Real mode allows direct access to memory without management or buffering, but has limitations such as only accessing 1MB of memory and being replaced by protected mode for security and flexibility. Programs using real mode can easily cause system crashes and data loss.
Real mode is a memory addressing scheme and operating state for computer microprocessors. In real mode, memory that can be accessed by a program—usually random access memory (RAM)—is not managed or buffered in any way by hardware, software, or basic input and output services (BIOS). . This means that a program is able to access all reachable memory addresses, regardless of memory usage, and must handle all aspects of reading from and writing to memory locations itself. Several limitations arise from using real mode, including that the amount of accessible memory is limited to 1 megabyte, since the processor in this mode allows addresses to be only 20 bits long. From a practical standpoint, computer software no longer uses real mode, as it has been replaced by a more secure, extensible, and flexible addressing mode known as protected mode.
The central processing unit (CPU) of a computer is where real mode can be activated, and most aspects of real mode deal with problems usually encountered in applications written in assembly language, because they are of a quite atomic. Processor chips based on the original 8086 architecture start in real mode when powered up to be able to run programs written for older hardware, although this would often require emulation software to be successful. Protected mode has almost completely replaced real addressing, as of 2011, to the point where there are very few compilers that could compile a program that could use real addressing, and even fewer traditional operating systems that could run it.
Real mode memory is basically a single linear sequence of bytes that can be freely accessed with an address composed of a 16-bit segment address and a 4-bit offset within the segment, creating a full 20-bit address when combined. A program can access anywhere in memory and read or write anything, no matter what is in the location. This means that, without proper handling and knowledge, a program using real addressing mode could easily overwrite the operating system and system BIOS, trigger a physical hardware interrupt, or accidentally send a signal to a peripheral device. Not only could this cause a system to freeze or crash, but it could also lead to data loss or physical hardware damage.
As processor architecture advanced, protected mode eventually replaced real mode in almost all software. Over time, using real addressing mode became unnecessary, because it couldn’t access more than 1 megabyte of RAM and couldn’t use more than 20 system bus bits, which meant it could only use one fraction of the resources available on most computers. Real addressing also carries a significant security risk. One of the only ways to write a program that uses real addressing mode is to use assembly language and run the program in a special disk operating system (DOS) that does not automatically switch to protected mode when it starts.
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