Abstract: In order to more effectively use read only memory space of a personal computer system, ROM code is selectively located or mapped to either an address boundary which is an even or odd integer multiple of one half the capacity of the read only memory device in which the ROM code is stored. The ROM code is stored in the read only memory device in two fields. In a first field, the ROM code is broken up into two segments, and the first segment precedes the second segment. In the second field, the same two segments are stored, but the second segment is stored preceding the first segment. A register, for storing page select bits, provides an input to an address decoder and an input to an adder, which adds unity to the contents of the register and provides its outputs to the address decoder as well. Accordingly, the address decoder will respond to either the page which is identified by the page select bits or the page following the identified page.

Abstract: An all points addressable raster scan graphics display system is operable in two modes. In the first mode, data is extracted from a refresh store, serialized, modified, and applied to a display device at a first frequency. In the second mode, data is extracted from the refresh store, serialized and partially modified at said first frequency, but it is then further modified and passed to the display device at an even sub harmonic, for example half, of the first frequency. The further modification includes concatenation of successive groups of display data bits. Accordingly, with the raster scan device operating at a constant scan velocity, the first mode provides a high picture element definition but relatively low color definition display, and the second mode provides a display with an even submultiple, for example half, the picture element definition but considerably greater color definition.