Conference noted::hackers_v1

    You can have more than one DRCS in the terminal at one time, I beleive,
    but you can only have one at a time mapped into GR/GL.  That is
    to say, you can have one DRCS in GR and another in GL (I think),
    but then you have to figure out how to swap between them.  I'm not
    real clear on all this, you might want to try the real gurus in
    the terminals notes file.
    
    Willie
    

    From the VT220Z Programmers Reference Manual.
    
    Escape Sequence is:
    
    	DCS Pfn;Pcn;Pe;Pcms;Pw;Pt { Dscs Sxbp1;...Sxbpn ST
    
    Where:
    	DCS = (9/0) is the device control string introducer.
    	Pfn = Font Number
    	Pcn = Starting character number (from which character to start
    	      loading into)
    	Pe  = Erase Control. Selects which characters are erased before
    	      loading according to:
    		0 = erase all characters
    		1 = erase only the characters that are being reloaded
    		2 = erase all characters in all DRCS sets.
        ( I will bet that you have this parameter set to zero or ommitted
    	  and this is what is causing your trouble.)
    	.
    	.
    	.
    
    Warwick.

    You would lose the bet ... it's set to 1 (erase only those being
    loaded.) 
    
    Here is an analogy:
    
    DEC standard graphics characters (corners, T's, those things) can
    be printed on your terminal. When you swicth back to normal text,
    the graphics characters already on the screen are not changed. Any
    new characters are normal text, though.
                         
    
    How come when I swich to my_set#2, all characters from my_set#1
    *already*on*the*screen* are changed?
    

    I know the 'problem' you are having, and it's not something you
    can do anything about.  Take a look at P 56 in the VT-240 programmers
    reference manual if you have one handy. [Yes, I know you are talking
    about a VT-220, but it works the same way].
    
    You can only have 2 'invoked' character sets displayed on your terminal
    at one time, these are GL and GR (graphic left and right).  You can
    swap between them with SO and SI or setting the highbit of the
    character. 
    
    Each of these 2 'invoked' character sets can be mapped from a list
    of 'designated' character sets G0, G1, G2, G3.  Note that while
    you can have 4 'designated' character sets, you can still only display
    a maximum of two at one time.
    
    Each of the 'designated' character sets is in turn loaded from the
    'graphic repertory' of the terminal, including ROM fonts and downline
    loaded fonts.  Note that you can only have 4 of these mapped to
    'designated' character sets at any one time.
    
    For example:
    
    you have downloaded 2 soft fonts and 'designated' them into G1 and G2.
    G0 is still normal ASCII to give you text and such.  You 'invoke' soft
    font #1 [from G1] into GR and put some characters up on the screen. Now
    you want to display some characters on the screen from soft font #2, so
    you 'invoke' font #2 [from G2] into GR, _BUT_ as soon as you do, all
    the characters that are drawn from GR are changed, because G2 is
    now 'behind' GR and G1 is out of the picture.
    
    Does this help?  The terms in quotes ['designate','invoke', etc]
    are probably not the official DEC terms, but the manual is a bit
    unclear in this area, so I did what I could.
    
    Willie 

    The problem is not with "invoking" sets - that I 'understand' (read
    as "know the magic sequences").
    
    It's keeping them on the screen afterwards that I cannot do.
    
    I can have, at one time, standard characters, graphics characters,
    and sixel characters, all displayed on my screen. _Simultaneously_.
    All were written there 'through' G0 and GL. 
    
    Because, when you change G0 or GL, things already on the screen
    don't (should not) change. 
    
    But when I add a second sixel set, the characeters of the first
    set (not the graphics or the standard characeters) already on the
    screen change. 
    
    I suspect that by defining the second sixel set, the first definition
    stored in the terminal's memory is wiped. 
    
    It's getting to be more of a problem talking about this stuff than
    anything else. Oh, well. 
    
    I'm still interested in any advice out there.
    
    Kevin

Text terminals (and even graphics terminals, for text) don't store a bitmap
of the screen - they store each line as a series of encoded characters - more
or less as ASCII characters, though there are all sorts of extra bits to specify
things like character set, rendition, and so on.  To display a character, its
bitmap is located in the proper character set and sent to the screen.  This
happens each time the character is displayed - i.e., at every screen refresh,
or 60 times per second.

All the various "hard-wired" character sets are available all the time, indepen-
dent of whether they are designated or invoked.  There is ONE DRCS set:  A
character can be drawn from that one set, and whatever bitmap is currently in
character set will appear on the screen.  When you load a DRCS, you are changing
the actual bitmap.  The next time the screen is refreshed, the new bitmap will
become visible.
							-- Jerry

re .-1

I believe that the previous statement is true for the VTxxx series, but
it is a device dependency, not a requirement.  When I last looked at the
VSRM, it did not specify what happens to the screen when the DRCS is changed.
The PRO, for example, does NOT change the displayed characters, unless the
terminal subsystem needs to redisplay them (switching background from light
to dark, for example).

In short, don't depend upon one effect or the other.  You'll get burned
eventually.

/s/ Chuck

    Thanks, .6. That answers my question.
    
    Regardless of effect, I needed to know if two character sets can
    be loaded into a terminal at one time. They can't - the second
    over-rides the first. 
    
    Two questions, then: (just out of curiosity)
    
    1. What is the Font Number parameter (Pfn) in the down-load character
    set control sequence used for?
    
    2. What is the Dscs  parameter in the down-load character set and
    the Designating soft character set control sequences used for?
    
    Thanks, 
    
    Kevin

    From the VT220 programmers reference manual.
    
    Pfn		Font Number	Specifies the DRCS font buffer to be
    				loaded. The VT220 has only one DRCS
    				font buffer. This parameter has two
    				valid values: 0 and 1.
    
    Dscs defines the character set "name" for the soft font, and is
    used in the SCS (select character set) escape sequence.
    
    Warwick.