May 18, 2015

zxsp 0.8.pre27: Multiface ONE, 128 and +3


Three new members in the family of supported extensions: The Multiface ONE, 128 and +3 from Romantic Robots.

This time i scrutinized the circuits into depth and therefore the emulation should be near perfect. :-) There were different roms used in the MF1, evtl. also in the MF128, i don't know – i have included the most common.

The Multiface ONE had a Kempston-style joystick port which could be disabled by cutting a wire, in case another device also used this port. You can disable this port in zxsp too. If you attached the Kempston interface (e.g. you have enabled the auto-attach option in the preferences) and then at some time attach the Multiface ONE, then disabling this port is not strictly required, because an already attached USB joystick will happen to be magically plugged in both interfaces, and then both devices will react the same when a program reads a byte from the Kempston port. But disabling this port in the MF1 is probably more intuitive and leads to less unexpected misbehavior.

While looking at the circuits i noticed some interesting points:

The Multifaces do only react to an NMI generated by the own red button, not to any NMI.

The presence of the Multiface ONE could be detected by the running program if it paged in the MF1 rom, and probably at some point games started to test for it and complained if they detected this device.

The presence of the Multiface 128 could no longer be detected by the running software; Romantic Robots had added a flag which stored the visibility state of the interface, i called it the camouflage flag. Same time they wrote "PIRACY IS ILLEGAL! MULTIFACE 128 is NOT designed to encourage piracy!" into their manual – Words and actions are not always the same.

The Multiface +3 was unusable if a program mapped ram to address $0000. Romantic Robots silently did not mention this in their manual. But the circuit of the MF+3 is very clear: It has a flag which monitors the "special mode" bit written to the MMU of the Spectrum +3 and, if set, ignores any press of the button. The reason is quite clear: Amstrad had reworked the ZX Spectrum bus connector of the +3 and added a second line to disable the internal rom at address $0000 with highest comfort, but no possibility to disable ram at this address. Yes, the Spectrum was not their most beloved child…

zxsp displays the state of the most interesting flags in the Multiface, which is, for the MF1, whether an NMI is pending (and the NMI routine very likely already has been executed and any more pressing this button has no effect), for the MF128 whether it is visible and for the +3 whether the ZX Spectrum MMU was set to "special mode" which means ram at $0000 and a disabled Multiface.

The red button of all Multifaces is gated by the output of the NMI pending flip flop which is set by this button: If the button set the FF, then the FF's output immediately disabled the button. To my best understanding this is unnecessary and it does not work:
  • I believe it was meant to immediately stop the signal from the button, so that recursive NMI processing in the CPU could not happen. But the NMI is edge triggered: once active, it can be held active for years without triggering another NMI.
  • Once the FF was reset (by software) this disable signal disappeared and the FF was immediately set again, if the button was still pressed.
The last point might be tested with the real device if you press the red button and keep it pressed while you type 'r' on the keyboard to exit the Multiface menu. Then the Multiface should be immediately entered again. But it's not sure whether it will, because for this the NMI must go away for a long enough time so another edge can be detected by the Z80 CPU, but it only goes away for a tiny moment, just the round-trip time through one gate and the flip flop. Note: this is not reproduced in zxsp.

So, to all of you who owned a Multiface: Test your skills and revive memories. ;-).

     ... Kio !