DOS Memory Management

1024 bytes) (KiB) of "conventional memory". The 640 KiB limit was specific to the IBM Laptop and close compatibles; other machines working MS-DOS had different limits, Memory Wave for instance the Apricot Computer may have as much as 768 KiB and the Sirius Victor 9000, 896 KiB. DOS, whereas permitting pc customers to take advantage of massive amounts of low-cost memory and new generations of processors. Since DOS has given way to Microsoft Home windows and different 32-bit operating methods not restricted by the original arbitrary 640 KiB restrict of the IBM Pc, managing the memory of a private computer now not requires the user to manually manipulate inside settings and parameters of the system. The 640 KiB limit imposed great complexity on hardware and software supposed to avoid it; the bodily memory in a machine could be organized as a mixture of base or typical memory (together with decrease memory), higher memory, excessive memory (not the same as upper memory), prolonged memory, and expanded memory, all dealt with in different ways.

The Intel 8088 processor used in the original IBM Pc had 20 address strains and so might instantly tackle 1 MiB (220 bytes) of memory. Completely different areas of this tackle house were allotted to different kinds of memory used for various functions. Starting at the bottom end of the address area, the Pc had learn/write random entry memory (RAM) installed, which was utilized by DOS and software applications. The first a part of this memory was installed on the motherboard of the system (in very early machines, sixty four KiB, later revised to 256 KiB). Additional memory might be added with playing cards plugged into the growth slots; each card contained straps or switches to control what part of the tackle area accesses memory and units on that card. On the IBM Laptop, all of the handle house up to 640 KiB was available for RAM. This a part of the tackle space known as "conventional memory" since it is accessible to all versions of DOS robotically on startup.

Section 0, the first 64 KiB of conventional memory, can also be known as low memory area. Usually expansion memory is set to be contiguous within the address area with the memory on the motherboard. If there was an unallocated hole between motherboard memory and the enlargement memory, the memory would not be mechanically detected as usable by DOS. The higher memory space (UMA) refers to the tackle area between 640 and 1024 KiB (0xA0000-0xFFFFF). The 128 KiB area between 0xA0000 and 0xBFFFF was reserved for VGA screen memory and legacy SMM. The 128 KiB region between 0xC0000 and 0xDFFFF was reserved for system Choice ROMs, including Video BIOS. The sixty four KiB area between 0xE0000 to 0xEFFFF was reserved for BIOS as decrease BIOS space. The 64 KiB area 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 display adapter was used, the address area between 0xA0000 and 0xAFFFF could be used for RAM, which would be contiguous with the typical memory.

The system BIOS ROMs must be at the upper finish of the address house as a result of the CPU starting handle is fastened by the design of the processor. The beginning handle is loaded into the program counter of the CPU after a hardware reset and must have a defined worth that endures after energy is interrupted to the system. On reset or power up, the CPU loads the handle from the system ROM after which jumps to a defined ROM location to begin executing the system power-on self-take a look at, and finally load an working system. Since an growth card akin to a video adapter, onerous drive controller, or network adapter could use allocations of Memory Wave Method in most of the upper memory areas, configuration of some combos of playing cards required cautious studying of documentation, or Memory Wave Method experimentation, to find card settings and memory mappings that labored. Mapping two devices to use the same physical memory addresses could end in a stalled or unstable system.

Not all addresses within the higher memory area have been used in a typical system; unused physical addresses would return undefined and system-dependent knowledge if accessed by the processor. As memory costs declined, utility programs corresponding to spreadsheets and computer-aided drafting were changed to take advantage of more and more bodily memory within the system. Virtual memory within the 8088 and 8086 was not supported by the processor hardware, and disk expertise of the time would make it too sluggish and cumbersome to be practical. Expanded memory was a system that allowed software programs to access extra RAM than immediately seen to the processor's handle space. The method was a form of bank switching. When further RAM was needed, driver software would temporarily make a bit of expanded memory accessible to the processor; when the info in that piece of memory was updated, one other half could be swapped into the processor's tackle space.