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From: uhclem@trsvax
Newsgroups: net.micro.trs-80
Subject: Re: Again: What makes the Model 4 work?
Message-ID: <70700051@trsvax>
Date: Fri, 15-Nov-85 11:37:00 EST
Article-I.D.: trsvax.70700051
Posted: Fri Nov 15 11:37:00 1985
Date-Received: Mon, 18-Nov-85 06:30:20 EST
References: <408@houem.UUCP>
Lines: 53
Nf-ID: #R:houem.UUCP:408:trsvax:70700051:000:2414
Nf-From: trsvax!uhclem    Nov 15 10:37:00 1985


<I'm dreaming of a fixed notes handler, just like the ones I used to know....>

How to set your Model 4 screen to 80x24 or 40x24 while staying in Model III
mode.

This is pretty easy.  There is a port called OPREG$ (0x84) which
controls the memory mapping and the CRTC-program select.  It looks liks this:

B7       B6       B5       B4       B3       B2       B1       B0
Page	Fix       Memory   Memory   Invert   80/64    Select   Select
	Upper (0) Bit 1    Bit 0    Video             Bit 1    Bit 0
	Memory    		    (Alt Set
				     Select)

Bit 2 controls if the CRTC is programmed for 80x24/40x24 or 64x16/32x16.
The half-width adjustment is in port 0xEC (MODOUT), bit 2 (MODSEL).

Bits 1 & 0 of OPREG$ decide what memory map to use.  Since you want to
stay in Model III mode, these bits should remain 0.  This keeps the ROMs
selected, and places the keyboard at 0x3800 and the video at 0x3C00.

The Page bit (Bit 7) selects which 1K of the 2K video RAM is addressable
at 0x3C00.  It does not affect which page is shown on the screen in any
mode.  So what you get to do is write code to break your screen requests
up and turn that bit on if you want the upper 1K and off if you want the
lower 1K.

OPREG$ is unknown to the Model III ROM and operating systems, so you
will need to keep a copy of its contents in RAM somewhere as it is a
write-only port.
MODOUT is known, and Model III systems maintain a copy of it at 0x4210.
So if you modify this port, be sure to read the copy at that address,
and/or bits in it, store it back, then write to the port.

(By the way, Bits 6-4 control banking when you have 128K.)

All of the above information can be found in the Model 4/4P Technical Reference
Manual 26-2110, and some of it was in Model III/4 Cassette BASIC Reference
Manual or something like that.  (Should have come with your 16K machine.)

<This information is provided by an individual and is not nor should be
 construed  as  being  provided  by  Radio  Shack or Tandy Corp.  Radio
 Shack/Tandy Corp has no obligation to support the information provided
 in  any way.  I mean, they didn't even show the Model 4D computer at the
 Annual stockholders meeting!  And it was only 16% of Tandy computer sales
 (According to the annual report, page 16)!  Gee Whiz!>
						
						"Thank you, Uh Clem."
						Frank Durda IV
						@ <trsvax!uhclem>

"If you took the bugs out it wouldn't be crashy, would it?!?"