Overview
The 8086 microprocessor has a 20-bit address bus, which means it can access up to 1 MB (2^20 bytes) of memory. The memory is divided into segments, each of which is 64 KB (2^16 bytes) in size. The 8086 has four segment registers – CS, DS, SS, and ES – which are used to access different segments of memory.
- Code Segment (
CS): This register points to the segment that contains the current code being executed. The code segment is read-only, and all instructions and constants used in the code must be located in this segment. - Data Segment (
DS): This register points to the segment that contains data used by the program, such as variables and arrays. - Stack Segment (
SS): This register points to the segment used for the program’s stack. The stack grows downward from the top of this segment. - Extra Segment (
ES): This register is an extra segment register that can be used to access additional data segments.
Each segment is further divided into smaller units called paragraphs, each of which is 16 bytes in size. The 8086 microprocessor uses a segmented memory model, where each memory address is composed of a segment address and an offset address. The effective address used by an instruction is computed by adding the segment address and the offset address together.
The 8086 also has a memory management unit (MMU) that maps logical addresses (segment and offset) to physical addresses in memory. The MMU performs address translation and protection, and it is responsible for mapping segments to physical memory.
Memory in the 8086 microprocessor is typically accessed using memory addressing modes, such as direct, indirect, based, and indexed addressing modes. These modes allow the microprocessor to access data stored in memory, and they are essential for operations that require large amounts of data or data that is not known at compile-time. However, memory access is slower than register access, so accessing data stored in memory can significantly affect program performance.
Program memory
The 8086 microprocessor has a program memory, which is also known as the code segment. The program memory is a section of memory that stores the machine code instructions that the microprocessor executes. The program memory is read-only, which means that the microprocessor cannot write to it. The instructions stored in the program memory must be fetched by the microprocessor and loaded into its instruction queue before they can be executed.
The program memory is accessed using the CS (Code Segment) register. The CS register holds a 16-bit value that points to the base address of the program memory segment. When the microprocessor executes an instruction, it uses the value in the CS register to calculate the memory address of the next instruction to execute.
The size of the program memory in the 8086 microprocessor is determined by the number of segments that can be addressed by the 20-bit address bus. Each segment is 64 KB in size, which means that the maximum size of the program memory is 1 MB (2^20 bytes). However, the actual size of the program memory is determined by the size of the program being executed.
The program memory in the 8086 microprocessor is organized into instruction blocks or code segments, which are composed of machine code instructions. The instructions are executed by the microprocessor in a sequential manner, starting from the address specified by the CS register.
To load a program into the program memory, the program must first be assembled or compiled into machine code instructions that can be executed by the microprocessor. The machine code instructions are then written to the program memory using an appropriate tool, such as a debugger or a programmer. Once the program is loaded into the program memory, the microprocessor can execute the instructions and run the program.
Data Memory
The 8086 microprocessor has a data memory, which is also known as the data segment. The data memory is a section of memory that is used for storing data used by a program. The data memory is read-write, which means that the microprocessor can read from and write to it. The data stored in the data memory can be modified by the program during its execution.
The data memory is accessed using the DS (Data Segment) register. The DS register holds a 16-bit value that points to the base address of the data memory segment. When the microprocessor needs to read or write data from the data memory, it uses the value in the DS register to calculate the memory address of the data to be accessed.
The size of the data memory in the 8086 microprocessor is also determined by the number of segments that can be addressed by the 20-bit address bus. Each segment is 64 KB in size, which means that the maximum size of the data memory is 1 MB (2^20 bytes). However, the actual size of the data memory is determined by the size of the data used by the program being executed.
The data memory in the 8086 microprocessor is organized into variables and arrays, which are used to store different types of data. The variables and arrays are declared and defined in the program code, and the microprocessor uses their memory addresses to read and write data from the data memory.
To access data in the data memory, the microprocessor uses different addressing modes, such as direct, indirect, based, and indexed addressing modes. These addressing modes allow the microprocessor to access data stored in memory using different types of memory addresses and offsets.
The data memory in the 8086 microprocessor can be modified during program execution, allowing the program to manipulate data as needed. However, modifying data stored in memory can be slower than accessing data stored in registers, so it is important to use registers for frequently accessed data whenever possible to improve program performance.
In summary, the data memory in the 8086 microprocessor is a section of memory used for storing data used by a program. It is read-write, and its size is determined by the number of segments that can be addressed by the 20-bit address bus. The data memory is accessed using the DS register, and it is organized into variables and arrays that are accessed using different addressing modes.
Stack memory
The 8086 microprocessor has a stack memory, which is a section of memory used for storing data during program execution. The stack memory is a Last-In-First-Out (LIFO) structure, which means that the most recently pushed item is the first to be popped from the stack. The stack memory is implemented using a section of the data memory, and it is used to store function call information, local variables, and temporary data.
The stack memory in the 8086 microprocessor is accessed using the SS (Stack Segment) and SP (Stack Pointer) registers. The SS register holds a 16-bit value that points to the base address of the stack memory segment, while the SP register holds a 16-bit offset that points to the top of the stack.
When data is pushed onto the stack, the SP register is decremented to point to the next available memory location on the stack. When data is popped from the stack, the SP register is incremented to point to the next item on the stack. The SS register is used to specify the segment of the stack memory being accessed.
The size of the stack memory in the 8086 microprocessor is determined by the size of the stack segment. The stack segment is typically 64 KB in size, but it can be smaller depending on the size of the data memory segment.
The stack memory in the 8086 microprocessor is used for several purposes, including:
- Storing function call information – When a function is called, its return address and other function call information are pushed onto the stack. This allows the program to return to the correct location after the function has completed.
- Storing local variables – Local variables used by a function are typically stored on the stack. This allows multiple instances of a function to be called simultaneously without interfering with each other’s data.
- Storing temporary data – The stack memory can be used to store temporary data that is used during program execution. This data is typically pushed onto the stack and then popped off once it is no longer needed.
In summary, the stack memory in the 8086 microprocessor is a section of memory used for storing data during program execution. It is a LIFO structure implemented using a section of the data memory, and it is accessed using the SS and SP registers. The stack memory is used for storing function call information, local variables, and temporary data.
