So to use IM2 the way it on MSX, should for some added hardware gizmo, this
] devices would have to put its dev-ID on the bus during acknowledgment, AND
] that resistor must be removed. Just out of curiosity, is this easily done?
You don't need to remove the pull-up resisters. They have a high resistance
so that a device that wants to put something on the bus can do so. They are
only there to ensure a well-defined state when no device writes something to
the bus while the Z80 reads from it. For example if you do something like
inp(30), while there is no device at i/o port 30. The Z80 reads from the bus,
all data lines are up, so you will see 255.
For the rest, it is not so difficult to build such a device. Just read a book
about the Z80 for the exact timing and behaviour of the different signals on
the Z80 bus. Though, when you build such a device you should make sure that
you only have one such device in your computer. If you put a second one in
the second cartridge slot for example, you will get bus conflicts if they
generate an interrupt at the same time... Or to be more precise, if they both
generate an interrupt before the Z80 acknowledges it (the interrupt does not
get acknowledged as long as the Z80 is in DI mode...)
]
] > When you combine 2) and 3) you can safely conclude that any MSX computer has
] > 255 on the databus at the moment that the Z80 reads it during IM 2.
]
] > So, it is sufficient to write your pointer to (I + 0xFF, I + 0x100), as shown
] > in Laurens code example.
] Thanks for explaining.
]
] > Kind regards,
] > Alex Wulms
] >
] > Ps. I have used IM 2 as well on several occasions.
] What added use is it over using the regular 038h?
To bypass the standard interrupt routine when the ROM-BIOS is enabled. Can be
very usefull when playing with line interrupts and you want to have a very
fast reaction to the IRQ without the delay of all registers being pushed onto
the stack...
Kind regards,
Alex Wulms
--
Alex Wulms - Visit http://www.inter.nl.net/users/A.P.Wulms for info
on XelaSoft, Merlasoft, Quadrivium, the XSA Disk image distribution
format, the MSX Hardware list, SD-Snatcher on fMSX and lot's more
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