[TI z80 asm] Interrupt modes

King Harold · Post by **King Harold** » Mon 02 Jul, 2007 12:47 pm

I've posted about it before, there are 4 interrupt modes!
They are as follows:
IM 0 ED86
IM ? ED8E <-- NOT a NOP!
IM 1 ED96
IM 2 ED9E

The unknown IM has thus far been "documented" as an undocumented NOP, but when executed it changes the Interrupt Mode!
Try it out on your calc..
The program will run normally but when it exits the calculator will not use the IM1 interrupt anymore so the keys won't be checked and the cursor won't blink etc.
I haven't yet found a use for it though.. But perhaps someone else could find out more about it?

Jim e · Post by **Jim e** » Mon 02 Jul, 2007 1:29 pm

It's probably just im 0, since the switching bit seems to be dependent on bit 4 of the second opcode. Problem is that im 0 isn't easy to test. The z80 will try to get byte(s) from the hardware but the hardware wont respond with anything so whatever is on the databus will be used for the interrupt.

King Harold · Post by **King Harold** » Mon 02 Jul, 2007 4:13 pm

Ah I see..
So definitly not a NOP

CoBB · Post by **CoBB** » Tue 03 Jul, 2007 11:07 am

It's generally believed to be an IM 0 mirror.

King Harold · Post by **King Harold** » Tue 03 Jul, 2007 12:50 pm

How would we find out if it isn't, though?
Is it even possible?

driesguldolf · Post by **driesguldolf** » Sun 15 Jul, 2007 11:14 am

Why can't we simply ask the manifacturer (not ti

but zilog or something)?

King Harold · Post by **King Harold** » Sun 15 Jul, 2007 12:01 pm

Why should they care? They'd probably tell you that they "sadly do not have the information you require" or some such. Since that's cheaper than actually looking it up.
You could try of course

Post by **Dwedit** » Sun 15 Jul, 2007 10:53 pm

Usually it's called IM 0/1, because the people who make the undocumented instruction lists have no clue which one it actually selects.