DOS Memory Management > 자유게시판

1024 bytes) (KiB) of "conventional memory". The 640 KiB limit was specific to the IBM Laptop and close compatibles; different machines working MS-DOS had completely different limits, for example the Apricot Laptop might have up to 768 KiB and the Sirius Victor 9000, 896 KiB. DOS, while permitting pc customers to reap the benefits of large amounts of low-cost memory and new generations of processors. Since DOS has given technique to Microsoft Windows and different 32-bit operating systems not restricted by the unique arbitrary 640 KiB restrict of the IBM Pc, managing the memory of a private pc now not requires the consumer to manually manipulate internal settings and parameters of the system. The 640 KiB restrict imposed great complexity on hardware and software supposed to avoid it; the bodily memory in a machine might be organized as a mix of base or conventional memory (together with decrease memory), upper memory, excessive memory (not the identical as higher memory), extended memory, and expanded memory, all dealt with in alternative ways.

The Intel 8088 processor used in the unique IBM Computer had 20 tackle traces and so could directly address 1 MiB (220 bytes) of memory. Completely different areas of this deal with house have been allotted to different kinds of memory used for various functions. Beginning at the lowest end of the tackle house, the Computer had read/write random entry Memory Wave Experience (RAM) put in, which was utilized by DOS and application packages. The first a part of this memory was installed on the motherboard of the system (in very early machines, 64 KiB, later revised to 256 KiB). Additional Memory Wave may very well be added with playing cards plugged into the expansion slots; each card contained straps or switches to manage what a part of the handle area accesses memory and devices on that card. On the IBM Computer, all of the handle area up to 640 KiB was accessible for RAM. This a part of the deal with house is called "conventional memory" since it is accessible to all variations of DOS mechanically on startup.

Segment 0, the primary sixty four KiB of typical memory, can also be referred to as low memory space. Usually growth memory is ready to be contiguous in the deal with house with the memory on the motherboard. If there was an unallocated gap between motherboard memory and the growth memory, the memory would not be automatically detected as usable by DOS. The upper memory area (UMA) refers to the tackle space between 640 and 1024 KiB (0xA0000-0xFFFFF). The 128 KiB area between 0xA0000 and 0xBFFFF was reserved for VGA display screen memory and legacy SMM. The 128 KiB area between 0xC0000 and 0xDFFFF was reserved for system Choice ROMs, including Video BIOS. The sixty four KiB region between 0xE0000 to 0xEFFFF was reserved for BIOS as decrease BIOS area. The 64 KiB region between 0xF0000 and 0xFFFFF was reserved for BIOS as higher BIOS area. For example, the monochrome video adapter memory space ran from 704 to 736 KiB (0xB0000-0xB7FFF). If solely a monochrome show adapter was used, the deal with space between 0xA0000 and 0xAFFFF might be used for RAM, which would be contiguous with the conventional memory.

The system BIOS ROMs have to be at the higher end of the address house because the CPU beginning deal with is fastened by the design of the processor. The starting tackle is loaded into this system counter of the CPU after a hardware reset and must have an outlined worth that endures after energy is interrupted to the system. On reset or energy up, the CPU loads the handle from the system ROM after which jumps to an outlined ROM location to begin executing the system power-on self-take a look at, and finally load an operating system. Since an enlargement card similar to a video adapter, arduous drive controller, or community adapter may use allocations of memory in many of the higher memory areas, configuration of some combos of cards required careful reading of documentation, or experimentation, to search out card settings and memory mappings that labored. Mapping two devices to use the identical physical memory addresses might lead to a stalled or unstable system.

Not all addresses in the higher memory space were utilized in a typical system; unused bodily addresses would return undefined and system-dependent data if accessed by the processor. As memory prices declined, application programs equivalent to spreadsheets and pc-aided drafting were changed to make the most of increasingly more physical memory within the system. Digital memory within the 8088 and 8086 was not supported by the processor hardware, and disk know-how of the time would make it too slow and cumbersome to be practical. Expanded memory was a system that allowed application applications to access more RAM than straight visible to the processor's handle space. The process was a form of financial institution switching. When additional RAM was needed, driver software program would quickly make a bit of expanded memory accessible to the processor; when the information in that piece of memory was updated, one other part may very well be swapped into the processor's tackle area.