Update. Found an interesting piece of text - "Breaching into the upper memory area".
"One technique used on early IBM XT computers was to install additional RAM into the video memory address range and push the limit up to the start
of the Monochrome Display Adapter (MDA). Sometimes, a custom address decoder was required for this to work. This moved the barrier to 704 KB
(with MDA/HGC) or 736 KB (with CGA). Memory managers on 386-based systems, such as QEMM or MEMMAX (+V) in DR-DOS, could achieve the same
effect, adding conventional memory at 640 KB and moving the barrier to 704 KB (up to segment 0xB000,[a] the start of MDA/HGC) or 736 KB (up to
segment 0xB800, the start of the CGA). Only CGA could be used in this situation, because Enhanced Graphics Adapter (EGA) video memory was
immediately adjacent to the conventional memory area below the 640 KB line; the same memory area could not be used both for the frame buffer
of the video card and for transient programs.
AllCard, an add-on memory management unit for XT-class computers, allowed normal memory to be mapped into the 0xA0000-0xEFFFF address range,
giving up to 952 KB for DOS programs. Programs such as Lotus 1-2-3, which accessed video memory directly, needed to be patched to handle this
memory layout. Therefore, the 640 KB barrier was removed at the cost of hardware compatibility.
It was also possible to use console redirection (either by specifying an alternative console device like "AUX:" when initially invoking COMMAND.COM
or by using CTTY later on) to direct output to and receive input from a dumb terminal or another computer running a terminal emulator.
Assuming the System BIOS still permitted the machine to boot (which is often the case at least with BIOSes for embedded PCs), the video card could
then be removed completely, and the system could provide a total of 960 KB of continuous DOS memory for programs to load.
Similar usage was possible on many DOS- but not IBM PC compatible computers with a non-fragmented memory layout, for example the
Victor 9000/Sirius 1 which supported up to 896 KB continuous DOS memory to be used under its custom version of MS-DOS. "
Source: https://www.wikizero.com/en/Conventional_memo … per_memory_area
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