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Memory Devices
18 questions By Tony R. Kuphaldt
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Question 13 of 18
Suppose you need a memory array with 4k × 4 organization, but all you have on hand are 1k × 4 memory chips. Explain how you could build a memory array of this size using multiple 1k × 4 chips.
Reveal answerUse four memory chips: parallel the ten address lines from each 1k × 4 memory chip, then connect the [CS] line of each chip to the output of a 2-line to 4-line decoder. The two decoder input lines will then become address lines A10 and A11 of the 4k × 4 memory array.
Notes:Although no schematic is given here in the answer, I expect my students to be able to draw one on their own.
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Question 14 of 18
Dynamic RAM chips often contain more addresses than they have address lines to select them with. For example, the MCM516100 DRAM chip has an organization of 16M × 1, yet it only has twelve address lines.
Explain how it is possible to select one out of 16 million unique addresses while using only twelve address lines. Hint: the technique is known as address multiplexing. Be sure to refer to one or more dynamic RAM datasheets when doing your research!
Reveal answerWith address multiplexing, the address lines going in to the memory chip are used twice to select any arbitrary address, bringing in 12 bits worth of the 24-bit address at a time.
Follow-up question: explain how the memory chip “knows” which 12 bits of the address are being read at any given time.
Notes:Explain to your students that address multiplexing is not technically limited to application in dynamic RAM chips only, but that it is usually applied there because of the high address density afforded by dynamic RAM technology. Most static RAMS, by contrast, are not dense enough to require address lines serve double-duty!
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Question 15 of 18
After a ROM memory has been programmed with data, it is good to verify that the data now stored is okay, and not corrupted with any errors. A popular method of doing this is to calculate a checksum on the stored data, and compare that against the checksum for the original data. If the checksum numbers are identical, chances are there are no corruptions in the stored data.
Explain exactly what checksum is, and how it works as an error-detection strategy.
Reveal answerOne way to think about checksum is to recall the error-detection strategy of parity bits. At root, the two processes are very similar. As for the details of what checksum is and how it is calculated, I leave that for you to research!
Notes:Once again, there is little I can reveal in the answer without giving everything away. There are enough resources available for students to learn about checksum on their own, that you should not have to supply additional information.
