Code_ The Hidden Language of Computer Hardware and Software - Charles Petzold [124]
The 2102 chip is known as static random access memory, or SRAM (pronounced ess ram), to differentiate it from dynamic random access memory, or DRAM (pronounced dee ram). SRAM generally requires 4 transistors per bit of memory (not quite as many transistors as the flip-flops I used for memory in Chapter 16). DRAM, however, requires only 1 transistor per bit. The drawback of DRAM is that it requires more complex support circuitry.
An SRAM chip such as the 2102 will retain its contents as long as the chip has power. If the power goes off, the chip loses its contents. The DRAM is also similar in that respect, but a DRAM chip requires also that the contents of the memory be periodically accessed, even if the contents aren't needed. This is called a refresh cycle, and it must occur several hundred times per second. It's like periodically nudging someone so that the person doesn't fall asleep.
Despite the hassle of using DRAM, the ever-increasing capacity of DRAM chips over the years has made DRAM the standard. In 1975, Intel introduced a DRAM chip that stored 16,384 bits. In accordance with Moore's Law, DRAM chips have quadrupled in capacity roughly every three years. Today's computers usually have sockets for memory right on the system board. The sockets take small boards called single inline memory modules (SIMMs) or dual inline memory modules (DIMMs) that contain several DRAM chips. Today you can buy a DIMM containing 128 megabytes of memory for under $300.
Now that you know how to make memory boards, you don't want to fill up the entire memory space of your microprocessor with memory. You want to leave some memory space for your output device.
The cathode-ray tube (CRT)—a familiar sight in homes for the last half century in its guise as the television set—has become the most common output device for computers. A CRT attached to a computer is usually known as the video display, or monitor. The electronic components that provide the signal to the video display are usually known as the video display adapter. Often the video display adapter occupies its own board in the computer, which is known as the video board.
While the two-dimensional image of a video display or a television might seem complex, the image is actually composed of a single continuous beam of light that sweeps across the screen very rapidly. The beam begins in the upper left corner and moves across the screen to the right, whereupon it zips back to the left to begin the second line. Each horizontal line is known as a scan line. The movement back to the beginning of each of these lines is known as the horizontal retrace. When the beam finishes the bottom line, it zips from the lower right corner of the screen to the upper left corner (the vertical retrace) and the process begins again. For American television signals, this happens 60 times a second, which is known as the field rate. It's fast enough so that the image doesn't appear to be flickering.
Television is complicated somewhat by the use of an interlaced display. Two fields are required to make up a single frame, which is a complete still video image. Each field contributes half the scan lines of the entire frame—the first field has the even scan lines, and the second field has the odd scan lines. The horizontal scan rate, which is the rate at which each horizontal scan line is drawn, is 15,750 Hertz. If you divide that number by 60 Hertz, you get 262.5 lines. That's the number of scan lines in one field. An entire frame is double that, or 525 scan lines.
Regardless of the mechanics of interlaced displays, the continuous beam of light that makes up the video image is controlled by a single continuous signal. Although the audio and video components of a television program are combined when they're broadcast or transmitted through a cable television system,