About IM2 mode, from the Thomas Scherrer Z80 FAQ :

Quote:

Mode 2: Is the most powerful, in that it allows for an indirect call to any location in memory. With this mode the CPU forms a 16-bit memory address where the upper 8-bits are the content of register I and the lower 8-bits are supplied by the interrupting device. This address points to the first of two sequential bytes in a table where the address of the service routine is located. The CPU automatically obtains the starting address and performs a CALL to this address. The RETI is also used to return to the main programm.

The ugly thing is that in common Z80 machines, the LSB (the one that comes from the device) never has a constant value; so you can't predict the value of the built jumping address (256 different addresses). The only solution is to set up a vector table whith 257 identical values, say $C1 for example, and then you'll be sure that when we get the interrupt address, it will be always $C1C1.

As you can see it's only a bit more work (and 257 bytes free less). The advantage is the total control of the CPU.

Oh I get it now. So register I combined with a the lower byte from the device points to the address of the interrupt routine... This must be specially useful on machines with more than 1 interrupt!

For some reason I just assumed that the MSX didn't have a IM2 mode (just as it doesn't use NMI's), but it turns out you just have to waste a bunch of memory to make sure it'll jump to the correct place. Thanks a bunch jltursan!

If you are more interested about working of databus, you may want to check out this program, that I wrote for monitoring it:
http://www.msx.org/forumtopic3816.html

Quote:

Oh I get it now. So register I combined with a the lower byte from the device points to the address of the interrupt routine... This must be specially useful on machines with more than 1 interrupt! For some reason I just assumed that the MSX didn't have a IM2 mode (just as it doesn't use NMI's), but it turns out you just have to waste a bunch of memory to make sure it'll jump to the correct place. Thanks a bunch jltursan!

As far as I know, MSX always contains pull-up resistors on the data bus. It means that the Z80 will read the value 0xFF for the high-part of the pointer as there is no device to put any other value on the bus.

In the thread that NYYRIKKI posted above, some MSX models will have random values. Or didn't I get what you just said? =)