System Maintenance | Explain three modes of operation of CPU. (i) Real mode (ii) Virtual real mode (iii) Protected mode.

 (i)   Real mode:

The original IBM PC could only address 1 MB of system memory, and the original versions of DOS created to work on it were designed with this in mind.  

 The DOS operating system requires the microprocessor to operate in the real mode.  

 Real mode operation allows application software written for the 8086/8088, which contains only 1 M byte of memory, to function in the 80286 and above without changing the software. 

 DOS is by its nature a single-tasking operating system, meaning it can only handle one program running at a time.  

 The upward compatibility of software is partially responsible for the continuing success of the Intel family of microprocessors.  

 In all cases, each of these microprocessors begins operation in the real mode by default whenever power is applied or the microprocessor is reset. 

 When a processor is running in real mode, it accesses memory with the same restrictions of the original 8088: a limit of 1 MB of addressable RAM, and slow memory access.

(ii)  Virtual real mode:

 Virtual real is essentially a virtual real mode with 16-bit environment that runs inside 32-bit protected mode. 

 When you run a DOS prompt window inside Windows, you have created a virtual real modesession. 

 Note that any program running in a virtual real mode window can  access up to only 1MB of memory, which that program will believe is the first and only megabyte of memory in the system.  

 If you run a DOS application in a virtual real window, it will have a 640 KB limitation on memory usage. That is because there is only 1 MB of total RAM in a 16-bit environment, and the upper 384KB is reserved for system use. The virtual real window fully emulates an 8088 environment, so that aside from speed, the software runs as if it were on an original real mode–only PC. Each virtual machine gets its own 1 MB address space, an image of the real hardware basic input/output system (BIOS) routines, and emulation of all other registers and features found in real mode.

(iii) Protected mode:

 This mode is the native state of the processor. 

 In this mode all instructions and architectural features are available, providing the highest performance and capability. This is the recommended mode for all new applications and operating

systems.  

 Among the capabilities of protected mode is the ability to directly execute “real-address mode” 8086 software in a protected, multitasking environment.

 It has full access to all of the system's memory. There is no 1 MB limit in protected mode.  

 It has the ability to multitask, meaning having the operating system manage the execution of multiple programs simultaneously.  

 It supports virtual memory, which allows the system to use the hard disk to emulate additional system memory when needed.   

 It has faster (32-bit) access to memory, and faster 32-bit drivers to do I/O transfers.