[Coco] **JUNK** Re: 2022-06-10 Update on my CoCoIOn Ethernet Card Project

[RETRO Innovations]

On 6/11/2022 5:51 PM, gene heskett wrote:
> Are you sure, I've been under the impression that the irq vectors at the
> top of the address space, were not switched out under any conditions
> including all ram.  Have I read the docs wrong since '85?

Here's what the schematics note to me:

Coco 1:

Technical manual, page 65.  SLENB (pin 40) goes into G1 of the 74ls138 
decoder.  If that line goes low, all decoder outputs go to high, which 
includes the ROM select lines (Y1 and Y2)

Coco 2:

Service Manual, page 87/88.  74ls138 (IC11), ROMs are connected to Y1 
and Y2 again, and SLENB is connected to G1

Coco 3:

Service Manual, page 104. SLENB connected to G1 again, though combined 
ROM is connected to Y0.

In all cases, dropping SLENB low completely removes the entire ROM from 
the memory map.  It's a hardware removal and cannot be overridden by the 
SAM/VDG/GIME/etc.

Mind you, this is using the SLENB line.  IN the absence of that line 
being used, normal rules apply.  SAM protects the 16 upmost bytes in ROM 
from being replaced by RAM, as shown on page 4-647 of the SN74ls783 
datasheet, even is "all RAM" mode.

Jim

-- 
RETRO Innovations, Contemporary Gear for Classic Systems
www.go4retro.com
store.go4retro.com