🚀 go-pugleaf

In article <442@uwmacc.UUCP> curtis@uwmacc.UUCP (Alan Curtis) writes:
>I have been perusing through some assembler code written who knows where,
>and I noticed that when a particular routine writes to screen memory,
>it first reads the video controller scan status and makes sure it is 
>low, then after it goes low it waits until it goes high before writing
>to the screen buffer in memory.  The associated comment remarks that
>it must go high before it is safe to write directly to screen buffer
>memory.
>
>Is it possible to damage the monitor if the screen writes are not 
>synchronized with the scans, or is there some other explanation.  

	The word "safe" here refers to avoiding "snow" on the screen.
This is the procedure to avoid creating snow when updating the screen
on older video cards.  Newer cards have no problem with direct screen
writes.

---
Kenneth R. Ballou		...!ucbvax!cartan!brahms!ballou
Dept. of Mathematics
University of California
Berkeley, California  94720

    Writing to the screen memory during the retrace period avoids getting
snow on the screen while the memory update is going on.


                                       John Kelley
                                       AT&T

> I have been perusing through some assembler code written who knows where,
> and I noticed that when a particular routine writes to screen memory,
> it first reads the video controller scan status and makes sure it is 
> low, then after it goes low it waits until it goes high before writing
> to the screen buffer in memory.  The associated comment remarks that
> it must go high before it is safe to write directly to screen buffer
> memory.
> Is it possible to damage the monitor if the screen writes are not 
> synchronized with the scans, or is there some other explanation.  

I wrote a program sometime ago that writes to and reads from the screen
memory extensively and did not check the video controller scan status.
Never had any problems of any kind (the program has been in use for a
couple of years).

I have never seen any warning about doing this anywhere either. The
only thing I can guess is that this may prevent "snow". Some other
netlander may have a better guess.

Venu P. Gopal, ihnp4!ihuxk!vg55611

In article <442@uwmacc.UUCP>, curtis@uwmacc.UUCP (Alan Curtis) writes:
> I have been perusing through some assembler code written who knows where,
> and I noticed that when a particular routine writes to screen memory,
> it first reads the video controller scan status and makes sure it is 
> low, then after it goes low it waits until it goes high before writing
> to the screen buffer in memory.  The associated comment remarks that
> it must go high before it is safe to write directly to screen buffer
> memory.
> 
> Is it possible to damage the monitor if the screen writes are not 
> synchronized with the scans, or is there some other explanation.  

The reason that programs check before they write to screen memory is
because the IBM color graphics adapter (cga) was very poorly designed.
If you update the screen without the check, the screen will flash with
very annoying snow.  The check you saw waits until the monitor is in
retrace, (the beam is always turned off during retrace, for obvious
reasons) and then updates memory.

-- 

Ben Broder
{ihnp4,decvax} !hjuxa!catnip!ben
{houxm,topaz}/