[Oberon] Sherlock Holmes looking for ben

Skulski, Wojciech skulski at pas.rochester.edu
Thu Dec 19 01:37:30 CET 2019


trying to decipher the firmware I am now puzzled with the individual byte access logic in RISC5 and RISC5top. For the individual byte access, the processor is outputting "ben" described in PO.Computer page 2 as follows: "ben indicates a byte (rather than word) access". There is no further information in PO.Computer whatsoever. There is no truth table. Using my ingenuity I am guessing "ben" is active LOW because otherwise it would have been named "be". But this is not a strong basis to be sure.

Exploring the open source did not bring definite knowledge. Namely, inside RISC5.v "ben" was made wonderfully clear:

assign ben = p & ~q & v & ~stallX & ~stall1;  // byte enable

where both "p" and "q" are clear to everyone:

assign p = ins[31];
assign q = ins[30];

where "ins" is obvious to at least some students, as follows:

assign ins = PMsel ? pmout : IR;  // decoding

After exploring all this great information I am not sure whether "ben" is active LOW or HIGH, and where to find the answer. Does anyone know?



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