Under DOS, Quake will try to grab 64MB

Although minimum is only 12MB, I believe Links LS 1997 Edition (DOS) would use as much as you could give it. It also took advantage of extra VRAM to render at up to 1600x1200x32bpp (provided graphics card and monitor supported it).

As far as I know most DOS era games allocated just the RAM thats needed.

Games derived from DOOM engine (require 4MB, can use up to 8MB) used the extra RAM to improve cache retention.

All allocated RAM goes to a custom memory allocator providing memory to the engine. Some no longer needed memory allocations can be marked as purgeable. The memory manager can purge data in order to satisfy other allocations but might keep them around for longer and reuse them later. Having 8MB instead of 4MB could improve retention of these blocks. Such as some level features, less frequently used sounds, etc.

However, as the cost of RAM before 1995 was very high, DOS games are unlikely to be complex enough to require or even be optimised for large amount of RAM. Even if they are capable to allocate the RAM this is unlikely to noticeably improve performance.

auron wrote on 2020-03-22, 12:31:

also was virtual memory actually ever a thing in DOS extender games, as afaik DOS/4G supports it?

Absolutely. Many games will use a pagefile if you have less than 8MB.
Red Alert will always allocate a 20MB pagefile even if it's not needed.