I guess I was slightly wrong:

Quote:
I / O 043 Bh
Name 16 MB Space Memory Control
Undocumented
Target PC-9821 Af · Ap 2 · As 2 · Bp · Bs · Be · Bf · Cs 2 · Ce 2 · Ts · An · Xt · Xa · Xn · Xp · Xs · Xe
PC-9821 Np · Ns · Ne 2 · Nd · Es
PC-9801 BA 2, BS 2, BX 2, BA 3, BX 3, BX 4, NL / A, NS / A
function
[READ / WRITE]
Bit 7 to 3: not used (always 00000 b)
Bit 2: 16 MB space control
1 = normal memory space
0 = system used
Bit 1, 0: unknown (always 00b)
Comment o Whether the system uses memory space of F00000 to FFFFFFh or normal memory space
As shown in FIG.
O When 16 MB space is used as normal memory, it is possible to expand the expansion in F00000 to F7FFFFh
Graphics VRAM can not be used. However, even in this case FFF 00000 -
Extended graphics VRAM can be used if address of FFF7FFFFh is used.
In the case of
O On PCI bus-based models, besides this I / O, PCMC, PCI-C bus bridge, PCI-R
It is necessary to make settings for the Carbus bridge as well.
Related I / O 881 Eh bit 0 ■ [98NOTE, 98FiNE]
I / O 8F1Eh bit 0 ■ [Other than 98NOTE, 98FiNE]


Looks like the vram window at 0xF00000 can be disabled but the window at 0xFFF00000 is always available.