[XCSSA] XCSSA Feb 19th Meeting, 7pm at SAC

[xcssa@xcssa.org]

On Feb 17, 2007, at 9:21 AM, xcssa-admin@xcssa.org wrote:

> o Z80 and 68000 code "transcoding"

I'm not exactly sure what you mean by this.  Though I am porting  
stuff over that used to be on the Z80 (and the 6502 as well), I  
pretty much just re-wrote everything from scratch.  The Z80 code was  
so full of pushes and pops due to its small register set that it  
wasn't worth "transcoding" anything.  About the only thing I kept  
intact was the data structures, though I did adjust them for word  
alignment.

Really, the most important thing was that the Genesis (and Sega  
Master System) uses a video chip design that is derived from the TMS  
9918A (a variant of which is used in the Colecovision).  It  
quadruples the memory, changes all graphics to 4-bit pixels, adds  
more sprites, palette registers, and a simple copy/fill DMA, but it  
still uses the same basic principles.  It even has the same sound  
chip (more or less) as the Coleco, plus an FM synth chip with a PCM  
channel.


Random trivia: one minor annoying thing about the Genesis is that it  
doesn't have a bus error timeout.  Most CPUs, and basically all 8-bit  
CPUs, expect data transfers to happen in the proper clock window  
unless a wait state is asserted.  In contrast, the 68000 has a DTACK  
(data acknowledge) input which is used by the memory or I/O device to  
indicate when a transfer has been completed.  Apparently, if DTACK  
never gets asserted, the 68000 will just sit there forever, waiting  
for a response.  Because of this, there needs to be a circuit to  
detect the condition and assert BERR (bus error).  From what I've  
seen, this needs to be a sort of watchdog timer.

But the Genesis doesn't have anything to generate BERR.  If you read  
an address in a range that isn't assigned to anything, it just locks up.

However, you can ground DTACK and have the 68000 behave just like an  
8-bitter with a faster bus, which is where the old 68000 newsletter  
"DTACK Grounded" got its name.

  - Bruce -