xms memory dos
The upper memory area (UMA) is memory in the range between 640 kb and 1 Mb. 15 Mb XMS MS-DOS 6.0 : 3.07, recognizes max. Also the UMBPCI driver uses this feature to create UMBs through Shadow-RAM. (todo: expand), HIMEM.SYS: enables XMS (except UMBs) with 286 up to 15 Mb, with 386/486 up to 1023 Mb. The latest EMS 4.0 allowed to use up to 32 Mb as expanded memory. By default such drivers are placed in conventional memory. It has to be loaded in config.sys as second driver after HIMEM.SYS. The earlier 8086/8088 processors can make use of more than 1 MB of RAM, if one employs special hardware to make selectable parts of it appear at addresses below 1 MB. Later two additional methods were developed allowing direct access to a small portion of extended memory from real mode. Load comments. This can reduce available UMBs considerably. Only these chips can address more than 1 megabyte of RAM. The 64 kB region within the Upper Memory Area is referred to as EMS Page Frame. The Expanded Memory Solution. HIMEM.SYS: enables XMS (except UMBs) with 286 up to 15 Mb, with 386/486 up to 1023 Mb Versions: MS-DOS 5.0 : 2.77, max. A common use of Shadow-RAM is copying the BIOS ROMs to the Shadow-RAM below and mapping out the ROM afterwards. Architectural reasons force the memory for DMA transfers of ISA cards to be reserved below 1 Mb. Total: 65,967,536 Used 64,**8,960 Free 1,048,576. To achieve this you have to use memory managers that allow e.g. These blocks are called Upper Memory Blocks (UMB) and are treated from DOS as High Memory. Some TSRs do not work correctly when loaded to upper memory, resulting in crashs or erratic system behavior. The Expanded Memory Specification (EMS) is a standard developed by Lotus, Intel and Microsoft. Optimizing your system to attain enough free memory for your programs is called memory management. Mainboard BIOS, graphic card BIOS and also additional cards as mass storage controllers or network cards use the memory area between 640K and 1 MB sharing space with UMBs. The return value is a 32-bit linear address, and if I have understood this correctly it is simply not possible to use that from a 16-bit environment. Virtually every AT-class or better computer has some XMS memory and nearly all users install the device driver that enables access to XMS. Extended memory can also be accessed directly by DOS programs running in protected mode using VCPI or DPMI, two (different and incompatible) methods of using protected mode under DOS. If no contiguous free Upper Memory Block is available the driver will be loaded to Conventional Memory. EMM386.EXE: introduced with MS-DOS 5.0, uses XMS to create UMBs in UMA, uses XMS to create EMS, MEMMAKER.EXE: introduced with MS-DOS 6.0, utility to optimize free conventional memory, QEMM386: a complete package of memory management driver and utilities, last version 9.0 / QEMM97, QEMM386.SYS memory management driver implementing XMS, EMS, UMA, VCPI and DPMI, enhanced features include support for up to 256 MB RAM, QuickBoot, Stealth, dynamically load drivers from command line and specific support for popular drivers as DoubleSpace, Stacker. The extended memory manager is also responsible for managing allocations in the high memory area (HMA) and the upper memory area (UMA; also referred to as upper memory blocks or UMBs). On x86-based PCs, extended memory is only available with an Intel 80286 processor or higher. If you notice any instability in your DOS system, try to move TSRs back into conventional memory to find the culprit. This functionality can usually enabled by BIOS (Shadow System BIOS and Shadow Video BIOS) or as parameter of a memory manager. Conventional memory or base memory is the memory range between 0 kb and 640 kb. The default memory management driver that enables this mapping through XMS memory is EMM386.EXE. MS-DOS 6.2 6.21 6.22 : 3.10, recognizes max. If DOS=HIGH was specified but loading to HMA fails HMA not available/loading DOS low is reported. Accessing memory above the High Memory Area requires switching the CPU to Protected Mode, thus HIMEM.SYS requires at least a 286 CPU. MS-DOS 6.0 : 3.07, recognizes max. enabling of UMBs and installation of APIs for XMS and EMS access. The Extended Memory Specification (XMS) allows to map blocks of mainboard RAM in the remaining UMA memory range. This prevents programs running in protected mode from interfering with each other's memory. Directly after an XMS driver is loaded in config.sys with DEVICE= and the HMA is still unoccupied DOS will move parts of code there. As the BIOS code is executed now from RAM instead of slow ROM the BIOS calls get accelerated. Memory manager DOS memory manager XMS manager Himem Memory Emm386 Jemmex. The High Memory Area (HMA) are the 64 kb directly above 1 Mb and are part of XMS. Ack, I have to answer my own question. In DOS memory management, extended memory refers to memory above the first megabyte (2 bytes) of address space in an IBM PC or compatible with an 80286 or later processor. Functionality is chipset dependent, see UMBPCI homepage for a list of supported chipsets. The amount of free conventional memory is reported by CHKDSK or MEM. When using the mem /c command from a DOS window in Windows, no upper blocks are seen, as shown in the above example. The driver tries to detect common unused blocks and maps XMS memory there creating Upper Memory Blocks. EMM386 switches the CPU to Virtual 8086 mode. So what they did was leave a hole in memory between 640K and 1 MB, then put the rest of their memory up above 1 megabyte. Only applications executing in protected mode can use extended memory directly. The most common memory management driver for this functionality is HIMEM.SYS that has to be loaded as first driver in config.sys.
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