You could bridge the ULA and Z80. But hobbling a reasonably powerful ARM SoC by hanging it off a far less capable system seems counterintuitive. It would be more effective to emulate the system on the Raspberry Pi?
http://www.e-basteln.de/computing/65f02/65f02/
100mhz 6502!
ula+ has timex hi rez 512x192 but no attributes yes? could ink n paper colours be altered for a word processor?
sam has line interrupt register but only 128 lines per frame ula+ doesnt have line interrupt?
what are the video modes supported by ula+ please? do u understand the floating bus? does this mean zx spectrum cant access 64k of ports?
sam haz hi res mode 3 512x192 but only 2bits per pixel so 4 colours but mode 4 with 4bits per pixel uses a colour lookup table with 16 7bit valus ie 128 colour palette but also the hmpr port has 2 bits so that in mode3 you can switch to 4 x 4 sets of colours this can be done along a scan line according to an article in world of sam every 19 pixels but a dma is 8 times faster ram to port write 2tstates wed nearly get 16 colours per scan line at 512x192