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Rev

BTW, for systems that had backing RAM at those (ROM) address, but you wanted to use as RAM and not to shadow the BIOS, there were some hacks like 'UMBPCI' but as noted there, whether it worked was chipset dependant.
– got trolled too much this week
Commented Feb 4 at 21:30
There were apparently some '286 era chipsets that had shadow RAM support too retrocomputing.stackexchange.com/a/7788/12212
– got trolled too much this week
Commented Feb 4 at 22:04
Intel's own 82340DX chipset for the 386DX processor mentions shadow RAM support as well. (Programming info for that controller seems harder to find though, these days. According to Wikipedia "it is the Topcat chipset licensed from VLSI". Earlier VLSI chipsets for the '286 though, for which I can find some details don't seem to mention shadow RAM though.
– got trolled too much this week
Commented Feb 4 at 22:51
1

The shadow RAM feature seems to have appeared in the "mid-life" of '286 processor. The VLSI chipset 82C101B (prev. link) doesn't have/mention it, but the 82C202 does. On this one you could separately enable (or not) the write protection after the shadowing. (Aside: accessing the configuration registers had a bit of an "open sesame" aspect on this chip: you had to write 9 times the same value to a register at I/O 09FH.)
– got trolled too much this week
Commented Feb 4 at 23:26
Shadow RAM appeared when it became common to have 1MB or more of RAM. Real Mode can't use more that 1MB of RAM, and the address space used by the ROM had to shadow the RAM anyway. So using the faster RAM for the BIOS is one possible use. Other uses are to remap the area and use it as extended or expanded memory.
– RalfFriedl
Commented Feb 5 at 15:20

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