[Bug] x128 REU emulation is broken in C64 mode with VICE 1.16 (and before)

The information in this article applies to the following versions of VICE:

VICE 1.16, and most likely all older releases

SYMPTOMS

On some programs which try to access the REU, the program does not work at all, crashes, or waits forever. This only occurs on x128 in C64 mode. Neither x64 nor x128 in C128 are affected.

CAUSE

For the REU, there is a so-called $FF00 option. The actual transfer to or from the REU is delayed until $FF00 has been written. On a C128, there is a MMU on $FF00. This allows to change the memory configuration when trying to write to or read from the REU.
Unfortunately, in C64 mode of x128, the access to $FF00 is never catched. Thus, every program which relies on this does not work with the REU emulation in C64 mode of x128.

SOLUTION

Do not use the C64 mode of x128 if you want to access the REU. Use x64 instead. The next version of VICE will have this bug corrected.

STATUS

The VICE team has confirmed this to be a bug in the VICE versions listed at the beginning of this article. It has been corrected in all subsequent versions.
Neither x64 nor x128 in C128 mode are affected by this bug!

ACKNOWLEDGEMENT

Thanks to Sid Davis for reporting this bug.

REVISION

February 12th, 2005: Original release of this document

Last Reviewed: February 12, 2005