Re: Assign#2: PC Datapath

Ed Casas (edc@ece.ubc.ca) Thu, 2 Nov 2000 20:37:33 -0800 

Date: Thu, 2 Nov 2000 20:37:33 -0800
From: Ed Casas <edc@ece.ubc.ca>
Subject: Re: Assign#2: PC Datapath



> In the PC Datapath section of the CPU description, the
> specification states that the program counter, as a multiplexed
> option, can be loaded with its current PC register value.
> However, there is no (opcode) instruction requiring the PC
> register to directly load its own value.  Consequently, why is
> this option necessary?

> If I understand it correctly, all opcodes with exception of the
> 2 jump commands will simply load the PC with the next
> instruction (ie: PC+1) on the next clock cycle.

You're right, in this design the value of the PC is never held
between clock cycles, so this feature (next_PC <= PC) is never
used.  You have to include it in your design nevertheless.

I included this feature for future expansion.  Some processors
have separate fetch and execute cycle(s) and/or some instructions
may required multiple cycles to execute (e.g.  multiply and
divide instructions) and the PC has to be held during these
cycles.  I left out these features to simplify the design.

-- 
Ed Casas   edc@ece.ubc.ca   http://casas.ece.ubc.ca   +1 604 822-2592