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MEMO1001.TXT
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1992-12-02
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╔═════════════════════════════════════════════════════════════════════════╗
║ ║
║ Glossary of Common Memory Terms ║
║ ║
╚═════════════════════════════════════════════════════════════════════════╝
CONVENTIONAL MEMORY:
Conventional memory is the first 640K bytes of memory. DOS based programs
load and run from within the first 640K of memory.
Note: Application programs such as Lotus 123 ver 2.x or Wordperfect 5.1
must have enough conventional memory available to load and run before they
can access and use any expanded memory. The expanded memory they use is
used mainly for data storage, the program itself does not run from
expanded memory.
BASE MEMORY:
Base memory is another name for Conventional Memory. Whenever you come
across the phrases Conventional Memory or Base Memory, they both refer to
the first 640K bytes of memory.
EXPANDED MEMORY:
Expanded memory works by using some of the DOS reserved memory address
space to access memory on the Intel memory board. The area of reserved
address space used to access expanded memory is made up of 16K byte
sections called pages. A minimum of four contiguous 16K byte pages are
taken from the reserved DOS memory between 640K and 1 Megabyte. Typically,
the address space used is between 768K and 896K. This 64K byte area is
used as a mappable space and is known as the Expanded Memory Page Frame.
Additional unused 16K byte pages found in the reserved memory area can
also be assigned to expanded memory. Having expanded memory requires that
you have an Expanded Memory Manager (EMM.SYS for Intel memory boards), to
"manage" the expanded memory. The driver associates (or maps) the pages
of physical memory on the memory board to the pages in the reserved DOS
area. Only programs written to use expanded memory can utilize expanded
memory. Some examples of applications that use expanded memory are Lotus
123 ver 2.x and Wordperfect 5.x.
EXTENDED MEMORY:
Extended memory is memory above the 1Megabyte address boundary. Only
80286 (or greater) CPU's have the ability to address memory above the
1Megabyte boundary. The 80286 chip can access 16M bytes of total address
space, and the i386 chip can access 4 gigabytes of total address space.
The 8088 and 8086 microprocessors can only access 1Mb of address space,
(640K conventional and 384K reserved memory). The DOS operating system
was not designed to access memory above the 1M byte address boundary,
which means that DOS programs do not automatically have access to extended
memory.
However, many DOS based computers do have more than 1M byte of memory on
the motherboard. Certain DOS applications, (such as Windows 3.x), and the
OS/2 operating system do use extended memory because they were
specifically written to use extended memory. No special drivers are
needed to access extended memory. Only programs that are written to use
extended memory can utilize extended memory.
HIGH MEMORY AREA (HMA):
HMA refers to a specific way of accessing the first 64K of Extended
Memory. Normally, for an 80286 to access extended memory, the processer
has to switch into what is called "protected mode", make the extended
memory access, and then be reset so that it returns to "real mode" (which
is the mode that DOS runs under). This whole operation takes time. There
is, however, a "back door" method to access the first 64K of extended
memory without switching the processor into protected mode. What manages
this special way of accessing the first 64K of extended memory is
something called an HMA provider (also sometimes called an A20 handler).
Device drivers that create the HMA in 80286-based systems:
1. HIMEM.SYS provided with DOS 5.0
2. QEXT.SYS from Quarterdeck Office Systems
3. MOVEM.MGR from Qualitas
Device drivers that create the HMA in i386-based systems:
1. HIMEM.SYS provided with DOS 5.0
2. QEMM386 from Quarterdeck Office Systems
3. 386MAX from Qualitas
RESERVED MEMORY:
Reserved Memory is the address space between 640K and 1 megabyte and was
designed to be reserved for system operations. The system BIOS ROM, video
ROM, and ROMs of some add-in cards reside in the reserved memory address
space. Keep in mind that this is "address space", meaning that there
doesn't have to be any physical memory located at all the addresses from
640K to 1 megabyte. The system BIOS ROM can be found at hex address F0000
or 960K. If you have a VGA or EGA video card you can almost always find
video ROM at hex address C0000 or 768K.
Between the video ROM and the system BIOS ROM at hex address D0000 for
example, there might not be anything at all! This "address space" is
unused and available, and drivers such as Intel's EMM.SYS (Expanded Memory
Manager) can "map" blocks of memory within the C000-DFFF address range
(between 768K and 896K), which means that the memory EMM.SYS controls on
an Intel memory board will appear within that address range.
UPPER MEMORY BLOCKS (UMB'S):
Also known as High RAM and Upper-Memory. Normally expanded memory only
REQUIRES a 64K page frame. But, a lot of LIM 4.0 compatible expanded
memory boards (such as Intel memory boards) automatically try to provide
as large a page frame as possible, up to 128K (if there are no ROM's
within this range also using some of this space), within the C000-DFFF
range. These additional pages of expanded memory in the page frame can be
used to create Upper Memory Blocks, which are blocks of memory that can be
used to load device drivers and TSR's into them in order to free up more
conventional memory for your applications. Basically, every extra page of
expanded memory outside of the first 64K of the pageframe can be turned
into a UMB. There are memory managers available that create these UMB's.
UMB providers in 8088, 8086, and 80286 based systems:
1. QRAM
2. MOVEM
These memory managers require that the available Upper Memory Blocks first
be made mappable by the combination of a LIM 4.0 Expanded Memory Manager
and a memory card such as Intel's EMM.SYS device driver and an Intel Above
Board. The Above Board Plus and Above Board Plus 8 can map extra 16K
blocks of memory within the C000-DFFF address range beyond the first 64K
(which has to remain intact so that applications that use expanded memory
will still be able to use it).
UMB providers in i386 and i486 based systems:
1. EMM386.SYS provided with DOS 5.0
2. QEMM
3. 386MAX
These memory managers do not require LIM 4.0 expanded memory board because
the i386 and i486 microprocessors have "mapping" capabilities built into
them. 386 memory managers simply use extended memory to emulate expanded
and also UMB's.
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End of file Intel FaxBack # 1001 December 2,1992