The EIO Handler is being considered for removal/replacement.
M_Sim_v2.0 did not properly support the EIO handler which is necessary to load checkpoints. However, it appears that the EIO handler may not be necessary since the syscalls it emulates can be handled directly if the appropriate redirections are made. It is not clear if the EIO handler does anything else other than emulating input/output.
Furthermore, the EIO handler is extraordinary convoluted code. Seriously, raw bit access would have been easier to understand.
The EIO files contain the specification for the checkpoint data.