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Hugo Holden
April 16th, 2016, 09:45 PM
There are some software programs such as Draw Partner (part of Harvard Graphics) and the DIAG program that comes with the Video-7 EGA card, that are able to flip the EGA card back and forth between the low horizontal scan rate (15.750kHz) and the EGA 21.85 kHz scan rate. The video-7 or the IBM EGA card starts up in one mode or the other depending on where its board switches are set at boot up. If say it is set to the low scan rate mode initially, then a command in BASIC for screen 9 is an illegal function call and won't auto toggle it into that screen, but it is ok if it started/booted in the high scan rate mode initially.
Is there any command in DOS that could be used to toggle between the two scan rate options, rather than having to hard set it on the EGA card before the boot up ?

modem7
April 16th, 2016, 10:50 PM
If say it is set to the low scan rate mode initially, ...
Note that on the IBM EGA card, the card's switches are not 'power up scan rate' switches.
The switches inform the card's BIOS as to the monitor type attached to the card.

Set for EGA monitor: Hor. sync is 15.7 kHz or 21.85 kHz depending on the [video mode (http://www.minuszerodegrees.net/video/bios_video_modes.htm)] selected.
Set for CGA monitor: Hor. sync locked to 15.7 kHz (because 21.85 kHz is invalid for a CGA monitor)

Hugo Holden
April 17th, 2016, 12:22 AM
Thanks.
How do I select EGA mode 1 and EGA mode 2 from the DOS prompt ?

Hugo Holden
April 17th, 2016, 03:54 PM
or perhaps the question put better; how can I instruct the bios that the monitor type has changed, could it be done via an instruction in Debug ? or writing to an address using a BASIC OUT command ?
Perhaps I should try using the Checkit software to switch between the modes....do you know if it is set to a mode, it stays in that mode after the checkit program closes ? (I'm doing some experiments with a dual standard monitor and I need a quick way of switching between the modes)

modem7
April 18th, 2016, 12:06 AM
or perhaps the question put better; how can I instruct the bios that the monitor type has changed, ...
I do not know, but I do not think that is the correct route.


Perhaps I should try using the Checkit software to switch between the modes....do you know if it is set to a mode, it stays in that mode after the checkit program closes ?
I have not seen that behaviour.


(I'm doing some experiments with a dual standard monitor and I need a quick way of switching between the modes)
I have just found something pre-built that may meet your need. Attached at [here (http://www.minuszerodegrees.net/software/DOSFISH.ZIP)] is a ZIP of a small DOS application named DOSFISH. Extract the contents into a directory.

1. At the DOS prompt, the system is using mode 2 (21.85 kHz).
2. When you then run FISH.COM, a graphic will appear, using mode 1 (15.7 kHz).
3. Press ESC to return to mode 2.

Hugo Holden
April 18th, 2016, 12:51 AM
Thanks, I'll try it.
I also looked at IBM's document for the EGA card, and I couldn't see any obvious way to get the card toggle between the two modes but the software for that must be in programs like Video-7's Diag and within Fish.com. Also in Harvard graphics, regardless of how the monitor preferences are set, if it is in the 15.7kHz 640 x 200 graphics mode initially, once Draw Partner is opened from within Harvard Graphics, it flips it into the 21.85 kHz 640 x 350 graphics mode.

modem7
April 18th, 2016, 01:16 AM
With an EGA card and monitor, a program will not be directly instructing the system to use mode 1 or to use mode 2. Instead, the program selects a [video mode (http://www.minuszerodegrees.net/video/bios_video_modes.htm)]. Some video modes will result in a scan rate of EGA mode 1, and others, EGA mode 2.

So, FISH.COM will be selecting a video mode that is graphical and has colour. Looking at the EGA column at [here], and knowing that we observe EGA mode 1 in operation when FISH.COM runs, that video mode could be [04h] or [0Dh] or [0Eh].

When FISH.COM terminates, I expect that it is DOS that sets the video mode back to what it was before FISH.COM was run.

There are others on these forums who know way way more than I do on this subject. They will correct me if I am wrong.

vwestlife
April 18th, 2016, 04:40 AM
Try the SCREEN utility here:

http://www.pc-tools.net/dos/dosutils/

"Screen Explorer (SCREEN)
Lets you use the arrow keys to browse through the 256 screen modes. You can then stay in the mode you like."

Hugo Holden
April 18th, 2016, 06:02 AM
Thanks vwestlife, I will try that utility.
In the meantime I found that if the computer is booted with the EGA card switches set for an ega monitor, it comes up as expected in the 21.85kHz mode and screen 9 in Basic is the expected 640 x 350 graphics mode. But Basic then allows screen 8, which drops it into the 15.7 kHz 640 x 200 mode. (Yet doing the reverse by starting in the lower scan rate mode, and trying to select screen 9 in Basic doesn't work).
Also I think Modem7 is right about DOS. Coming out of any program, DOS selects the screen mode it was booted in.