Why did the Z80 producers leave some code operations undocumented?  Did they not have time to "test" these instructions, did they come about accidentally in the design of the processor, or what?

Did TI not want to release them?

:O Didn't know about it, then no idea.

I'm not exactly sure why the instructions are undocumented. The best guess I can make is that they are optionally supported. So, it's common knowledge that they are there, but if the very early models or late models don't support them, then they won't be disobeying the z80 spec.

I think I remember reading something to that extent somewhere some time ago.

There's a couple of reasons I can suggest as to why they're undocumented.

First and probably the main reason, instructions that weren't originally in the instruction set worked predictably due to the logic that the Z80 performed. The Z80 is quite a logical chip, so under most circumstances, logic would dictate a few things that weren't intended. Like being able to load the high and low bytes of the IX/IY registers independently.

Well, I guess I don't have a second reason.

There was a site that explained all this clearly, but it was on the Geocities thing that got shut down like last year or so. Wished I could've saved it.

@ Iambian: So basically, you're saying that the undocumented ones worked based upon how the Z80 is structured?

There's a couple of reasons I can suggest as to why they're undocumented.

First and probably the main reason, instructions that weren't originally in the instruction set worked predictably due to the logic that the Z80 performed. The Z80 is quite a logical chip, so under most circumstances, logic would dictate a few things that weren't intended. Like being able to load the high and low bytes of the IX/IY registers independently.

Well, I guess I don't have a second reason.

There was a site that explained all this clearly, but it was on the Geocities thing that got shut down like last year or so. Wished I could've saved it.

Could it be the RichFiles? I think it's still available elsewhere, but I forgot the link.