[rasz]

1992 Cirrus Logic GD5402/AVGA2 (ISA Bus) allowed you to enable special 128KB linear window directly mapping A0000-Bffff to VGA memory. Only ISA VGA chip fully supporting linear framebuffer access was ATI Mach64. Since ISA bus is limited to 16MB address space you had a choice of either no more than 12MB of ram, or finding a motherboard supporting "Memory Hole At 15M-16M" Bios option. VLB and PCI graphic cards mostly didnt have this problem (some mapped itself at 64MB assuming nobody would be crazy enough to cram that much ram into a computer in 1993). It popped once again at the end of AGP life/start of PCIE when we ran out of 32bit memory, this is why fitting 4GB of ram often resulted in 3.5GB usable at most.

Of course this problem is with us even today. PCIE Resizable BAR is a brand spanking new feature, first proposed in 2008, enabling expanding directly mapped buffer from previous limit of 256MB to full capacity of VRAM. AMD tried to repackage it under different name (Smart Access Memory) and upsell as exclusive to only highend brand new parts, Intel new lol GPUs require it to work reasonably fast, but lock out support on non Intel platforms.