stack-related operations.
BP (Base Pointer): A reference point for local variables and parameters. Commonly used in function calls. BP is often used as a reference point within the stack frame for accessing local variables and parameters. It provides a convenient way to access data within a function's stack frame. Each function has local memory associated with it to hold incoming parameters, local variables, and (in some cases) temporary variables. This region of memory is called a stack frame and is allocated on the process' stack. The BP register is particularly useful for providing a stable reference point within the stack frame, facilitating efficient access to local data.
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