| 摘要 |
<p>A method and apparatus is provided for accessing a large number of control information locations with a series of memory digital values of relatively short n-bit length. In an embodiment applied to color raster graphics, 4-bit pixel values are serially output from the frame buffer to a ''color extender''. The color extender shifts the pixel values through an 8-bit working register and outputs the contents of the working register to a color palette having a look-up table containing more than 24 (e.g. 28 or 256) slots storing RGB triple values. In this embodiment, the working register contains a high pad 4-bit register and a low pad 4-bit register. The high order portion value selects a group of 16 slots in the look-up table, while the low order portion value selects a particular slot from the selected group. To change to a different color group a selection sequence of one or more special pixel values is passed to the color extender, placing the extender in a special state. For example, the selection sequence may comprise two successive pixel values. The first value, a flag value, places the extender in the special state. The pixel value succeeding the flag value is transferred to the high order portion to identify a group of slots while the low order portion remains unchanged. The working register then returns to normal operation with succeeding pixel values transferring to the low order portion. Thus, the present invention allows for selection of up to 2m color control information locations or devices using n-bit pixel values, where m > n. The method and apparatus is particularly useful in graphic window systems where the color look-up table is a shared resource.</p> |
