Re: pcb quiery
In a practical sense, the more 'ground' the better; otherwise, wait until EMI testing and find out otherwise!
I drew up and produced a board (needed to show a proof-of-concept at that stage) using a 'simple' Z80A uP (early 1990's time frame) and the 'ground' was really composed only of a .1" wide buss between and among the 5 or 6 other chips (incl PIO, CTC, EPROM, SRAM etc), and the design was noisy RF emissions-wise.
The succeeding layout (2nd version) employed literal 'ground plane' to provide a termination for 'E field' emanating from the digital address and data lines as well provide a low impedance path for digital signals rather than only the .1" bus between ICs ... that design emanated much less RFI up to and past 30 MHz than the first layout ...
Jim