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The mode gets its name from the fact that addresses in real mode always correspond to real locations in memory. Real mode is characterized by a 20-bit segmented memory address space (giving 1 MB of addressable memory) and unlimited direct software access to all addressable memory, I/O addresses and peripheral hardware. Real mode provides no ...
Three segments in real mode memory (click on image to enlarge). There is an overlap between segment 2 and segment 3; the bytes in the turquoise area can be used from both segment selectors. In real mode or V86 mode, the size of a segment can range from 1 byte up to 65,536 bytes (using 16-bit offsets).
In real mode, in order to calculate the physical address of a byte of memory, the hardware shifts the contents of the appropriate segment register 4 bits left (effectively multiplying by 16), and then adds the offset. For example, the logical address 7522:F139 yields the 20-bit physical address:
This allowed 16-bit real-mode DOS programs to access several megabytes of RAM through a hole in real memory, typically (0xE000–0xEFFF). A program would then have to explicitly request the page to be accessed before using it. These memory locations could then be used arbitrarily until replaced by another page.
Real Address mode, [37] commonly called Real mode, is an operating mode of 8086 and later x86-compatible CPUs. Real mode is characterized by a 20-bit segmented memory address space (meaning that only slightly more than 1 MiB of memory can be addressed [ p ] ), direct software access to peripheral hardware, and no concept of memory protection or ...
In a computer using virtual memory, accessing the location corresponding to a memory address may involve many levels. In computing, a memory address is a reference to a specific memory location in memory used by both software and hardware. [1] These addresses are fixed-length sequences of digits, typically displayed and handled as unsigned ...
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In real mode, the interrupt table is called IVT (interrupt vector table). Up to the 80286, the IVT always resided at the same location in memory, ranging from 0x0000 to 0x03ff, and consisted of 256 far pointers. Hardware interrupts may be mapped to any of the vectors by way of a programmable interrupt controller.