Hi. How much thought has there been on what operations the ALU will perform (floating point).
I can see that we need add, sub and multiply. I think that divide and inverse square root would be nice to have, but I understand that they are costly in terms of area etc.. There are some other functions that we could have in hardware, but we can possibly work around with the other operations (power, sin, cos). If we decide not to have full divide, square root, etc. has anyone thought about having a half-way instruction that would produce a good first estimate. Other chips (IA64?) have these instructions. If we can get half or a quarter of the bits of accuracy that we need from and initial instruction then completing the result is much easier. I thought about adding a reciprocal instruction, but that's not much easier to calculate that a full divide. I was also wondering if it was feasible to split the divide up into two opcodes, the first opcode would do half of the work and would leave the partial result in some special flip-flops. The second op-code would complete the operation and put the result in the correct place (I know that this is a bit nasty, but it lets us extend our pipeline to almost any depth we need). I'm keen to hear your thoughts. Mark Marshall.
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