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<!doctype HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<title> pAVR TODOs </title>
<b> Documentation </b>
   <li> Add some AVR kernel schematics.
   <li> Add some AVR general considerations.
<b> Coding style </b>
   <li> Replace `next_...' signals family with a (pretty wide) state decoder.
<b> Testing </b>
   <li> Exercise string manipulating routines - C test for pAVR
   <li> C compiler - test for pAVR
   <li> Burn pAVR into an FPGA.
<b> Design </b>
   <li> Branch prediction with hashed branch prediction table and 2 bit predictor.
   <li> Super-RAM interfacing to Program Memory. <br>
A super-RAM is a classic RAM with two supplemental lines: a mem_rq input and a mem_ack output. The device that writes/reads the super-RAM knows that it can place an access request when the memory signalizes it's ready via mem_ack. Only then, it can place an access request via mem_rq.
A super-RAM is a super-class for classic RAM. That is, a super-RAM becomes classic RAM if the RAM ignores mem_rq and keeps continousely mem_ack to 1.
The super-RAM protocol is so flexible that, as an extreme example, it can serially (!) interface the Program Memory to the controller. That is, about 2-3 wires instead of 38 wires, without needing to modify anything in the controller. Of course, that would come with a very large speed penalty, but it allows choosing the most advantageous compromise between the number of wires and speed. The only thing to be done is to add a serial to parallel converter, that complies to the super-RAM protocol.
After pAVR is made super-RAM compatible, it can run anyway from a regular RAM, as it runs now, by ignoring the two extra lines. Thus, nothing is removed, it's only added. No speed penalty should be payed.
A simple way to add the super-RAM interface is to force nops into the pipeline as long as the serial-to-parallel converter works on an instruction word.
   <li> Modify stall handling so that no nops are required after the instruction wavefront. The instructions could take care of themselves. The idea is that a request to a hardware resource that is already in use by an older instruction, could automatically generate a stall.
That would:
      <li> generally simplify instruction handling
      <li> make average instruction execution slightly faster.

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