Unformatted text preview: hen pas s through
GateALU onto the s y s tem bus (the thic k blac k wire) to be s ent bac k to the regis ter file (DR) and to the c ondition c odes (NZP). In
order for this operation to ex ec ute c orrec tly , the blue c irc led s ignals mus t have the c orrec t values . The ALU will perform the ADD operation only when the ALUK bits s elec t the c orrec t operation and GateALU s ends the output of the
ALU to the s y s tem bus . The regis ter file mus t als o s elec t the right s ourc e and des tination regis ters by c orrec tly s etting DRMUX and
SR1MUX. The regis ter file mus t als o update the c ontents of the des tination regis ter by c orrec tly s etting LD.REG. It als o mus t s et
the c ondition c odes with the LD.CC s ignal. Finally , we need to mak e s ure that no other gate s ignal is s ending information to the
s y s tem bus , s o GateMARMUX, GatePC, or GateMDR s hould all be 0. You will now add thes e s ignals to y our waveform to find the
bug. W e wall als o us e the table below to identify the c orrec t values for eac h of the s ignals . Clic k on the CPU c omponent in the W ork s pac e window.
Add LDCC, LDREG, GateALU, GateMARMUX, GateMDR, GatePC, DRMUX, SR1MUX, and ALUK to y our waveform.
Res tart the s imulation by c hoos ing Simulate-> Res tart.
Ty pe the c ommands "do sta rt.do" and "run 1000" again.
Bas ed on our analy s is of the ADD operation and the table above, we k now that we want LD.REG to be 1 in order to update the c ontents of the des tination regis ter R0. In y our new waveform, y ou s hould notic e that the s ignal "LDREG" is 0 ins tead of 1. W e will
now s how y ou how to c hange this s ignal in the Control ROM. Editing the Contr ol ROM
The Control ROM is the information s torage devic e that s tores all of the information about what eac h c ontrol s ignal s hould be during
eac h s tate. W e us e a ROM bec aus e of the number of c ontrol s ignals and the c omplex ity of eac h of their Boolean ex pres s ions . The
c ontrol s ignals are ordered ac c ording to the following ta...
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- Fall '14