] > Maybe to prevent accidental page-switches? Anyway, for megaROMs it's
] > not difficult: most of the IC's used, simply get connected to all the
] > higher address lines, so that they can decode ANY combination of for
] > instance address lines A12-A15. What is done with that, is decided by
] > the IC design.
]
] I know, the IC was designed this way, but what was the idea behind block
] numbers can't be selected via pages 0 and 3? Nobody knows.
I have a theory about this, which might make some sense:
Megaroms mostly contain games. And the gamedevelopers want three things:
1) They want to have easy and fast access to the BIOS, so they keep the BIOS
always switched on in page 0
2) They want to have easy and fast access to some workmemory and to the MSX
system memory. So they keep the RAM always enabled in page 3
3) This leaves them with only page 1 and 2 available for the megarom.
However, they want flexibility with the blockswitching in the megaroms. So,
they use 8KB blocks in their megaroms. And those blocks are always enables in
page 1 and page 2 so why bother about page 0 and 3.
This basic set-up (page 0 = rom bios, page 1 and 2 = cartridge, page 3 = ram)
was a set-up that the designers of the MSX had already in their mind. That is
why they made life easy for cartridge designers by providing the CSxx
signals. Furthermore, as they assumed that all cartridge designers will use
this set-up anyway, they also only searched in page 1 and page 2 for
extensions. Later, when the MSX2 was introduced with the subrom in page 0,
the MSX also started looking in page 0. But in page 0, they look for 'CD' as
token in stead of 'AB'. This is to prevent that an old cartridge that is
mirrored at address 0 is accidentally recognized as a MSX2 subrom.
Kind regards,
Alex Wulms
--
Alex Wulms/XelaSoft - MSX of anders NIX - Linux 4 ever
See my homepage for info on the *** XSA *** format
http://www.inter.nl.net/users/A.P.Wulms
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