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Software limitations to usable physical RAM may be present. An operating system may only be designed to allocate a certain amount of memory, with upper address bits reserved to indicate designations such as I/O or supervisor mode or other security information. Or the operating system may rely on internal data structures with fixed limits for ...
With 4 GiB or more of RAM installed, and with RAM occupying a contiguous range of addresses starting at 0, some of the MMIO locations will overlap with RAM addresses. On machines with large amounts of video memory, MMIO locations have been found to occupy as much as 1.8 GB of the 32-bit address space. [12]
An x86-64 processor acts identically to an IA-32 processor when running in real mode or protected mode, which are supported modes when the processor is not in long mode.. A bit in the CPUID extended attributes field informs programs in real or protected modes if the processor can go to long mode, which allows a program to detect an x86-64 processor.
SoftPerfect RAM Disk can access memory available to Windows, i.e. on 32-bit systems it is limited to the same 4 GB as the 32-bit Windows itself, otherwise for physical memory beyond 4 GB it must be installed on 64-bit Windows. Multiple RAM disks can be created, and these can optionally be made persistent by automatically saving contents to and ...
For example, a single link PCIe 3.0 interface has an 8 Gbit/s transfer rate, yet its usable bandwidth is only about 7.88 Gbit/s. z Uses 8b/10b encoding , meaning that 20% of each transfer is used by the interface instead of carrying data from between the hardware components at each end of the interface.
Windows 3.0 actually had several modes: "real mode", "standard mode" and "386-enhanced mode"; the latter required some of the virtualization features of the 80386 processor, and thus would not run on an 80286. Windows 3.1 removed support for real mode, and it was the first mainstream operating environment which required at least an 80286 processor.
The increasing popularity of Windows 3.0 made the necessity of the upper memory area less relevant, as Windows applications were not directly affected by DOS' base memory limits, but DOS programs running under Windows (with Windows itself acting as a multitasking manager) were still thus constrained.
A 386 CPU can be put back into real mode by clearing a bit in the CR0 control register, however this is a privileged operation in order to enforce security and robustness. By way of comparison, a 286 could only be returned to real mode by forcing a processor reset, e.g. by a triple fault or using external hardware.