Using a DS'4x0 you can generate bit-mapped 256 x 256 NTSC pretty nicely with only external SRAM (page mode) used as the frame buffer.

TVT

Using Craig Steiner's suggestion to index into an array of NOPs sized to compensate for maximum interrupt latency, the display is quite stable.

Each component of Sync Generation occurs in an Timer Interrupt routine (blanking/front porch/sync/back porch). The interface bits are flipped and the routine exits after re-loading a sequence specific value into the Timer.

Each pixel line is blast of byte fetch, rotate, and emit operations padded with NOPs for equal time per pixel. Pity the poor interrupt that's waits for this to complete.

Using this technique the foreground has no more display obligation but to decide where in the frame buffer to write a pixel.

I don't use interlacing as @ 30Hz single horizonal pixel lines "wiggle" observably and doesn't benefit the little TV used.

The implementation shown accept 19.2bps serial keyboard input data (PC Terminal Program) and display's std. VGA 8 bit patterns for text char's and posts graphics for "magic key's".

I wrote a test characterizing the # of cycles available for the foreground.

I wait for a flag that toggles every time a frame occurs (60Hz).
The foreground uses a 24 bit adder inc'ing by 1 in loop until the flag toggles again. Turns out the DS'420 is utilized @ 37% which beats my old Timex Sinclair generating in Text Mode 32x16 chars.

Now the circuit executes @ 44MHz which isn't factory approved but does underscore what could be done with an even faster ucontroller.

As for VGA, the frame buffer is what scares me 640 x 480 is 38,400 bytes for monochrome.

The VGA Horz Sync is aprox. twice the NTSC rate but the pixel rate can be accomodated by an outboard shift register clocked at the pixel rate. The Vertical moves from 60 to 72Hz not all that scary.

What may not be intuitive is that the number of pixels could be held to values such as 256 within the faster sync frame. There goes the square pixel and not typical VGA pixel depth that, but it would easily move my current display technique to say a 15" LCD monitor with acceptable result for sign posting, machine control display and of course, 8052 RetroPC.

regards,
p