I posted an answer to the question MSX's primary and secondary slots but while researching some references in the msx.org wiki to ensure my answer was accurate, I found some discrepancies between my recollection and the information I found.

There was an incompatibility between the way the Hotbit (HB-8000) and the Gradiente Expert mapped slots into each of the four 16kb memory pages. This I remember vividly because I had to patch some games to make them work on my machine.

What I recall is that the I/O Port A8h only worked on the Expert, while the HB-8000 required writing to a "magic" address somewhere in the address space. The mechanism on the HB-8000 actually resembles what's documented for secondary slot mapping in MSX2.

The msx.org wiki states

the Hotbit followed MSX standards strictly unlike the competing Gradiente Expert

But my recollection is just the opposite: A8h I/O was the MSX Standard way of controlling the slot mapping, and the HB-8000 implementation was non-standard.

Where can I find documentation on the HB-8000 way of doing it? And which one was standard?

1 Answer 1


The MSX specification was originally defined by The MSX Technical Data Book (that particular copy being branded by Sony, with no consequence).

Page 40 provides the IO address map, including PPI (8255) between A8 and B0. Skipping forwards to page 43 shows that port A of the 8255 is indeed intended to do primary slot selection. A peek at the 8255 data sheet shows that A0 = A1 = 0 selects the value of port A for reading or writing.

So, in net, A8 is defined to be the IO port that controls slots in the original MSX specification.

That being said, checking the openMSX XML descriptions of the HB-8000 and the Gradiente Expert Plus I notice that:

  • the HB-8000 simply fills slot 3 with 64kb of RAM, like most machines; whereas
  • the Gradiente Expert Plus adds secondary slots to slot 3; secondary slot 0 is its 64kb of RAM, secondary slots 1 and 2 are unfilled and slot 3 contains a demo ROM.

So that would appear to be the opposite to your recollection; the HB-8000 should always present RAM when primary slot 3 is active whereas the Gradiente Expert Plus would require you both to switch in primary slot 3 and then select secondary slot 0 within primary slot 3.

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