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Unformatted text preview: h-> dec ode-> ex ec ute c y c le for our brok en ins truc tion ADD R0, R6, 5 s hould oc c ur from 300-600 ns after the
s tart of the s imulation. Run the s imulation.
Ty pe do sta rt.do at the vs im prompt
Ty pe run 1000 at the vs im prompt to run the s imulation for 1000 ns .
If y ou want, z oom in to where the ADD ins truc tion ex ec utes by s elec ting Zoom-> Zoom Range and entering 300 ns for the
s tart time and 640 ns as the s top time. If y ou do this s tep, y our window s hould look lik e the following. Here are s ome things that y ou s hould notic e about this waveform.
1. The CARout tells us whic h s tate the c ontrol unit is in. As a reminder, the fetc h s tages of the c ontrol unit oc c ur during s tates
18, 33, and 35. The dec ode s tage oc c urs during s tate 32. The ex ec ute s tage for the ADD ins truc tion oc c urs in s tate 1.
Notic e how the value of CARout progres s es through the s tates 18, 33, 35, 32, and 1 to ex ec ute the ADD ins truc tion and then
return to s tate 18 to begin fetc hing the nex t ins truc tion.
2. The c ontent of R6 is x 300D when the ADD ins truc tion is fetc hed. If we ex ec ute the *ADD R0, R6, 5 *ins truc tion, we s hould
s ee the c ontents of R0 c hange to after x 3012 (= x 300D + x 0005) after we leave the ex ec ute s tate of the ADD ins truc tion:
s tate 1. Notic e that value of R0 never c hanges ! Debugging the micr oinstr uction To debug the mic roins truc tion, we need to figure out whic h s ignals c ould potentially c aus e an error with the ex ec ute phas e of the
ADD ins truc tion. Sinc e we are adding an immediate value of 5, we us e the s ec ond ADD ins truc tion from above to determine that the
ADD ins truc tion s hould ex ec ute the following RTL ins truc tions : DR< -SR1+ SEXT(im m 5), se t NZP. Let's look at the datapath to
determine whic h s ignals we need to s et to ex ec ute this RTL ins truc tion. The ADD ins truc tion us es the ALU to add the c ontents of
SR1 (R6 in this c as e) to the immediate s tored in the ins truc tion regis ter (IR). The output of the ALU s hould t...
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This document was uploaded on 03/22/2014.
- Fall '14