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Random Access A random access memory (RAM) chip stores binary data in arrays The data can be addressed (selected) from anywhere in the matrix Data is easily changed and stored back in RAM, in whole or in part A RAM chip is sometimes called a read/write memory An example of the use of RAM is a word-processing computer file that you are actively working on This paragraph, this chapter, and in fact the whole text of this book was written in semiconductor RAM in small sections before being incrementally stored on the computer hard drive, and ultimately on external media There are two major categories of RAM: dynamic RAM (DRAM) and static RAM (SRAM) A DRAM chip contains transistors and capacitors, and data is stored as charges on the capacitors The charge must be replenished frequently, or it will be lost through discharge Replenishing is done automatically several hundred times per second An SRAM chip uses a flip-flop to store the data This gets rid of the need for constant replenishing of charge, but the tradeoff is that SRAM ICs require more elements than DRAM chips to store a given amount of data With any RAM chip, the data in it will vanish when power is removed, unless some provision is made for memory backup The most common means of memory backup is the use of a small cell or battery with a long shelf life Modern IC memories need so little current to store their data that a backup battery lasts as long in the circuit as it would on the shelf Memory that disappears when power is removed is called volatile memory If memory is retained when power is removed, it is nonvolatile Read Only By contrast to RAM chips, the data in a read-only memory (ROM) chip can be easily accessed, in whole or in part, but not easily written over A standard ROM chip is programmed at the factory This permanent programming is known as firmware But there are also ROM chips that you can program and reprogram yourself The data contents of ROM chips are generally nonvolatile That means a power failure is no cause for concern An erasable programmable ROM (EPROM) chip is an IC whose memory is of the read-only type, but that can be reprogrammed by a certain procedure It is more difficult to rewrite data in an EPROM than in a RAM; the usual process for erasure involves exposure to ultraviolet (UV) An EPROM IC can be recognized by the presence of a transparent window with a removable cover, through which the UV is focused to erase the data The IC must be taken from the circuit in which it is used, exposed to UV for several minutes, and then reprogrammed The data in some EPROM chips can be erased by electrical means Such an IC is called an electrically erasable programmable read-only memory (EEPROM) chip These do not have to be removed from the circuit for reprogramming.

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s you ve already seen in the previous chapters, the primary reason to use Ajax is that it enables web pages to be updated without requiring a page refresh, which makes the page update rather seamless to the viewer An important consideration when using this technique is how you present the data downloaded from the server So far, you ve been presenting the downloaded data simply by overwriting a <div> element As this chapter explains, you have many more options available in how you present that new data, enabling you to draw the reader s attention to the changes Because the way you present your downloaded data is such a big part of Ajax applications, that s the subject of this chapter For example, a frequent question among professional Ajax developers is, will the user notice the download when it appears in the page In pages that present tables of data, where you re updating only one or two items, that can be an issue As you ll see, one way of handling this issue is to display downloaded text with a flash of color You ll also see that you don t have to display downloaded text in simple <div> elements You can display such data anywhere in a page, as you re going to see while creating a pop-up menu system with text that s downloaded interactively with Ajax, allowing the menu items to be updated in real time In this chapter, updating the page with Ajax-downloaded data is going to take place using Cascading Style Sheets, CSS Familiarity with CSS is essential for the Ajax developer, and you re going to get a good introduction to the topic in this chapter CSS is one of the most important Ajax topics because updating web pages is so important in Ajax CSS is a formal specification for arranging and styling items in web pages You can set data s font, color, appearance, visibility, and placement using CSS You re going to get a good CSS foundation in this chapter, but for full, complete details on CSS, see the CSS specification at wwww3org/TR/CSS21/.

Refer to the text in this chapter if necessary A good score is at least 18 correct Answers are in the back of the book

One of the main issues of Ajax applications is also its main advantage there s no page refresh when you download data That s great, but it can also mean the user misses data updates, especially in pages with lots of content

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