Assignment3 : 3 ways to design ROM

Jun Wang (junwan73@hotmail.com) Sat, 04 Mar 2000 21:34:24 PST 

From: "Jun Wang" <junwan73@hotmail.com>
Subject: Assignment3 : 3 ways to design ROM 
Date: Sat, 04 Mar 2000 21:34:24 PST



I got 3 version of ROM  design
1. using conditional statement
               data <=    instruction   when address =  xx
2. using with and select statement which is similar to the first design.
3. using array element in MaxplusII  for example
         type byte_array  is array of (0 to 31)  ;
         signal      ROM : byte_array;
         ROM(i) <= instruction .....
         data <= ROM(address);
It seems to me the first 2 designs has more glitches in output than the 3rd 
design and I guess that's because the structure created by first 2 design is 
complicated.
Unlike using the array which is specificly for the ROM or RAM is simple and 
efficient in structure. However, there's a problem in using ROM. As we only 
use 32-byte ROM but the address is 8 bit long which would allocate a 
256-byte ROM. Of course, we can ignor MS 3 bit in input address and can 
access the ROM contents by index, but u will recieve a warning of the input 
pins are ignored etc. u can use these 3-bit to do some dummy job to avoid 
the warning but that seems unreasonable to me and also I find the3rd design 
takes long time to comple. So I wonder what is the best design that fits our 
needs.



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