A video sub-
system features reduced
power consumption by integrating a
video memory onto the same
chip as the
video memory controller. The
video memory is preferably a small
DRAM sufficiently large to store all pixel data for lower resolutions, but insufficient for higher resolutions. At higher resolutions, an external
DRAM supplements the internal
DRAM. The amount of external DRAM needed depends upon the resolution to be supported. The internal DRAM has a wide data
bus and thus
high bandwidth, since no external I / O pins are needed. The external DRAM is narrow to minimize pincount and
power consumption. Since the external DRAM is slower and lower in bandwidth, pixel data from both internal and external DRAMs are interleaved together for each horizontal
scan line. Thus the lower bandwidth of the external DRAM is masked by the
high bandwidth of the wide internal DRAM. Either the internal or the external DRAM, or both, are automatically tested with a pseudo-random
number generator that writes pseudo-random numbers to the DRAM while simultaneously supplying pixel data to the
graphics data path for display. A
checksum of the pixel data output from the
graphics data path is generated for the first screen of pixels or frame, while on the second frame the pseudo-random
number generator is disabled and the DRAM supplies the same pixel data that was written to it by the pseudo-random
number generator during the first frame. The checksums for the first and second frames should match if the DRAM is free of faults.