8080 Assembler-Text Editor (ATE)

On 4/21/2006 at 12:49 PM Holger Veit wrote: Those upper-lower IX/IY undocumented instructions seem to be very straightforward. DD/FD prefixes simply indicate whether IX or IY should be used in place of HL. Undocumented EDs on the Z80 are another kettle of fish, however. On the 8085, one important use of RIM and SIM is handling of TRAP conditions. I've got a system here that uses TRAP to diagnose accesses of non-existent peripherals and memory parity errors as well a keyboard entry akin to (Ctrl+Alt+Del). Out of curiosity, I got my old Grid laptop with a NEC V30 CPU out and started to try some of the "undocumented" 8085 opcodes on it. Well, the few that I've tried aren't no-ops, but on the the other hand, they don't do anything particularly exciting. For example, 28 seems to be identical in operation to DAD H (29), while 38 seems to act identically to INR H (24). FD appears to read the next byte and use it to set or clear the parity and half-carry flags. My guess is that part of the CPI instruction logic's being used, although the other flags (C, Z and S) don't seem to be affected. I haven't tried any others--it's pretty obvious that undefined opcodes in V30 8080 emulation mode simply take flying leaps into the logic array. How this compares with, say, a NEC C8080A is something that I'm not equipped to answer. After all these years, I'm still not certain that those who exploited the undocumented instructions in any of these chips saved much in the way of speed or space. About the only significant exception I can think of is the 80286 LOADALL instruction, which was only semi-documented by Intel. Cheers, Chuck