ViTi95 wrote on 2021-07-20, 17:23:
Edit: Do exist any ISA video card that supports LFB? The only card I have that is supposed to support it is a Cirrus Logic GD-5429, but UniVBE doesn't enable it at all (even with less than 16Mb of RAM). The same motherboard works fine with a GD-5426 and LFB enabled. Maybe someone here can try this new mode with a compatible ISA one.
dug out my old comments answering similar question:
"afaik only ISA Tseng ET4000 and ATI Mach64 supported LFB, with ATI being only one allowing mapping more than 1MB at once. ISA is limited to 24 bits of address space = up to 16MB, so new Bios option appeared in 386 and later motherboards called "Memory Hole At 15M-16M" to make room for 1MB of ISA card address space when computer has more than 15MB of ram. If you had 4MB ATI Mach64 and wanted it all mapped linearly you had to contend with 12MB ram max installed etc"
"Bios companion book covers VGA LFB ISA bios options https://books.google.com/books?id=69648Y1sOew … &q=LFB+ISA+card Page 270
ET4000: 3d4h index 36h (R/W): Video System Configuration 4 Linear addressing if set (4000) Video Memory is mapped as a 1 Meg block above 1MB. 5 ? Enable Tseng Addressing Mode (W32x) If set enables Memory Mapped registers. Three MMU areas exist.... 3CDh (R/W): Segment Select 6-7 Segment Configuration. 2 1M linear memory 3d4h index 30h (R/W): (W32x ?) bit 0-? Linear Frame Buffer Address in units of 4Mb"
"looking up VBE/AF documentation they only supported banked modes for ET4000/6000 chips"
tldr: there is a possibility ISA ATI Mach64 will support LBF