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Random freezes, missing sound and display glitches usually trace back to one bad driver. Find and replace yours safely.Free scan · under a minuteReal-mode code is x86 code intended to run while the processor is in real-address mode. It is a processor mode, not a separate programming language: assembly is common in examples, but the term describes the execution environment and how addresses are formed.
What does real-mode code mean?
On the Intel 80386, the processor starts in real-address mode after reset. Intel describes it as resembling a fast 8086, with extensions. System startup code can use this mode before initializing the processor for protected mode.
Real mode is often associated with 16-bit code, but the mode is more accurately defined by the processor state and its execution rules—not by instruction width alone. The Intel 80386 Programmer’s Reference Manual treats real-address, protected, and virtual 8086 modes as distinct modes.
How does real-mode addressing work?
In the 80386 real-address model, a 16-bit segment value is shifted left by four bits to form a segment base, then the effective address (offset) is added. For example, segment 0x1000 and offset 0x0020 produce the address 0x10020.
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The sum can use up to 21 significant bits on the 80386. Paging is not used in real-address mode, so the manual treats the resulting linear address as the physical address. This 21-bit detail describes the 80386’s real-address calculation; it should not be generalized to every x86 generation.
How does real mode differ from protected and virtual 8086 modes?
| Mode | Purpose and execution context | Addressing and protection |
|---|---|---|
| Real-address mode | The 80386’s mode immediately after reset; used for 8086-style execution and often during early startup. | Forms addresses from a segment and offset. Paging is not used, and protected-mode segment and page protection mechanisms are absent. |
| Protected mode | The 80386’s native 32-bit environment. | Uses segment descriptors and may use paging; it provides protection mechanisms not present in real mode. |
| Virtual 8086 mode | Runs an 8086 program while the processor is in protected mode, then returns to protected-mode execution. | It is not the same processor mode as real-address mode, even though it runs 8086-style code. |
These descriptions follow the Intel 80386 manual; details can differ across later x86 generations. A modern operating system’s virtualized 16-bit process should not be assumed to have the same privileges as bare real mode.
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How does a processor enter or leave real mode?
On the 80386, setting the PE bit in CR0 enters protected mode. Returning to real mode is a systems-programming transition, not a casual application-level setting change. The 80386 manual’s sequence includes clearing paging if it is enabled, preparing segment state, disabling interrupts, clearing PE, performing a far jump, loading the real-mode interrupt vector table, and restoring interrupts. The order and processor state matter; consult the architecture manual rather than applying an improvised sequence.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How do you disassemble real-mode BIOS code?
Microsoft’s debugger documentation says the ur command displays an assembly translation of specified 16-bit real-mode code. On an x86 processor, both ur and the ordinary u command can correctly examine 16-bit real-mode code. ur is useful when the code is somewhere the debugger does not expect, such as x86 BIOS code emulated on a non-x86 computer.
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Because ur decodes as 16-bit code, using it on 32-bit or 64-bit code produces meaningless output. Select the disassembly mode that matches the code being examined.
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