Operating Modes of 80386 Microprocessors

The Intel 80386 microprocessor, often known as the i386, was a popular x86-based microprocessor. Its capacity to run in numerous modes, which allowed varied levels of memory management and protection, as well as backward compatibility with previous processors, was one of its primary characteristics. 

The 80386 Modes of Operation:

• Real Mode
• Protected Mode 
• Virtual 8086 Modes
•  System Management Mode 

Each mode provided unique features and was designed for certain use cases, such as running older applications, advanced memory management, and system management duties. Understanding these operating modes is critical for understanding the 80386 microprocessor’s capabilities and limitations, as well as how it was employed in early personal computers and servers.

Real Mode:

When the 80386 is turned on for the first time, the Real mode is its default operating mode. Software created for the 8086 and 8088 processors will work on the 80386 without needing to be modified because it is fully backward-compatible with these older CPUs. The 80386 has a 20-bit address space in Real mode, giving it access to 1MB of memory. The memory is separated into 64K segments, and a 16-bit segment register can be used to access each section. The processor uses segment-based memory addressing while operating in this mode and is not given access to sophisticated memory management and security functions. Real mode likewise has a constrained set of instructions and does not support protected or virtual memory.

Protected Mode: 

A 32-bit address space is available in protected mode, a sophisticated operating mode that gives users access to up to 4GB of memory. Additionally, it offers sophisticated memory management and security features including segmentation and paging. Pages are fixed-size units of memory that can be moved in and out of physical memory as needed. Paging enables this. Memory can be separated into logical units called segments through segmentation, which can be used to restrict access to particular memory locations. The 80386 also has access to a number of privileged instructions and registers in a protected mode that are not present in regular mode. The protected mode also supports virtual memory which allows the system to use more memory than the physical memory available by swapping memory pages to and from the disk.

Virtual 8086 Modes:

The 80386 may operate numerous virtual environments that are compatible with the 8086 simultaneously in virtual 8086 modes. Although they share the same physical memory, each virtual environment has its own set of registers and memory space. This mode enables the 80386 to mimic the actions of an 8086 processor, which is frequently used to run older applications on more modern computers. The 80386 may move between many virtual environments as needed thanks to the system for handling interrupts and exceptions that are provided by virtual 8086 modes. The virtual 8086 mode allows for the simultaneous operation of numerous virtual machines without interfering with one another while simulating the behavior of a real 8086 CPU in a protected virtual memory environment.

Operating Mode	Address Space	Memory Management Features	Backward Compatibility
Real Mode	20-bit	None	Fully compatible with 8086/8088 processors
Protected Mode	32-bit	Segmentation, paging, privileged instructions	Partial compatibility with 8086/8088 processors
Virtual 8086 Mode	20-bit (per virtual machine)	Virtualization of 8086 environment	Fully compatible with 8086/8088 processors