[Coco] Question re: Cart slot decode

[jdaggett at gate.net]

On 2 Feb 2009 at 18:50, Darren A wrote:

> No.  The standard SCS is the 32-byte range of FF40 - FF5F.  The CoCo
> 3's non-standard SCS removes the bottom 16 bytes from that range,
> restricting it to FF50 - FF5F.
> 
> Darren

Okay 

I went back and reread the 1986 Kevin darling text file and yes the SCS is from $FF40 to 
$FF5F when the MC2 bit is "1". When the MC2 bit is "0" then $FF40 to $FF4F is internal. 

Thus a "0" on MC2 restricts the SCS to $FF50 to $FF5F to the external cartridge. 

The MC2 bit controls what is outputed to the SBUS. With MC2 cleared the output from the 
SBus is "100" during addresses $FF40 to $FF4F and that asserts pin 4 of the LS138. When 
MC2 is set then the output of the SBUSS is "110" and pin 6 of the LS138 is asserted. 

james