DOOM

In article <50F33642.5070405 at gmail.com>, Josh Dersch <derschjo at gmail.com> writes: he standard RS232 port. See <http://bitsavers.trailing-edge.com/pdf/aed/AED_1024/990024-01B_Model_1024_Color_Graphics_Terminal_User's_Manual.pdf> However, DMA != memory-mapped.

Memory-mapped means the pixel changes as soon as I diddle the bits and the next refresh cycle comes around. Memory-mapped means the CPU can determine the contents of a pixel by reading a memory location. DMA means a chunk of host memory can be transmitted to the device while the CPU is doing some other task. The AED 512/617/1024 can interface to a host machine by means of a parallel interface with DMA capability. While this is certainly faster than using a serial interface and an ASCII command stream, it is by no means as fast as a memory-mapped display. There is no abaility for the AED to send data back to the host over the DMA interface, it is unidirectional. The above PDF says the bandwidth is 3 MB/s across the DMA interface. At 800x600 256 color VGA mode, refreshed at 30 Hz, DOOM uses: 800*600*30/1024/1024 = 13 MB/sec I believe DOOM actually goes at 60 Hz, not 30, so it's even worse. However, 30 Hz is generally considered the minimum for interactive real-time graphics.

Assuming using the maximum 3 MB/sec bandwidth of the AED and 4 bits/pixel instead of 8, gives you enough bandwidth to do 512x384 (4:3 aspect ratio). The AED actually stores 8bpp, so I'd have to dig further into the docs to see if you could transmit 4bpp and have it locally expand that to 8; you probably can with some downloaded microcode (i.e. 6502 assembly language). That's a calculation based purely on the DMA speed listed in the document, you'd have to do an actual end-to-end analysis to see how fast you could update the AEd from a Qbus PDP-11; the DMA transfer rate may not be the limiting factor.