X Window server for NEC 7220

--- On Tue, 8/14/12, Andrew Lynch <LYNCHAJ at yahoo.com> wrote: Hi I did a quick look at the IBM PGC and it is enormous. I doubt very much it is a viable option unless there were later models that were a lot more compact. The original IBM PGC spans *three* ISA boards! Even using modern DRAMs there is not practical way to fit it on to an S-100 board. While we are "thinking outside the box" though it did occur to me that the S-100 uPD7200 board could adopt a similar shared memory architecture. Modify the board to be a dumb frame buffer in addition to graphics processor. The VRAM could be dual access by the 7220 and the put it on the bus for the CPU. Make sure either the 7220 has a flag for blanking interval or use a latch to capture that signal on the bus. The VRAM could appear on the S-100 bus as a 256KB SRAM board. It would use four 74LS157's to toggle back and forth between local 7220 and bus CPU access. If the CPU only writes to the VRAM during the blanking interval there would not be any on-screen artifacts. Unfortunately the current S-100 uPD7220 design completely fills the board so the VRAM and related circuitry would have to be exported to a mezzanine board. That would effectively double the builder cost of the board so maybe that's not practical. However, a dumb frame buffer mode on the 7220 board would improve the raster ops necessary for decent X window or other GUI performance. It is something to think about for sure. Thanks and have a nice day! Andrew Lynch