SUPDUP GRAPHICS information (DCP 26-Aug-81) Contains: * Quick reference * NWG/RFC# 746 SUPDUP Graphics Extension * Raster bits extension Quick reference section (numbers in octal): Graphics mode entered by %TDGRF (231) and exited by anything with 200 bit set (%TDNOP (210) is the prefered way). %TDRST (230) resets all graphics modes. DRAW/ERASE commands: %GO... 100 ...D.. + (draw: 000, ...E.. erase: 040) .....R + (relative: 000, .....A absolute: 020) + function code: 00 undefined ....L. 01

line ....P. 02

point ....R. 03

rectangle ....CH 04 <0> characters (text) ....SC 05 Scan lines ....RN 06 Run-length encoded lines 07-17 undefined Miscellaneous Commands: Symbol Value no modifier modifier (with 020 bit set) 000 no-op %GOMVR 001

Move relative %GOMVA 021

Move absolute %GOXOR 002 XOR mode ON %GOIOR 022 XOR mode OFF %GOSET 003 Select set %GOMSR 004

Move set origin relative %GOMSA 024

Move set origin absolute %GOINV 006 Make current set invisisble %GOVIS 026 Make current set visible %GOBNK 007 Make current set blink %GOCLR 010 Erase whole screen %GOCLS 030 Erase current set %GOPSH 011 Push status info %GOVIR 012 Use virtual coordinates %GOPHY 032 Use physical coordinates %GOHRD 013 Divert output to subdevice %GOGIN 014 Request graphic input %GOLMT 015 Limit graphics to subrectangle

is a point; 14-bit two's complement signed number. relative: , low seven bits of each eg, (logand dx 177) absolute: , 14 bits of each, low seven first eg, (logand dx 177) followed by (logand (lsh dx -7) 177) TTYSMT variable and screen dimensions: %TQHGT,, 076000,,0 character height in dots %TQWID,, 001700,,0 character width in dots %TQVIR,, 000040,,0 terminal implements virtual coordinates %TQBNK,, 000020,,0 terminal implements blinking %TQXOR,, 000010,,0 terminal implements XOR mode %TQREC,, 000004,,0 terminal implements rectangle commands %TQSET,, 000002,,0 terminal supports multiple sets %TQGRF,, 000001,,0 terminal understands graphics protocol %TRGIN 0,,400000 terminal can provide graphics input %TRGHC 0,,200000 terminal has hard copy device %TRSCN 0,,040000 terminal implements raster commands (scan and run) Screen width in dots is * Screen height in dots is * (0,0) is in the center of the screen. ( -, -) is lower left (/2,/2) is upper right (two's complement fashion) Following are the SUPDUP GRAPHICS EXTENSION paper written by RMS, and following that is the RASTER BITS description written by DCP. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Network Working Group Richard Stallman Request for Comments 746 MIT-AI NIC 43976 17 March 1978 The SUPDUP Graphics Extension ... extends SUPDUP to permit the display of drawings on the screen of the terminal, as well as text. We refer constantly to the documentation of the SUPDUP protocol, described by Crispin in RFC 734 "SUPDUP Protocol". Since this extension has never been implemented, it presumably has some problems. It is being published to ask for suggestions, and to encourage someone to try to bring it up. The major accomplishments are these: * It is easy to do simple things. * Any program on the server host can at any time begin outputting pictures. No special preparations are needed. * No additional network connections are needed. Graphics commands go through the normal text output connection. * It has nothing really to do with the network. It is suitable for use with locally connected intelligent display terminals in a terminal-independent manner, by programs which need not know whether they are being used locally or remotely. It can be used as the universal means of expression of graphics output, for whatever destination. Programs can be written to use it for non-network terminals, with little loss of convenience, and automatically be usable over the ARPA network. * Loss of output (due, perhaps, to a "silence" command typed by the user) does not leave the user host confused. * The terminal does not need to be able to remember the internal "semantic" structure of the picture being displayed, but just the lines and points, or even just bits in a bit matrix. * The server host need not be able to invoke arbitrary terminal-dependent software to convert a standard language into one that a terminal can use. Instead, a standard language is defined which all programmable terminals can interpret easily. Major differences between terminals are catered to by conventions for including enough redundant information in the output stream that all types of terminals will have the necessary information available when it is needed, even if they are not able to remember it in usable form from one command to another. Those interested in network graphics should read about the Multics Graphics System, whose fundamental purpose is the same, but whose particular assumptions are very different (although it did inspire a few of the features of this proposal). NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension SUPDUP Initial Negotiation: One new optional variable, the SMARTS variable, is defined. It should follow the other variables sent by the SUPDUP user process to the SUPDUP server process. Bits and fields in the left half-word of this variable are given names starting with "%TQ". Bits and fields in the right half are given names starting with "%TR". Not all of the SMARTS variable has to do with the graphics protocol, but most of it does. The %TQGRF bit should be 1 if the terminal supports graphics output at all. Invoking the Graphics Protocol: Graphics mode is entered by a %TDGRF (octal 231) code in the output stream. Following characters in the range 0 - 177 are interpreted according to the graphics protocol. Any character 200 or larger (a %TD code) leaves graphics mode, and then has its normal interpretation. Thus, if the server forgets that the terminal in graphics mode, the terminal will not long remain confused. Once in graphics mode, the output stream should contain a sequence of graphics protocol commands, each followed by its arguments. A zero as a command is a no-op. To leave graphics mode deliberately, it is best to use a %TDNOP. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Co-ordinates: Graphics mode uses a cursor position which is remembered from one graphics command to the next while in graphics mode. The graphics mode cursor is not the same one used by normal type-out: Graphics protocol commands have no effect on the normal type-out cursor, and normal type-out has no effect on the graphics mode cursor. In addition, the graphics cursor's position is measured in dots rather than in characters. The relationship between the two units (dots, and characters) is recorded by the %TQHGT and %TQWID fields of the SMARTS variable of the terminal, which contain the height and width in dots of the box occupied by a character. The size of the screen in either dimension is assumed to be the length of a character box times the number of characters in that direction on the screen. If the screen is actually bigger than that, the excess may or may not be part of the visible area; the program will not know that it exists, in any case. Each co-ordinate of the cursor position is a 14-bit signed number, where zero is at the center of the screen (if the screen dimension is an even number of dots, then the visible negative points extend one unit farther than the positive ones, in proper two's complement fashion). Excessively large values of the co-ordinates will be off the screen, but are still meaningful. An alternate mode is defined, which some terminals may support, in which virtual co-ordinates are used. The specified co-ordinates are still 14-bit signed numbers, but instead of being in units of physical dots on the terminal, it is assumed that +4000 octal is the top of the screen or the right edge, while -4000 octal is the bottom of the screen or the left edge. The terminal is responsible for scaling these virtual co-ordinates into units of screen dots. Not all terminals need have this capability; the %TQVIR bit in the SMARTS variable indicates that it exists. To use virtual co-ordinates, the server should send a %GOVIR; to use physical co-ordinates again, it should send a %GOPHY. These should be repeated at intervals, such as when graphics mode is entered, even though the terminal must attempt to remember the state of the switch anyway. This repetition is so that a loss of some output will not cause unbounded confusion. The virtual co-ordinates are based on a square. If the visible area on the terminal is not a square, then the standard virtual range should correspond to a square around the center of the screen, and the rest of the visible area should correspond to virtual co-ordinates just beyond the normally visible range. Graphics protocol commands take two types of cursor position arguments, absolute ones and relative ones. Commands that take address arguments generally have two forms, one for each type of address. A relative address consists of two offsets, delta-X and delta-Y, from the old cursor position. Each offset is a 7-bit two's complement number occupying one character. An absolute address consists of two co-ordinates, each 14 bits long, occupying two characters, each of which conveys 7 bits. The X co-ordinate or offset precedes the Y. Both types of address set the running cursor position which will be used by the next address, if it is relative. It is perfectly legitimate for parts of objects to go off the screen. What happens to them is not terribly important, as long as it is not disastrous, does not interfere with the reckoning of the cursor position, and does not cause later objects, drawn after the cursor moves back onto the screen, to be misdrawn. Whether a particular spot on the screen is specified with an absolute or a relative address is of no consequence. The sequence in which they are drawn is of no consequence. Each object is independent of all others, and exists at the place which was specified, in one way or other, by the command that created it. Relative addresses are provided for the sake of data compression. They are not an attempt to spare programs the need for the meagre intelligence required to convert between absolute and relative addresses; more intelligence than that will surely be required for other aspects of the graphics protocol. Nor are relative addresses intended to cause several objects to relocate together if one is "moved" or erased. Terminals are not expected to remember any relation between objects once they are drawn. Most will not be able to. Although the cursor position on entry to graphics mode remains set from the last exit, it is wise to reinitialize it with a %GOMVA command before any long transfer, to limit the effects of lost output. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Commands: Commands to draw an object always have counterparts which erase the same object. On a bit matrix terminal, erasure and drawing are almost identical operations. On a display list terminal, erasure involves searching the display list for an object with the specified characteristics and deleting it from the list. Thus, on such terminals you can erase an object only if precisely that object was drawn, and was specified the same way when drawn as when erased. (presumably; a very hairy program might allow more). Any terminal whose %TOERS bit is set must be able to erase to at least that extent. The commands to draw objects run from 100 to 137, while those to erase run in a parallel sequence from 140 to 177. Other sorts of operations have command codes below 100. Meanwhile, the 20 bit in the command code says which type of addresses are used as arguments: if the 20 bit is set, absolute addresses are used. Graphics commands are given names starting with "%GO". Graphics often uses characters. The %GODCH command is followed by a string of characters to be output, terminated by a zero. The characters must be single-position printing characters. On most terminals, this limits them to ASCII graphic characters. Terminals with %TOSAI set in the TTYOPT variable allow all characters 0-177. The characters are output at the current graphics cursor position (the lower left hand corner of the first character's rectangle being placed there), which is moved as the characters are drawn. The normal type-out cursor is not relevant and its position is not changed. The cursor position at which the characters are drawn may be in between the lines and columns used for normal type-out. The %GOECH command is similar to %GODCH but erases the characters specified in it. To clear out a row of character positions on a bit matrix terminal without having to respecify the text, a rectangle command may be used. Example: The way to send a simple line drawing is this: %TDRST ;Reset all graphics modes. %TDGRF ;Enter graphics. %GOCLR ;Clear the screen. %GOMVA xx yy ;Set cursor. %GODLA xx yy ;Draw line from there. << repeat last two commands for each line >> %TDNOP ;Exit graphics. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Graphics Input: The %TRGIN bit in the right half of the SMARTS variable indicates that the terminal can supply a graphic input in the form of a cursor position on request. Sending a %GOGIN command to the terminal asks to read the cursor position. It should be followed by an argument character that will be included in the reply, and serve to associate the reply with the particular request for input that elicited it. The reply should have the form of a Top-Y character (code 4131), followed by the reply code character as just described, followed by an absolute cursor position. Since Top-Y is not normally meaningful as input, %GOGIN replies can be distinguished reliably from keyboard input. Unsolicited graphic input should be sent using a Top-X instead of a Top-Y, so that the program can distinguish them. Instead of a reply code, for which there is no need, the terminal should send an encoding of the buttons pressed by the user on his input device, if it has more than one. Sets: Terminals may define the concept of a "set" of objects. There are up to 200 different sets, each of which can contain arbitrarily many objects. At any time, one set is selected; objects drawn become part of that set, and objects erased are removed from it. Objects in a set other than the selected one cannot be erased without switching to the sets that contain them. A set can be made temporarily invisible, as a whole, without being erased or its contents forgotten; and it can then be made instantly visible again. Also, a whole set can be moved. A set has at all times a point identified as its "center", and all objects in it are actually remembered relative to that center, which can be moved arbitrarily, thus moving all the objects in the set at once. Before beginning to use a set, therefore, one should "move" its center to some absolute location. Set center motion can easily cause objects in the set to move off screen. When this happens, it does not matter what happens temporarily to those objects, but their "positions" must not be forgotten, so that undoing the set center motion will restore them to visibility in their previous positions. Sets are not easily implemented on bit matrix terminals, which should therefore ignore all set operations (except, for a degenerate interpretation in connection with blinking, if that is implemented). The %TQSET bit in the SMARTS variable of the terminal indicates that the terminal implements multiple sets of objects. On a terminal which supports multiple sets, the %GOCLR command should empty all sets and mark all sets "visible" (perform a %GOVIS on each one). So should a %TDCLR SUPDUP command. Thus, any program which starts by clearing the screen will not have to worry about initializing the states of all sets. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Blinking: Some terminals have the ability to blink objects on the screen. The command %GOBNK meaning make the current set blink. All objects in it already begin blinking, and any new objects also blink. %GOVIS or %TOINV cancels the effect of a %GOBNK, making the objects of the set permanently visible or invisible. %TQBNK indicates that the terminal supports blinking on the screen. However, there is a problem: some intelligent bit matrix terminals may be able to implement blinking a few objects, if they are told in advance, before the objects are drawn. They will be unable to support arbitrary use of %GOBNK, however. The solution to the problem is a convention for the use of %TOBNK which, together with degenerate definitions for set operations, makes it possible to give commands which reliably work on any terminal which supports blinking. On a terminal which sets %TQBNK but not %TQSET, %GOBNK is defined to cause objects which are drawn after it to be drawn blinking. %GOSET cancels this, so following objects will be drawn unblinking. This is regardless of the argument to the %GOSET. Thus, the way for a program to work on all terminals with %TQBNK, whether they know about sets or not, is: to write a bliniking picture, select some set other than your normal one (set 1 will do), do %GOBNK, output the picture, and reselect set 0. The picture will blink, while you draw things in set 0. To draw more blinking objects, you must reselect set 1 and do another %GOBNK. Simply reselecting set 1 will not work on terminals which don't really support sets, since they don't remember that the blinking objects are "in set 1" and not "in set 0". Erasing a blinking object should make it disappear, on any terminal which implements blinking. On bit matrix terminals, blinking MUST always be done by XORing, so that the non-blinking background is not destroyed. %GOCLS, on a terminal which supports blinking but not sets, should delete all blinking objects. Then, the convention for deleting all blinking objects is to select set 1, do a %GOCLS, and reselect set 0. This has the desired effect on all terminals. This definition of %GOCLS causes no trouble on non-set terminals, since %GOCLS would otherwise be meaningless to them. To make blinking objects stop blinking but remain visible is possible with a %GOVIS on a terminal which supports sets. But in general the only way to do it is to delete them and redraw them as permanent. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Rectangles and XOR Mode: Bit matrix terminals have their own operations that display list terminals cannot duplicate. First of all, they have XOR mode, in which objects drawn cancel existing objects when they overlap. In this mode, drawing an object and erasing it are identical operations. All %GOD.. commands act IDENTICALLY to the corresponding %GOE..'s. XOR mode is entered with a %GOXOR and left with a %GOIOR. Display list terminals will ignore both commands. For that reason, the program should continue to distinguish draw commands from erase commands even in XOR mode. %TQXOR indicates a terminal which implements XOR mode. XOR mode, when set, remains set even if graphics mode is left and re-entered. However, it is wise to re-specify it from time to time, in case output is lost. Bit matrix terminals can also draw solid rectangles. They can thus implement the commands %GODRR, %GODRA, %GOERR, and %GOERA. A rectangle is specified by taking the current cursor position to be one corner, and providing the address of the opposite corner. That can be done with either a relative address or an absolute one. The %TQREC bit indicates that the terminal implements rectangle commands. Of course, a sufficiently intelligent bit matrix terminal can provide all the features of a display list terminal by remembering display lists which are redundant with the bit matrix, and using them to update the matrix when a %GOMSR or %GOVIS is done. However, most bit matrix terminals are not expected to go to such lengths. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension How Several Process Can Draw On One Terminal Without Interfering With Each Other: If we define "input-stream state" information to be whatever information which can affect the action of any command, other than what is contained in the command, then each of the several processes must have its own set of input-stream state variables. This is accomplished by providing the %GOPSH command. The %GOPSH command saves all such input-stream information, to be restored when graphics mode is exited. If the processes can arrange to output blocks of characters uninterruptibly, they can begin each block with a %GOPSH followed by commands to initialize the input-stream state information as they desire. Each block of graphics output should be ended by a %TDNOP, leaving the terminal in its "normal" state for all the other processes, and at the same time popping the what the %GOPSH pushed. The input-stream state information consists of: The cursor position the state of XOR mode (default is OFF) the selected set (default is 0) the co-ordinate unit in use (physical dots, or virtual) (default is physical) whether output is going to the display screen or to a hardcopy device (default is to the screen) what portion of the screen is in use (see "Using Only Part of the Screen") (default is all) Each unit of input-stream status has a default value for the sake of programs that do not know that the information exists; the exception is the cursor position, since all programs must know that it exists. A %TDINI or %TDRST command should set all of the variables to their default values. The state of the current set (whether it is visible, and where its center is) is not part of the input-stream state information, since it would be hard to say what it would mean if it were. Besides, the current set number is part of the input-stream state information, so different processes can use different sets. The allocation of sets to processes is the server host's own business. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Using Only Part of the Screen: It is sometimes desirable to use part of the screen for picture and part for text. Then one may wish to clear the picture without clearing the text. On display list terminals, %GOCLR should do this. On bit matrix terminals, however, %GOCLR can't tell which bits were set by graphics and which by text display. For their sake, the %GOLMT command is provided. This command takes two cursor positions as arguments, specifying a rectangle. It declares that graphics will be limited to that rectangle, so %GOCLR should clear only that part of the screen. %GOLMT need not do anything on a terminal which can remember graphics output as distinct from text output and clear the former selectively, although it would be a desirable feature to process it even on those terminals. %GOLMT can be used to enable one of several processes which divide up the screen among themselves to clear only the picture that it has drawn, on a bit matrix terminal. By using both %GOLMT and distinct sets, it is possible to deal successfully with almost any terminal, since bit matrix terminals will implement %GOLMT and display list terminals almost always implement sets. The %TDCLR command should clear the whole screen, including graphics output, ignoring %GOLMT. Errors: In general, errors in graphics commands should be ignored. Since the output and input streams are not synchronized unless trouble is taken, there is no simple way to report an error well enough for the program that caused it to identify just which command was invalid. So it is better not to try. Errors which are not the fault of any individual command, such as running out of memory for display lists, should also be ignored as much as possible. This does NOT mean completely ignoring the commands that cannot be followed; it means following them as much as possible: moving the cursor, selecting sets, etc. as they specify, so that any subsequent commands which can be executed are executed as intended. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Extensions: This protocol does not attempt to specify commands for dealing with every imaginable feature which a picture-drawing device can have. Additional features should be left until they are needed and well understood, so that they can be done right. Storage of Graphics Commands in Files: This can certainly be done. Since graphics commands are composed exclusively of the ASCII characters 0 - 177, any file that can hold ASCII text can hold the commands to draw a picture. This is less useful than you might think, however. Any program for editing, in whatever loose sense, a picture, will have its own internal data which determine the relationships between the objects depicted, and control the interpretation of the programs commands, and this data will all be lost in the SUPDUP graphics commands for displaying the picture. Thus, each such program will need to have its own format for storing pictures in files, suitable for that program's internal data structure. Inclusion of actual graphics commands in a file will be useful only when the sole purpose of the file is to be displayed. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Note: the values of these commands are represented as 8.-bit octal bytes. Arguments to the commands are in lower case inside angle brackets. The Draw commands are: Value Name Arguments 101 %GODLR

Draw line relative, from the cursor to

. 102 %GODPR

Draw point relative, at

. 103 %GODRR

Draw rectangle relative, corners at

and at the current cursor position. 104 %GODCH <0> Display the chars of starting at the current graphics cursor position. 121 %GODLA

Draw line absolute, from the cursor to

. The same effect as %GODLR, but the arg is an absolute address. 122 %GODPA

Draw point absolute, at

. 123 %GODRA

Draw rectangle absolute, corners at

and at the current cursor position. The Erase commands are: Value Name Arguments 141 %GOELR

Erase line relative, from the cursor to

. 142 %GOEPR

Erase point relative, at

. 143 %GOERR

Erase rectangle relative, corners at

and at the current cursor position. 144 %GOECH <0> Erase the chars of starting at the current graphics cursor position. 161 %GOELA

Erase line absolute, from the cursor to

. 162 %GOEPA

Erase point absolute, at

. 163 %GOERA

Erase rectangle absolute, corners at

and at the current cursor position. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension The miscellaneous commands are: Value Name Arguments 001 %GOMVR

Move cursor to point

021 %GOMVA

Move cursor to point

, absolute address. 002 %GOXOR Turn on XOR mode. Bit matrix terminals only. 022 %GOIOR Turn off XOR mode. 003 %GOSET Select set. is a 1-character set number, 0 - 177. 004 %GOMSR

Move set origin to

. Display list terminals only. 024 %GOMSA

Move set origin to

, absolute address. 006 %GOINV Make current set invisible. 026 %GOVIS Make current set visible. 007 %GOBNK Make current set blink. Canceled by %GOINV or %GOVIS. 010 %GOCLR Erase whole screen. 030 %GOCLS Erase entire current set (display list terminals). 011 %GOPSH Push all input-stream status information, to be restored when graphics mode is exited. 012 %GOVIR Start using virtual co-ordinates 032 %GOPHY Resume giving co-ordinates in units of dots. 013 %GOHRD Divert output to output subdevice . =0 reselects the main display screen. 014 %GOGIN Request graphics input (mouse, tablet, etc). is the reply code to include in the answer. 015 %GOLMT Limits graphics to a subrectangle of the screen. %GOCLR will clear only that area. This is for those who would use the rest for text. NWG/RFC# 746 RMS 17-MAR-78 43976 SUPDUP Graphics Extension Bits in the SMARTS Variable Related to Graphics: Note: the values of these bits are represented as octal 36.-bit words, with the left and right 18.-bit halfword separated by two commas as in the normal PDP-10 convention. Name Value Description %TQGRF 000001,,0 terminal understands graphics protocol. %TQSET 000002,,0 terminal supports multiple sets. %TQREC 000004,,0 terminal implements rectangle commands. %TQXOR 000010,,0 terminal implements XOR mode. %TQBNK 000020,,0 terminal implements blinking. %TQVIR 000040,,0 terminal implements virtual co-ordinates. %TQWID 001700,,0 character width, in dots. %TQHGT 076000,,0 character height, in dots. %TRGIN 0,,400000 terminal can provide graphics input. %TRGHC 0,,200000 terminal has a hard-copy device to which output can be diverted. Proposed extensions to the SUPDUP Graphics Extension (NWG/RFC# 746 set forth by RMS 08-MAY-78). Written by DCP, summer 1981. Objective: Allow rasters to be sent to terminals that support it -- useful for the transmission of pictures. Rasters can include raw scan lines and run-length encoding. Extensions to the SMARTS variable related to graphics: Name Value Description %TRSCN 0,,040000 terminal implements raster operations (raw scan lines being sent to it, as well as run length encoding). Extensions to the DRAW and ERASE commands: Value Name Arguments 105 %GODSC Draw scan bits starting at the current graphics cursor position. 106 %GODRN <0> Draw bits determined by starting at the current graphics cursor position. 145 %GOESC Erase scan bits starting at the current graphics cursor position. 146 %GOERN <0> Erase bits determined by starting at the current graphics cursor position. Functional description: Scans: Nearly all graphics systems in use today that can support writing bits onto the graphics memory deal with multiples of 8 bits (if they have any such restriction at all). The PDP-11 family normally deals with 16.-bit words. VAXes deal with 16.-bit or 32.-bit data. The PDP-10 has the capacity to deal with 36.-bit values, but usually discards the low 4 bits to be compatible with PDP-11s which often talk with the PDP-10. It is therefore desirable to send data to graphics terminal in some multiple of 8.-bits. The graphics protocol does not allow raw 8.-bit data to be sent, because if the 200 (octal) bit is set graphics mode is exited and the code is interpreted as a SUPDUP %TD code. Therefore, a non-8.-bit scheme must be used to transfer the scan line. The way this is to be done is to use 16.-bits as the basic unit of data, and send it in chunks of 6-bits 6-bits 4-bits. As always, if the 200 bit is set, it is a %TD code and should be interpreted as such. If the 100 bit is set on a character, the character is gobbled, not used for the scan line, and SCAN mode is exited. The byte with the 100 bit set is the mentioned in the %GODSC and %GOESC commands. In PDP-10s scans are usually kept in the high 32. bits of 36. bit words. To convert this to the 6-6-4 scheme the following works and is believed to be efficient (A must be an accumulator): HRLI A,440600 ;SIX BIT BYTE POINTER FOR NOW LOOP: ILDB B,A ;GET SIX BITS ILDB B,A ;GET ANOTHER 6 BITS TLC A,000600#000400 ;CONVERT FROM 6 BITS TO 4 ILDB B,A ;GET 4 BITS TLC A,000600#000400 ;CONVERT FROM 4 BITS TO 6 Run length encoding: Run length encoding is an efficient method to draw a long sequence of OFF bits or ON bits. Its commands are basically draw dots of OFF bits and draw dots of ON bits. Therefore new information about a scan line needs to be sent only when the scan line changes from an OFF bit to an ON bit, or vice versa. After run-length-encoding mode is entered, a zero byte will exit, a byte with the 200 bit will exit in the usual fashion and be reinterpreted as a %TD code -- all other bytes are run-commands. The low six bits of the byte are a count determining how many bits to draw. If the 100 bit is on, ON bits are drawn. If the 100 bit is off, OFF bits are drawn. Suggestions: DO NOT use vitual coordinates for setting the graphic cursor and then sending raster information -- the picture will probably be wrong. Instead, always use physical coordinates. Considerations: OFF (or zero) bits for raster lines should do the same thing as the OFF bits in characters do. This probably amounts to nothing, i.e., the bits are not cleared, but rather left alone. This allows character drawing not to erase a graphic line that may be under the character. In SCAN mode this means the scan bits are ORed with the existing bits already on the screen. (Of course, erase mode does bit clearing (BIC on PDP-11, ANDCAM on PDP-10) and XOR mode does XORing.) In RUN-LENGTH-ENCODING mode this means that OFF bits simply skip the corresponding display bits without affecting them.