SUMMARY
1. The HOLD input is used to request a DMA action, and the HLDA output signals that the hold is in effect. When a logic 1 is placed on the HOLD input, the microprocessor (1) stops executing the program; (2) places its address, data, and control bus at their high-impedance state; and (3) signals that the hold is in effect by placing a logic 1 on the HLDA pin.
2. A DMA read operation transfers data from a memory location to an external I/O device. A DMA write operation transfers data from an I/O device into the memory. Also available is a memory-to-memory transfer that allows data to be transferred between two memory locations by using DMA techniques.
3. The 8237 direct memory access (DMA) controller is a four-channel device that can be expanded to include an additional channel of DMA.
4. Disk memory comes in the form of floppy disk storage that is found as 31⁄2" micro-floppy
disks. Disks are found as double-sided, double-density (DSDD), or as high-density (HD) storage devices. The DSDD 31⁄2" disk stores 720K bytes of data and the HD 31⁄2" disk stores 1.44M bytes of data.
5. Floppy disk memory data are stored using NRZ (non-return to zero) recording. This method saturates the disk with one polarity of magnetic energy for a logic 1 and the opposite polarity for a logic 0. In either case, the magnetic field never returns to 0. This technique eliminates the need for a separate erase head.
6. Data are recorded on disks by using either modified frequency modulation (MFM) or run- length limited (RLL) encoding schemes. The MFM scheme records a data pulse for a logic 1, no data or clock for the first logic 0 of a string of zeros, and a clock pulse for the second and subsequent logic 0 in a string of zeros. The RLL scheme encodes data so that 50% more information can be packed onto the same disk area. Most modern disk memory systems use the RLL encoding scheme.
7. Video monitors are either TTL or analog. The TTL monitor uses two discrete voltage levels of 0 V and 5.0 V. The analog monitor uses an infinite number of voltage levels between 0.0 V and 0.7 V. The analog monitor can display an infinite number of video levels, while the TTL monitor is limited to two video levels.
8. The color TTL monitor displays 16 different colors. This is accomplished through three video signals (red, green, and blue) and an intensity input. The analog color monitor can display an infinite number of colors through its three video inputs. In practice, the most common form of color analog display system (VGA) can display 16 M different colors.
9. The video standards found today include VGA (640 × 480), SVGA (800 × 600), and EVGA or XVGA (1024 × 768). In all three cases, the video information can be 16M colors.