Re: 6502 illegal opcodes questions

[Bruce Tomlin <bruce#fanboy.net@127.0.0.1>]

In article <b_2dnVaBwopOtvjZRVn-uQ@comcast.com>,
 "Michael J. Mahon" <mjmahon@aol.com> wrote:

> That was DMA for video RAM acces, right?  If so, they just didn't get
> how trivial it is to share RAM with a 6502--it only accesses RAM during
> half the clock signal!

As I understand it, they actually halted the CPU for DMA so as to get 
more memory bandwidth than "half the clock signal", and they needed to 
know when the CPU had actually stopped so that they could start DMA.  
This was also done on the Atari 7800.

I've heard that in the original 400/800 they actually used a regluar 
6502, but the extra signal allowed them to save all the external 
circuitry that figured this out.

Again, I grew up on the TRS-80, so all of this is stuff that I've 
learned about the old Atari hardware in the past five years or so.