knz/mgsim — pull request #6 by lhelwerd (2012-10-04 16:03:46+00:00)
Title: Map invalid register to the physical null address

This solves problems with Immediate type instructions, which do not
set a register address for e.g. Rb, which causes INVALID_REG to be
used, which was set to register 0x1F or 31. This is correct for Alpha, but
for MIPS it triggers an illegal instruction error.

Define the invalid register index to 0 on MIPS targets, which is
cleaner than setting them explicitly in the decode logic or
translating register 31 to zero in GetRegisterClass.