SPEC1.mtx is original Speculator .mtx file
SPEC1A.mtx is patched Speculator .mtx file that uses MTX ports + 1F,7E,7F,FB,FE,FF

Speculator.mtx is SPEC1A.mtx, patched to ask MEMU to do .tap file load and save.

save(bc,de,a)    06a2    (offset 013e in Speculator.mtx)
load(bc,de,a)    0738    (offset 01d4 in Speculator.mtx)
load2(bc,de,a)   0804    (offset 02a0 in Speculator.mtx)

Normal Spectrum operation is in IM 1 with I=3F, but some games use IM 2.

A real Spectrum ROM has FFs from 386E to 3CFF.
We can't assume the low byte of the IM 2 interrupt vector is FF,
but fetching from any 38xx, 39xx, 3Axx and 3Bxx will return FFFF.

The Gosh Wonderful ROM only ensures there is FFFF at 38FF,39FF,3AFF,3BFF,
so it appears it is assuming/relying on the low byte of the vector being FF.

Speculator doesn't have FFFF at 38FF,39FF,3AFF,3BFF, which can prevent
some games using IM 2 from working, such as jswx.
