Understanding ROM Implementation: A Troubleshooting Guide

[transgalactic]

http://img211.imageshack.us/my.php?image=img86301qt1.jpg

i have a trouble in understanding ROM
i know that it has addresses and words and that its used in computers in order
to store info.

but when i get to the example i can't understand the functionality of it.

in the example that i present here we have a truth table and then
the Rom implementation
in the previos question i knew what the Full Adder does

i knew what its for
so i knew how to use itbut here i can't understand what its doing??

again i don't have any problem to build a KARNO table
and to build a big circuit out of it

but i need to implement ROM here

 

[rbj]

Read Only Memory is non-volatile. the data does not go away when you turn the computer off. and being read-only, if some program running in the computer accidently starts executing random (and bad) instructions and tries to write to the ROM, the data in the ROM will not change. so that data in the ROM is safe. it's always there and can be counted on to be there (unless you smoke the chip or a static discharge or similar).

the main function of ROM is for the boot program (what the CPU executes when the reset line is toggled) and enough drivers in it to be able to grab the next bunch of code to be executed. some computers will have other drivers or utilities (that know how to talk to other hardware devices) that applications might use, but often those are saved on the disk (or other writable and non-volatile memory) so they can be updated with new versions with bugs fixed or additional functionality.

ROMs can also be thought of as a fixed logic gate array. for each data line out, it can be defined as any static logical combination of the address lines going in. it's the same thing (but i think a ROM is slower than a PLA or PAL or whatever they call it) as a big mess of logic with no states (flip-flops) inside.

 

[transgalactic]

so how can i solve this question??

i understand the role ROM plays in our PC's
but that doesn't help me much in solving this example

 

[rbj]

listen, all ROMs are, are a piece of arbitrary combinatoric logic (without states inside).

name your data (out) line, let's say D5. so D5 is some boolean function of A0, A1, A2, A3... (all of the address lines). for each permutation of address bits, from 00000000 to 11111111, the data bits are all some function that turns on the data bit (or turns it off) depending on which address word was applied.

 

[transgalactic]

i noticed that they just put a big box on a certain part of karno table
and call it ROM
the leave out two lines because they can predict there out put
http://img410.imageshack.us/my.php?image=img86301qt1ka1.jpg

i could make a ROM out of the whole thing

am i correct?