Understanding ROM Implementation: A Troubleshooting Guide

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In summary, the trouble the person is having is that they don't understand what a ROM is, what it does, or how to use it. They are able to build a KARNO table and a circuit out of it, but they are having trouble understanding the example presented.
  • #1
transgalactic
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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
 
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  • #2
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.
 
  • #3
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
 
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  • #4
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.
 
  • #5

Related to Understanding ROM Implementation: A Troubleshooting Guide

1. What is ROM?

ROM stands for Read-Only Memory. It is a type of computer memory that stores data permanently and cannot be changed or erased by the user.

2. How is ROM different from RAM?

RAM (Random Access Memory) is a type of computer memory that stores data temporarily while the computer is running. Unlike ROM, the data in RAM can be changed or erased by the user.

3. What are the common examples of ROM?

Some common examples of ROM include BIOS (Basic Input/Output System) in computers, firmware in smartphones and tablets, and game cartridges in video game consoles.

4. How does data get stored in ROM?

Data is stored in ROM through a process called "burning". This involves using a special device to permanently write data onto the ROM chip.

5. Can data in ROM be updated or changed?

No, data in ROM cannot be updated or changed by the user. It can only be changed by the manufacturer through the process of burning.

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