[Coco] How does the SCS line / GIME Setting Work?

[sales at gimechip.com]

In the CoCo 3, if you set the GIME to Internal SCS*, the Y4 output of the 
74LS138 goes low on $FF4x range and Y6 goes low on the $FF5x range. I agree 
that this was probably done with the intention of possibly having an 
internal FDC chip. Passing the $FF5x out to the program pak slot would still 
allow DISTO's M.E.B. products to function, but I really don't know the 
reason it was done this way.
----- Original Message ----- 
From: "Darren A" <mechacoco at gmail.com>
To: "CoCoList for Color Computer Enthusiasts" <coco at maltedmedia.com>
Sent: Saturday, June 19, 2010 8:06 AM
Subject: Re: [Coco] How does the SCS line / GIME Setting Work?



> On 6/19/10, Brett Gordon wrote:

>> I hope this isn't too silly of a question to ask.  I'm foggy on the 

>> workings

>> of the SCS line and how the SCS setting on the GIME chip effects it.

>>

>> >From what I gathered the SCS line is asserted when the address bus is 

>> >from

>> 0xFF40 and 0xFF5F.   I assuming it's a specialized address line used to

>> "help" the FDC out. I'm assuming a FDC pack could do this itself if it

>> wanted too.  I'm figuring Tandy wanted to make their FDC a bit cheaper, 

>> so

>> added this line to the CoCo's Cart port.  Is there any other *regular* 

>> uses

>> for this line?

>>

>> And exactly what does the GIME's SCS enable do in 0xFF90 ?  I seem to

>> remember playing with this setting once, and my FCD software stopped

>> working.  So if your GIME's SCS's bit in 0XFF90 is off, the SCS will 

>> never

>> be asserted?

>>

>

> --

>

> You are correct.  The SCS line is an Address Decode signal that

> 'normally' indicates an address access from FF40 to FF5F.  This

> originated from a feature of the SAM chip which outputs a unique

> address code for the ranges FF00-FF1F, FF20-FF3F and FF40-FF5F (among

> others).  The first two were used internally for selection of the

> PIAs. The third was considred a spare and thus given the name "Spare

> Chip Select".

>

> The GIME's SCS bit in FF90 will restrict the SCS range to FF50-FF5F

> when off.  Softawre which communicates with the FDC does so through

> addresses in the FF40-FF4F range, so turning off this bit will

> effectively disable the disk controller.

>

> Darren

>

> --

> Coco mailing list

> Coco at maltedmedia.com

> http://five.pairlist.net/mailman/listinfo/coco



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