RAM Guide

How do you write to Read-Only Memory?!?

Read-Only Memory. Sounds pretty definite, you're never writing to it. And, believe it or not, regular ROM is never written to. The information stored in regular ROM is placed there as the memory is being fabricated. So anything that's written into the memory is there for good. If it's wrong, the entire thing has to be done again.

This is where PROM has an advantage. Programmable ROM can be written after fabrication, so if you're not sure what you want on it, you can go ahead and make the physical memory and worry about what's stored on it later. But, like regular ROM, you can only write to PROM once, and after you've finished loading the information onto the memory, it's not coming off.

This disadvantage lead to the development of two different types of PROM. EPROM and EEPROM. Both of these types of memory are PROM's, since the information is written to them after fabrication. EPROM can be erased using ultra-violet rays. This allows the manufacturer of the memory to change any flaws in the information without having to replace the memory. The problem is that the user cannot change this information, unless he was an ultraviolet PROM burner.

The cure for this is EEPROM. Similar to EPROM, EEPROM can be erased, but, as the name implies (see the EEPROM section above), it is much easier to do so. EEPROM can be erased by electricity, making it the preferable memory for BIOS's. If you have a flash BIOS, your BIOS is using flash EEPROM, an even more advanced form of PROM.

So, as you can see, not all ROM is truly read-only. You could probably change your BIOS right now since I'll bet it has EEPROM inside. :)

Parity, Non-Parity, ECC - How does an extra chip correct errors with my memory?

In the glossary, I briefly described Parity and Non-Parity and ECC Memory. But I didn't really explain them in depth and tell you quite how they worked. I hope that this will help out.

Remember that Non-Parity RAM does no error checking for you. If some how your data was corrupted, Non-Parity will still feed it into your CPU as if nothing had happened. Parity RAM does. As mentioned in my definition of Parity RAM, there is an extra chip on a Parity module, known as the Parity Bit. This is because each byte coming from Parity RAM has 9, not 8 bits.

Naturally, if your computer doesn't support Parity RAM, your computer won't know what to do with that extra bit. Don't worry about your CPU though, this other bit of information, a 1 or 0 is removed before the data is sent to your processor.

Parity RAM works by comparing a byte before and after going to DRAM. As the byte is entering memory, the Parity Bit is changed to either a one or a zero to make the total number of ones in the 9-bit byte even or odd depending on what type of Parity memory it is.

Some types of Parity OK a byte if it has an even number of 1's, others if it has an odd number. Therefore, if you have even Parity, and you have a byte 10011011 your Parity Bit will be 1, making the number of 1's 6. When it is time for that address to go to the CPU, if the data was corrupted, making the bit 10010011 instead, the Parity Bit would make the number of 1's equal to 5, therefore indicating a memory error.

The biggest disadvantage to Parity RAM is that there is no way of knowing where the error occurred. In the example above, your computer just knows that the bit isn't the same as when it came in. So what can you do? Nothing. The information is gone, there isn't a record of the byte stored somewhere. That's why more important jobs are left to ECC.

ECC stores more information about your byte of memory. The module keeps information about the bits, so that it can identify errors in memory to an accuracy of 1 bit. These errors can even be correct, thus the name Error Correcting Code. This makes ECC a much more advanced type of error checking than Parity RAM, but is also much more expensive.