DOS memory management - Revision history

Enigma: /* Additional Remarks */

2013-04-11T14:17:36Z

‎Additional Remarks

Enigma: /* Software */

2013-04-11T13:54:49Z

‎Software

Enigma: /* Software */

2013-04-11T13:54:10Z

‎Software

Enigma: /* Getting more conventional memory */

2013-04-11T00:03:30Z

‎Getting more conventional memory

Enigma at 23:50, 10 April 2013

2013-04-10T23:50:47Z

Enigma: /* Additional Remarks */

2013-04-10T23:46:55Z

‎Additional Remarks

Enigma: /* Upper memory */

2013-04-10T23:35:53Z

‎Upper memory

Enigma at 23:34, 10 April 2013

2013-04-10T23:34:55Z

Enigma at 23:26, 10 April 2013

2013-04-10T23:26:16Z

Enigma: /* High memory area */

2013-04-10T23:06:52Z

‎High memory area

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 The DOS=HIGH and DOS=UMB statements in config.sys can be combined by writing DOS=HIGH,UMB. Specifying DOS=LOW can be used to prevent DOS from using the HMA.
-Shadow RAM is RAM in the same range as BIOS ROMs. It can usually enabled by BIOS or as parameter of a memory manager. Since ROM access is rather slow it is usually used to copy the BIOS ROM to the underlying RAM at the same location and then switching off the ROM. This accelerates all BIOS calls.
+Shadow-RAM is the RAM below the reserved UMA. Depending on chipset this RAM can be mapped in again in specific UMA memory ranges. A common use of Shadow-RAM is copying the BIOS ROMs to the Shadow-RAM below and mapping out the ROM afterwards. As the BIOS code is executed now from RAM instead of slow ROM the BIOS calls get accelerated. This functionality can usually enabled by BIOS (''Shadow System BIOS'' and ''Shadow Video BIOS'') or as parameter of a memory manager. Also the UMBPCI driver uses this feature to create UMBs through Shadow-RAM.
 Architectural reasons force the memory for DMA transfers of ISA cards to be reserved below 1 Mb. This is usually of 16 kB size and reserved in UMA.

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-'''UMBPCI''' Upper memory driver, partial replacement for EMM386.EXE. Uses Shadow-RAM instead of extended memory to create UMBs. Functionality is chipset dependent, see [http://www.uwe-sieber.de/umbpci_e.html UMBPCI homepage] for a list of supported chipsets. Also be aware of issues with ISA-DMA using the Shadow-RAM method that could lead to data corruption if not setup properly.
+'''UMBPCI''' Upper memory driver, partial replacement for EMM386.EXE. Uses Shadow-RAM instead of extended memory to create UMBs. Functionality is chipset dependent, see [http://www.uwe-sieber.de/umbpci_e.html UMBPCI homepage] for a list of supported chipsets. Also be aware of issues with ISA-DMA using the Shadow-RAM method that could lead to data corruption if not setup properly. This driver leaves the CPU in Real mode.
 [http://support.microsoft.com/kb/95555 Overview of Memory-Management Functionality in MS-DOS]
 [http://en.wikipedia.org/wiki/QEMM#DOS_equivalents Overview about features of different memory managers]

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 '''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
@@ Line 96: / Line 97: @@
 '''MagnaRAM''' a memory compression utility replacing a portion of Windows 3.1 virtual memory subsystem.
 [http://support.microsoft.com/kb/95555 Overview of Memory-Management Functionality in MS-DOS]
 [http://en.wikipedia.org/wiki/QEMM#DOS_equivalents Overview about features of different memory managers]

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 On 286 CPUs special chipset support is required to enable UMBs. Rather well supported is the NEAT chipset. The HIMEM.SYS driver takes control of the A20-gate, makes HMA available and installs a XMS API.
-It is also possible to emulate a EMS page frame with the EMM286 driver.
+It is also possible to emulate a EMS page frame with the EMM286 driver. Some chipsets have jumpers to set XMS and EMS assignment of the main memory.
 and later CPUs feature an protected mode extension called Virtual 8086 mode that allows mimicking a 8086 memory layout for applications. With MS-DOS 5 a second memory manager was included called EMM386.EXE that uses the advanced features of 386 CPUs. It is loaded after HIMEM.SYS and enables UMBs and EMS emulation. This allows to load TSRs to UMBs freeing conventional RAM. The corresponding load statement in config.sys is DEVICEHIGH= and in autoexec.bat LOADHIGH which can be shortened to LH. EMM386 switches the CPU to Virtual 8086 mode.
@@ Line 65: / Line 65: @@
 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. Especially newer graphic cards and mainboards are not designed with DOS in mind and are delivered with large BIOSes (> 32 kB). This can reduce available UMBs considerably. A classic DOS system has about 128 kB to 192 kB free UMBs for TSRs. Using EMS emulation requires a 64 kB area between 640K and 1 MB where the page frame of a virtual EMS card is mapped. If not required EMS emulation can be disabled freeing 64 kB of UMBs.
 (todo: expand)

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 === Upper memory ===
-The upper memory area is memory in the range between 640 kb and 1 Mb. By default there is no RAM in this range as it is reserved for use with hardware that is able to map own memory to this range. Usually present in this region is a part of the graphics cards RAM and the BIOS ROMs of the graphics card and mainboard. Additional hardware like mass storage controllers, network adapters... can use parts of the UMA for own BIOS ROMs or buffer RAM. The Extended Memory Specification (XMS) allows to map blocks of mainboard RAM in the remaining UMA memory range. These blocks are called Upper Memory Blocks (UMB) and are treated from DOS as High Memory. The default memory management driver that enables this mapping through XMS memory is EMM386.EXE. It has to be loaded in config.sys as second driver after HIMEM.SYS. The driver tries to detect common unused blocks and maps XMS memory there creating Upper Memory Blocks.
+The '''upper memory area''' ('''UMA''') is memory in the range between 640 kb and 1 Mb. By default there is no RAM in this range as it is reserved for use with hardware that is able to map own memory to this range. Usually present in this region is a part of the graphics cards RAM and the BIOS ROMs of the graphics card and mainboard. Additional hardware like mass storage controllers, network adapters... can use parts of the UMA for own BIOS ROMs or buffer RAM. The Extended Memory Specification (XMS) allows to map blocks of mainboard RAM in the remaining UMA memory range. These blocks are called '''Upper Memory Blocks''' (UMB) and are treated from DOS as High Memory. The default memory management driver that enables this mapping through XMS memory is EMM386.EXE. It has to be loaded in config.sys as second driver after HIMEM.SYS. The driver tries to detect common unused blocks and maps XMS memory there creating Upper Memory Blocks.
 By specifying DOS=UMB in config.sys MS-DOS allocates all UMBs through XMS and takes over memory management of them. this allows TSR-programs to be loaded to UMBs (in High Memory) in config.sys by using ''DEVICEHIGH=driver.sys'' or in autoexec.bat with the loadhigh statement, like ''LH driver.com''. If no contiguous free Upper Memory Block is available the driver will be loaded to Conventional Memory. Since UMA memory is managed in blocks the amount of free Upper Memory is usually larger than the largest contiguous free block. The external program MEM with parameter /C reports additionally the UMA RAM status.
 === High memory area ===
-The High Memory Area (HMA) are the 64 kb directly above 1 Mb and are part of XMS. To use HMA it is required to load HIMEM.SYS in config.sys. By specifying the line '''DOS=HIGH''' in config.sys DOS is asked to load parts to HMA. 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. If DOS=HIGH was specified but loading to HMA fails ''HMA not available/loading DOS low'' is reported.
+The High Memory Area ('''HMA''') are the 64 kb directly above 1 Mb and are part of XMS. To use HMA it is required to load HIMEM.SYS in config.sys. By specifying the line '''DOS=HIGH''' in config.sys DOS is asked to load parts to HMA. 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. If DOS=HIGH was specified but loading to HMA fails ''HMA not available/loading DOS low'' is reported.
 The external program MEM reports if DOS is using the HMA.
 === Expanded memory ===
-The Expanded Memory Specification (EMS) is a standard developed by Lotus, Intel and Microsoft. Expanded Memory can be either memory on an memory expansion card or a part of the main memory. The specification describes that this memory can be used by mapping a 64 kB large part to the Upper Memory Area between 640 kB and 1 Mb. The 64 kB region within the Upper Memory Area is referred to as EMS Page Frame. The latest EMS 4.0 allowed to use up to 32 Mb as expanded memory. The most commonly used memory management driver implementing EMS is EMM386.EXE that has to be loaded in config.sys as second driver after HIMEM.SYS. EMM386 emulates expanded memory by using main memory beyond the High Memory Area. To achieve this it has to switch the CPU to Protected Mode. The amount of free main memory beyond 1 MB can be shared between EMS and XMS by adding the parameter AUTO to EMM386.EXE.
+The '''Expanded Memory Specification''' ('''EMS''') is a standard developed by Lotus, Intel and Microsoft. Expanded Memory can be either memory on an memory expansion card or a part of the main memory. The specification describes that this memory can be used by mapping a 64 kB large part to the Upper Memory Area between 640 kB and 1 Mb. The 64 kB region within the Upper Memory Area is referred to as EMS Page Frame. The latest EMS 4.0 allowed to use up to 32 Mb as expanded memory. The most commonly used memory management driver implementing EMS is EMM386.EXE that has to be loaded in config.sys as second driver after HIMEM.SYS. EMM386 emulates expanded memory by using main memory beyond the High Memory Area. To achieve this it has to switch the CPU to Protected Mode. The amount of free main memory beyond 1 MB can be shared between EMS and XMS by adding the parameter AUTO to EMM386.EXE.
 In the case no EMS is required this feature can be deactivated by using the parameter NOEMS. This frees 64 kB in the Upper Memory Area for loading TSR-programs.
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 === Extended memory ===
-The Extended Memory Specification (XMS) describes an application interface that allows to copy memory between conventional and extended memory. Extended Memory is all main memory beyond 1 Mb. The most common memory management driver for this functionality is HIMEM.SYS that has to be loaded as first driver in config.sys. Accessing memory above the High Memory Area requires switching the CPU to Protected Mode, thus HIMEM.SYS requires at least a 286 CPU.
+The '''Extended Memory Specification''' ('''XMS''') describes an application interface that allows to copy memory between conventional and extended memory. Extended Memory is all main memory beyond 1 Mb. The most common memory management driver for this functionality is HIMEM.SYS that has to be loaded as first driver in config.sys. Accessing memory above the High Memory Area requires switching the CPU to Protected Mode, thus HIMEM.SYS requires at least a 286 CPU.
 Versions:

← Older revision		Revision as of 23:46, 10 April 2013
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	The DOS=HIGH and DOS=UMB statements in config.sys can be combined by writing DOS=HIGH,UMB. Specifying DOS=LOW can be used to prevent DOS from using the HMA.		The DOS=HIGH and DOS=UMB statements in config.sys can be combined by writing DOS=HIGH,UMB. Specifying DOS=LOW can be used to prevent DOS from using the HMA.
		+
		+	Shadow RAM is RAM in the same range as BIOS ROMs. It can usually enabled by BIOS or as parameter of a memory manager. Since ROM access is rather slow it is usually used to copy the BIOS ROM to the underlying RAM at the same location and then switching off the ROM. This accelerates all BIOS calls.
		+
		+	Architectural reasons force the memory for DMA transfers of ISA cards to be reserved below 1 Mb. This is usually of 16 kB size and reserved in UMA.

	== Memory management ==		== Memory management ==

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 === Upper memory ===
-The upper memory area is memory in the range between 640 kb and 1 Mb. By default there is no RAM in this range as it is reserved for use with hardware that is able to map own memory to this range. Usually present in this region is a part of the graphics cards RAM and the BIOS ROMs of the graphics card and mainboard. Additional hardware like mass storage controllers, network adapters... can use additional parts for own BIOS ROMs or buffer RAM. Mainboard chipsets allow to map blocks of mainboard RAM in the remaining unused memory range. These blocks are called Upper Memory Blocks (UMB) and are treated from DOS as High Memory. The default memory management driver that enables this mapping through XMS memory is EMM386.EXE. It has to be loaded in config.sys as second driver after HIMEM.SYS. The driver tries to detect common unused blocks and maps XMS memory there creating Upper Memory Blocks.
+The upper memory area is memory in the range between 640 kb and 1 Mb. By default there is no RAM in this range as it is reserved for use with hardware that is able to map own memory to this range. Usually present in this region is a part of the graphics cards RAM and the BIOS ROMs of the graphics card and mainboard. Additional hardware like mass storage controllers, network adapters... can use additional parts for own BIOS ROMs or buffer RAM. The Extended Memory Specification (XMS) allows to map blocks of mainboard RAM in the remaining UMA memory range. These blocks are called Upper Memory Blocks (UMB) and are treated from DOS as High Memory. The default memory management driver that enables this mapping through XMS memory is EMM386.EXE. It has to be loaded in config.sys as second driver after HIMEM.SYS. The driver tries to detect common unused blocks and maps XMS memory there creating Upper Memory Blocks.
 By specifying DOS=UMB in config.sys MS-DOS allocates all UMBs through XMS and takes over memory management of them. this allows TSR-programs to be loaded to UMBs (in High Memory) in config.sys by using ''DEVICEHIGH=driver.sys'' or in autoexec.bat with the loadhigh statement, like ''LH driver.com''. If no contiguous free Upper Memory Block is available the driver will be loaded to Conventional Memory. Since UMA memory is managed in blocks the amount of free Upper Memory is usually larger than the largest contiguous free block. The external program MEM with parameter /C reports additionally the UMA RAM status.
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 XMS 3.0: max. 4 Gb
 == Memory management ==
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 == Software ==
-'''HIMEM.SYS''': introduced with MS-DOS 5.0, enables XMS (except UMBs) with 286 up to 15 Mb, with 386/486 up to 1023 Mb
+'''HIMEM.SYS''': enables XMS (except UMBs) with 286 up to 15 Mb, with 386/486 up to 1023 Mb
 Versions:
@@ Line 69: / Line 76: @@
 '''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
+'''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
@@ Line 86: / Line 93: @@
 [http://support.microsoft.com/kb/95555 Overview of Memory-Management Functionality in MS-DOS]
 [http://en.wikipedia.org/wiki/QEMM#DOS_equivalents Overview about features of different memory managers]

@@ Line 57: / Line 57: @@
 == Software ==
-HIMEM.SYS: introduced with MS-DOS 5.0, enables XMS (except UMBs) with 286 up to 15 Mb, with 386/486 up to 1023 Mb
+'''HIMEM.SYS''': introduced with MS-DOS 5.0, enables XMS (except UMBs) with 286 up to 15 Mb, with 386/486 up to 1023 Mb
 Versions:
@@ Line 67: / Line 67: @@
 MS-DOS 6.2 6.21 6.22 : 3.10, recognizes max. 4 Gb, 1023 MB XMS useable.
-EMM386.SYS: introduced with MS-DOS 4.01, enables UMBs in UMA, enables EMS
+MEMMAKER.EXE: introduced with MS-DOS 6.0, utility to optimize free conventional memory
-EMM386.EXE: introduced with MS-DOS 5.0, uses XMS to create UMBs in UMA, uses XMS to create EMS
+'''QEMM386''': a complete package of memory management driver and utilities, last version 9.0 / QEMM97