On Fri, Sep 28, 2012 at 10:39:50PM -0400, Peter Gavin wrote: > Speaking of which, this is something that annoys me about the OR1K ISA > and I think should be changed for OR2K (if it hasn't already). It's > pretty much impossible to implement the current ISA with a stock > 32-bit multiplier, so you have to use a 64-bit multiplier if you don't > want to hand code a multiplier. > > And the MAC truncates to 32-bits *before* the add, is that right? But > it keeps the whole 64-bits of the accumulation? Is there a good > reason for that? >
I think there was some discussion about that sometime ago, Yann Vernier participated in that, perhaps he could chime in? (I believe he was of the same opinion as you) > I don't understand the justifications for this in the ISA, but it > certainly seems odd and difficult to implement, if you ask me. It > seems to me the best thing would just to have a single, 64-bit MAC > that takes 32-bit operands and can be used for both the MAC and for > regular multiply. > Yes, I agree, and you wouldn't loose much if the ISA would allow pushing out the full 64-bit result to a register pair.... Stefan _______________________________________________ OpenRISC mailing list [email protected] http://lists.openrisc.net/listinfo/openrisc
