Display terminal in a digital data system

Abstract: In a dumb terminal (103) to which an auxiliary device (109) may be attached, improvements in display definition, in the transfer of data between a host computer (101) and the terminal or an auxiliary device, and in character generation memory. In the area of display definition, pages (801) may be mapped onto the terminal's display memory (213) and viewports (811) may be mapped onto pages. Page and viewport definition are independent of each other and settable modes (623) permit a viewport to follow the active position. In the area of transfer of data, the source of data for the host computer and the destination of data from the host computer may be specified independently. Improvements in flow control ensure that the host computer can always change the destination of data and gain control of the terminal. Improvements in access control ensure that neither the terminal nor the auxiliary device can monopolize transfer between it and the host. Moreover, transfer can proceed transparently even when non-ASCII 8-bit codes or a 7-bit host-terminal link are involved. Finally, a protocol (1501) permits one portion of character generation memory to be copied to another portion thereof.

Description: The present invention relates to digital data and word processing systems and more particularly to terminals employed therein. One of the most common varieties of prior-art terminal is the "dumb" terminal. A "dumb" terminal is so called because it performs no local processing of data beyond what is required to receive the data in the terminal and provide it to a digital computer or to receive the data from the digital computer and display it on the terminal screen. Communication between the "dumb" terminal and the digital computer is by means of protocols. Different protocols specify the data transferred between the digital computer and the terminal, permit the digital computer to control the operation of the terminal, and indicate the state of the terminal to the digital computer. A common class of "dumb" terminals is made up of ANSI standard terminals which are controlled by means of protocols compatible with the ANSI X3.4, X3.41, and X3.64 standards. As microprocessors, display controllers, and memory have dropped in cost and grown in speed and power, the "dumb" terminals have become "smarter". An example of such a "smart" "dumb" terminal is the IBM 3163. This and similar ANSI standard terminals now have sufficient display memory to hold more than one terminal screen's worth of data. In such terminals, the display memory is divided into pages and a user of the terminal can specify which page of display memory is to be displayed. The 3163 and others of these terminals also permit the terminal screen to be divided into viewports. A viewport is a "window" onto a page, and consequently, a terminal with viewports can display data from more than one page simultaneously. Other areas where "dumb" terminals have grown smarter include the capability of transferring data not only between the terminal and the computer, but also between the computer and an auxiliary device connected to the terminal and the capability of displaying more than one character font, c.f. ANSI standard X3.64 - 1979. This standard was proposed for (9) input-output devices with auxiliary devices. Referring to page 42, one method for communicating to the auxiliary device is through the MEDIA COPY protocol (5.57). By use of this protocol, the data stream received by the display terminal may be directed to the auxiliary device, or a copy of the received data stream may be directed to the auxiliary device. Or the data stream transmitted from the display terminal may be received from the auxiliary device. This is further emphasized on page B-7, section B10. As can be seen, prior art display terminals, in essence, contained a switch which either connected the host computer to the display device, or to the auxiliary devive. In all of these areas, the prior art has left certain problems unsolved. These problems are addressed in the improved terminal of the present invention as defined in the appended claims. Summary of the Invention The present invention relates to terminals for use in digital computer systems and more specifically to "dumb" terminals. One or more pages may be defined in the improved terminal of the present invention. Varying amounts of display memory may be mapped to a defined page or a defined page may be mapped to no display memory at all. Any page may have a viewport assigned to a location on it. The viewport may have a varying number of lines or no lines at all. Independent protocols define pages, define viewports, and assign viewports to pages. Apparatus in the terminal insures that if a line in a viewport has no corresponding line in the page to which the viewport is assigned, the line in the viewport displays blanks. The improved terminal also permits a viewport to automatically follow the active position, i.e., the position in display memory at which operations are presently being performed and permits the active position to be detached from the cursor position. Finally, a region in a page may be scrolled and a region in a page may be copied to a region in any page. The improved terminal of the present invention further permits independent specification of the terminal and an auxiliary device attached thereto as a source of or destination of data transferred between a digital computer and the terminal. Thus, the terminal may be a destination of data received from the computer while the auxiliary device is a source of data for the computer and vice-versa. Other improvements in the area of transfer of data between the computer and the terminal or auxiliary device include improved flow control which generally permits the computer to switch flow from the auxiliary device to the terminal, improved access control which generally prevents the auxiliary device or the terminal from exluding the other device from communicating with the computer, and emplying the ANSI INT code to permit the computer to gain control of the terminal under any circumstances. A transparency mode further permits automatic translation of 8-bit protocols into ANSI equivalent prefix + code sequences, thus permitting operation of the terminal with a 7-bit link between the terminal and the computer or with auxiliary devices which employ non-ANSI codes. It is thus an object of the invention to provide an improved terminal; to provide improved control of transfer of data between a digital computer, a terminal, and an auxiliary device attached to the terminal, and to provide transparent transfer of data between the auxiliary device and the terminal. Other objects and advantages of the present invention will be understood by those of ordinary skill in the art after referring to the detailed description of a preferred embodiment contained herein and to the drawings, wherein: Brief Description of Drawings Fig. 1 is a conceptual block diagram of a system employing the improved terminal; Fig. 2 is a logical overview of the improved terminal; Fig. 3 is a block diagram of an implementation of a preferred embodiment of the improved terminal; Fig. 4 is a schematic diagram of data transfer paths in the improved terminal; Fig. 5 is a diagram of control data for the data transfer paths; Fig. 6 is a list of mode set and reset commands and the modes; Fig. 7 is a list of data transfer protocols; DESCRIPTION OF A PREFERRED EMBODIMENT The following Description of a Preferred Embodiment of the improved terminal of the present invention will first present an overview of a system in which the terminal is employed, then overviews of the logical structure and an implementation of the improved terminal, and finally, detailed descriptions of the data transfer and display components of the improved terminal. 1. System Employing the Improved Terminal: Fig. 1 The improved terminal of the present invention is a so-called "dumb" terminal for use with a digital computer system. The terminal is termed a "dumb" terminal because it performs no local processing of data beyond what is required to receive the data at the terminal and provide it a digital computer or to receive the data from the digital computer and display it on the terminal screen. Figure 1 shows a system using the improved terminal. Terminal TERM 103 is connected by serial link (SL) 111 to host computer (HC) 101. TERM 103 includes CRT display DISP 105 and is connected by further SL 113 to keyboard (KB) 107 and by SL 115 to auxiliary device (AD) 109. HC 101 may be any computer, for example, a VS computer manufactured by Wang Laboratories, Inc. AD 109 may be any device which can transmit or receive serial data, but will most generally be a printer or other hard-copy display device. In a preferred embodiment, TERM 103 is a character terminal, SL 111 is an RS 232 link, and communication across SL 111 with HC 101 is by means of protocols compatible with the ANSI X3.4, X3.41, and X3.64 protocols. The protocols include protocols for characters, control protocols which control the manner in which TERM 103 operates, protocols to control routing between HC 101 and TERM 103 or AD 109, and protocols by means of which HC 101 can receive information from TERM 103 about its status. Operation of TERM 103 is as follows: When a user of KB 207 strikes a key on KB 107, TERM 103 packages the character into the proper protocol and sends the protocol via SL 111 to HC 101. If a program executing on HC 101 wishes to display a character on TERM 103, it sends a protocol for the character via SL 113 to TERM 103, which displays the character at the proper position in DISP 105. If the program wishes to alter operation of TERM 103 or determine TERM 103's status, it employs the proper protocols for those ends. DISP 105 in TERM 103 may display data from up to 8 pages retained in TERM 103. What pages are displayed and what data on a page is displayed is determined by viewports. A viewport is a group of lines in DISP 105 which are associated with a group of lines on a given page. The lines associated with the viewport on the page appear on the lines in DISP 105 associated with the viewport. The size of pages, the size of viewports, the association between a viewport and a group of lines on a screen, and the association between a viewport and a group of lines on a page are all controlled by protocols from HC 101. AD 109 may be controlled either by the user of TERM 103 or by a program operating on HC 101. In the former case, AD 109 can print lines or screens of data displayed on DISP 105; in the latter case, HC 101 may provide data to or receive data from AD 109. When HC 101 is providing data, the data goes via SL 111 to TERM 103, which repackages it as required for AD 109, and then sends it via SL 115 to AD 109. When HC 101 is receiving data from AD 109, the data goes via SL 115 to TERM 103, which does any repackaging necessary for HC 101, and then sends the data on to HC 111 via SL 111. Destination and direction of flow of data between HC 101, TERM 103, and AD 109 is controlled by means of protocols sent by HC 101 to TERM 103 and by TERM 103 to HC 101. In the improved terminal, the protocols permit simultaneous transfer in one direction between HC 101 and TERM 103 and in the other between AD 109 and HC 101. 2. Logical Overview of TERM 103: Fig. 2 In logical terms, TERM 103 is implemented by means of a group of devices which share a common terminal address space. Figure 2 shows the devices, the common terminal address space (TAS) 203, and the manner in which the devices relate to each other by means of the address space and interrupt signals INT 209. Overall control of TERM 103 is provided by microprocessor (uP) 201. uP 201 is connected to TAS 203 by means of ADDR bus 205, which carries addresses from uP 201 to TAS 203, and DATA bus 207, which provides data from uP 201 to the address specified in TAS 203 by the address on ADDR 205 or fetches data from that location in TAS 203 to uP 201, depending on whether uP 201 is performing a read or write operation on TAS 203. uP 201 contains registers for retaining data to be output to DATA bus 207 or received from DATA bus 207 and addresses to be output to ADDR bus 205 and ALUs for manipulating data and addresses. Operations performed by uP 201 are determined by programs in PROG 235 of TAS 203. uP 201 further receives interrupt signals at INT 209 from other devices in TERM 103. In response to such an interrupt signal, uP 201 executes interrupt code in PROG 235. Data required for execution of the programs, for example, stacks and work areas, is contained in PROGD 237. CTLD 239 contains control data which indicates the present status of TERM 103 to uP 201. Transfer of data between TERM 103 and KB 107 is managed by keyboard port (KBP) 231. KBP 231 includes registers (KBPRS) 233 which are part of TAS 203, and hence accessible to uP 201. KBPRS 233 in a preferred embodiment include a transmit register for a byte of data, a receive register for a data byte, a register for a command word, and a register for a status word. When uP 201 is transferring data to KB 107, for example to set certain characteristics of KB 107, uP 201 first places a command word specifying that data is going to KB 107 and then places a sequence of data bytes in the transmit register. As KBP 231 receives each data byte, it converts the data byte from parallel to serial form and outputs it to KB 107. The status of the transfer is retained in the status register, where it may be read by uP 201. When a user hits a key on KB 107, KBP 231 receives the code corresponding to the keystroke via SL 113. KBP 231 converts the code from serial to parallel form, places the parallel value in the receive register, sets the status register to indicate that a keystroke code has been received, and sends an interrupt via INT 209 to uP 201, which responds to the interrupt by executing interrupt handler code in PROG 235. The interrupt handler code examines the status register in KBPRS 233 to determine the cause of the interrupt. If it was caused by the receipt of a keyboard code, the interrupt handler code moves the received keystroke code from KBPRS 233 to keystroke buffer (KEYB) 249, where it is available for further processing by uP 201. Transfer of data between HC 101 and TERM 103 is managed by host port (HOSTP) 223, and transfer of data between AD 109 and TERM 103 is managed by AUXP 227. These ports operate generally in the same manner as KBP 231: registers in auxiliary port registers (AUXPRS) 229 and host port registers (HOSTPRS) 225 contain data to be transferred, a command word, and a status word as described for KBP 231 and transfers occur in the same fashion. Thus, when TERM 103 sends bytes of data to HC 101, uP 201 writes a command word into HOSTPRS 225 and then provides a sequence of bytes to those registers; when HC 101 sends a byte of data to TERM 103, HOSTPRS 225 receives the byte, sets a status register to indicate that fact, and generates an interrupt signal on INT 209 to uP 201. The interrupt handler code which uP 201 executes in response to the interrupt copies the byte from HOSTPRS 225 to host receive queue (HRXQ) 241, where it is available for further processing. Other queues used in connection with HOSTP 223 and AUXP 227 in the fashion described above for HRXQ 241 are host transmit queue (HTXQ) 243, which contains data to be transmitted to HC 101 by HOSTP 223, auxiliary receive queue (ARXQ) 245, which contains data received in AUXP 227 from AD 109, and auxiliary transmit queue (ATXQ) 247, which contains data to be transmitted to AD 109 by AUXP 227. Additionally, terminal receive queue (TRXQ) 240 receives data from TERM 103 which is to be placed in HTXQ 243 for transmittal to HC 101. Displays on DISP 105 are generated by display generator (DISPG) 211. The data used to generate the display is contained in display memory (DISPM) 213 and character generator memory (CGM) 215. DISPM 213 contains rows of data to be displayed in DISP 105. Each character in a row is represented by a 1-byte character code and a 1-byte attribute code indicating how the character specified by the character code is to be displayed. As previously mentioned, DISPM 213 may be divided into up to 8 pages. CGM 215 contains a visual representation corresponding to each character code. When CGM 215 receives a character code as an input, it outputs the visual representation to DISPG 211 for display on DISP 105. Operation of DISPG 211 is controlled by display control registers (DCTLR) 219 in a manner similar to that described for KBP 231. When uP 201 wishes to display data on DISP 105, it first loads registers in DCTLR 219 to initialize DISPG 211. Then it loads the character codes for the data and the codes for the attributes into DISPM 213. Next, it loads pointers specifying the rows in DISPM 213 to be displayed into a rowtable in display data (DISPD) 221. Then it loads a pointer to the rowtable into DCTLR 219 and finally issues a command which turns DISPG 211 on. DISPG 211 responds to the command by fetching each character code and each corresponding attribute code in each row specified in the rowtable and displaying it on DISP 105, employing CGM 215 to convert the character code to its visual representation, and using the attribute code to modify the manner in which the visual representation is displayed. DISPG 211 continues in this manner until uP 201 inactivates the display or a condition arises which requires intervention by uP 201. While DISPG 211 is operating as just explained, uP 201 may modify the display on DISP 105 by changing the contents of DISPM 213 or by modifying the rowtable so that DISPG 211 displays different rows of DISPM 213 on display 105. When DISPG 211 requires intervention of uP 201, it provides an INT 209 to uP 201, to which uP 201 responds by executing interrupt code which examines the status registers in DCTLR 219 and performs whatever action is necessary to deal with the indicated status. As previously indicated, HC 101 controls TERM 103 by means of protocols sent via SL 111 to TERM 103. The protocols may be a single byte long or may be multi-byte protocols. The protocols include codes indicating characters to be displayed on DISP 105, control codes indicating operations to be performed by TERM 103, and commands. The commands are introduced by command control codes and contain parameter bytes representing parameters used in the command and command specification bytes. Certain codes used in the protocols are used to indicate that a protocol has a private meaning. Each time a byte is received in HRXQ 241 from HC 101, uP 201 examines the byte to determine how it should be processed. In the case of protocols indicating characters to be displayed, the characters are simply written to DISPM 213; in the case of single-byte control codes, state variables in CTLD 239 are altered to specify the new state of TERM 103 and uP 201 begins operating the terminal as required for the control code. In the case of a command, uP 201 responds to the command control code and the command byte by collecting the parameter bytes and the command specifier in command buffer (COMB) 251. uP 201 then executes the command as specified by the command specifier and the parameter bytes. Included in the command protocols to which TERM 103 responds are commands which determine the flow of data between HC 101, TERM 103, and AD 109, a command which define pages in DISPM 213, a command which defines view-ports, a command which assigns a viewport to a page, and commands which determine how page and viewport mapping interacts with changes in the the location of current interest in DISPM 213. 3. Implementation of Term 103: Fig. 3 Figure 3 presents an overview of a preferred implementation of TERM 103. uP 201 is implemented by means of uP 301, a MC 6809C microprocessor manufactured by Motorola, Inc. The MC 6809C is described in the MC6809-MC6809E Microprocessor Programming Manual, Motorola, Inc., 1981, incorporated herein by reference. uP 301 receives two different interrupt signals, interrupt request (IRQ) 339 and fast interrupt request (FIRQ) 338. In a preferred embodiment, IRQ 339 receives interrupts generated by the keyboard port (KPB 317) and the host and auxiliary ports (DUART 319), while FIRQ 338 receives interrupts from display controller (DC) 315, a component of the display generator. TAS 203 is implemented by means of ROM 307, RAM 309, non-volatile RAM (NVRAM) 311, display RAM (DISPR) 323, character generator memory (CGM) 329, and command, data, and status registers in KBP 317, DUART 319, and DC 317. Data is transferred between uP 301 and the components of TAS 203 by 8-bit DBUS 305, and addresses are provided to the components by 16-bit ABUS 303. ROM 307 contains programs executed by uP 301, and consequently corresponds to PROG 235. RAM 309 is used for general data storage, and corresponds to PROGD 237, HRXQ 241, HTXQ 243, TRXQ 240, ARXQ 245, ATXQ 247, COMB 251, KEYB 249, DISPM 213, and portions of CTLD 239 and DISPD 221. NVRAM 311 does not lose its contents when TERM 103 is turned off, and is consequently used to store those portions of CTLD 239 and DISPD 221 which are to be retained between uses of TERM 103. CGM 329, finally, corresponds to CGM 215. In a preferred embodiment, CGM 329 contains a ROM element and a RAM element and provision is further made in TERM 103 for the addition of two external ROM or RAM cartridge elements. The additional elements contain further alternate fonts or character sets. In the case of the RAM, uP 301 may load the RAM with the visual representations of an alternate font or character set in response to a protocol from HC 301 which contains the visual representations as parameters. Visual representations in CGM 329 are addressed as follows: two bits in the attribute byte corresponding to the character being output to CRT CTL 331 select one of the four elements of CGM 329; then the visual representation corresponding to the character code is selected by means of 12-bit addresses made up of 4 bits on RA0 325 and 8-bit character codes on CGD 337. As shown in Figure 3, the signals which select an individual element come via CGXEL 341 from CRT CTL 331; addresses on RA 325 may come either via a latch (not shown) from uP 301 or from DC 315, while those on CGD 337 may come either by way of DBUS 305 and a latch (not shown) from uP 301 or from DISPR 323. Addresses from uP 301 are used when uP 301 is reading CGM 329 or loading RAM in GCM 329; those from DC 315 and DISPR 323 are used when TERM 103 is displaying data on DISP 105. Data for loading RAM in CGM 329 comes via DBUS 305 from uP 301. KBP 317 corresponds to KBP 231. It is implemented by means of a MC 68B50 asynchronous communications interface adapter integrated circuit, manufactured by Motorola, Inc., and fully described at page 3-495 of 8-Bit Microprocessor and Peripheral Data Manual, manual no. DL133, Motorola, Inc, 1983. That manual is hereby incorporated into the present Specification by reference. DUART 319 has two ports, and corresponds to both AUXP 227 and HOSTP 223. DUART 319 is implemented by means of a SCN2681 dual asynchronous receiver/transmitter integrated circuit, manufactured by Signetics, Inc. DUART 319 is fully described in Microprocessor Data Manual, Signetics, Inc., 1986, pg. 2-89. That manual is also incorporated herein by reference. As may be seen from Fig. 3, the registers contained in KPB 317 and DUART 319 are addressed by means of bits 0-3 of ABUS 303 and data is provided to and received from the registers by means of DBUS 305. When KBP 317 receives a byte from KB 107, or DUART 319 receives a byte from either HC 101 or AD 109, it produces an interrupt signal on IRQ 339. DISP 105 is implemented in Fig. 3 by means of CRT 333 and DISPG 211 is implemented by means of display controller (DC) 315, row RAM (RRAM 323), CGM 329, and CRT control logic (CRT CTL) 331. DC 315 controls display of characters on CRT 333. In TERM 103, DC 315 is an advanced video display controller SCN2674, manufactured by Signetics, Inc. The SCN2674 is fully described at page 2-123 of the Microprocessor Data Manual. Operation of DC 315 is controlled by internal registers in DC 315 corresponding to DCTLR 219. Included in the internal registers are a pointer to the row table and a pointer to the row table entry containing the location of the row currently being displayed. Data for the registers is received from DBUS 305, and addresses from bits 0-2 of ABUS 303. Output from DC 315 is 14 bits of display address (DADD) 321. Bits 0-7 of DADD 321 provide addresses to RRAM 323, which contains the character codes and attributes for the row currently being output for display on CRT 333. Bits 0-14 provide addresses via a latch (not shown) and ABUS 303 to TAS 203, permitting DC 315 to copy rows of data (character codes and their attributes) from DISPM 213 in TAS 203 via DBUS 305 and a latch (not shown) to RRAM 323. During the period that DC 315 is copying a row of data, it emits HALT signal 340, which keeps uP 301 from using ABUS 303 and DBUS 305. After DC 315 has copied the row of data into RRAM 323, it causes RRAM 323 to output the data one byte at a time. The character code is output first and is latched. Then, the corresponding attribute byte is output and latched. The character code goes via CGD 337 to CGM 329, which outputs the corresponding visual representation from the element selected by the bits in the attribute byte. The visual representation goes via shift data (SHD) 343 to CRTCTL 331. At the same time, the attribute goes via ATTR 335 to CRT CTL 331. In response to the visual representation and the attribute, CRT CTL 331 produces the character on CRT 333. After the entire row has been output from RRAM 323 as described above, DC 315 fetches the next row specified in the row table from DISPM 213. When the end of the row table is reached, DC 315 begins the cycle again with the first row in the row table. If DC 315 requires intervention of uP 301 during operation, it generates a FIRQ signal 338 to uP 301, which responds to the signal by executing interrupt code which examines the registers in DC 315 to determine the cause of the interrupt. Operation of the implementation of TERM 103 shown in Fig. 3 is as described in logical terms above. A byte from HC 101 is received in DUART 319, an IRQ 339 is generated, and uP 301 executes interrupt code which places the byte in HRXQ 241. uP 301 examines the byte and the present state of TERM 103 as indicated by values in CTLD 239. If the state indicates that the byte is intended for AD 109, uP 301 places it in ATXQ 247, from whence uP 301 outputs it via DUART 319 to AD 109. If the present state of TERM 103 indicates that the byte is intended for TERM 103, uP 301 determines from its value and the present state of TERM 103 whether it is a character code, a control code, a parameter, or a byte of a command specifier. If it is a character code, uP 301 places the code at a location in DISPM 213 termed the active position. A pointer to the active position is maintained in DISPD 221. If it is a control code, uP 301 changes state in CTLD 239 or DISPD 219 as required; if it is a parameter byte, uP 301 places it in COMB 251. If it is a command specifier, uP 301 executes the specified command using the parameters. While uP 301 is executing commands, DC 315 operates as described above to display the lines from DISPM 213 specified by the rowtable. In response to a byte from KB 107, KBP 317 generates an interrupt, as previously explained. The interrupt handling routine places the byte in KEYB 249, uP 201 then examines it, and in most cases, places it in TRXQ 240, from whence it goes to HTXQ 243 for transfer to HOSTPRS 225. DUART 319 then transmits it to HOST 101. Essentially the same sequence of events occurs with a byte from AUXP 227, except that in that case, the interrupt handling routine moves the byte to ARXQ 245, where it is examined and in most cases, moved to HTXQ 243, from whence it is transmitted to HC 101 as just described. 4. Data Transfer Paths between HC 101 and TERM 103 or AD 109: Fig. 4 A feature of TERM 103 is that HC 101 can select either TERM 103 or AD 109 as the destination of the data it transfers to TERM 103 and independently select either TERM 103 or AD 109 as the source of the data it receives from TERM 103. In the following, the paths by which data may be transferred between TERM 103, AD 109, and HC 101 are described. Then the discussion explains how paths are selected, how the flow of data on them is controlled, and how TERM 103 and AD 109 are each guaranteed access to HC 101. Figure 4 is a schematic drawing of the paths by which data may be transferred between HC 101 and TERM 103 or AD 109. Dashed lines in the figure indicate the path 403 for data sent to HC 101 and solid lines indicate the path 401 for data received from HC 101. Beginning with path 401 from HC 101, data bytes on that path move from HC 101 to host input byte register (HIBR) 405, one of HOSTPRS 225, in HOSTP 223. As previously explained, HOSTP 223 generates an interrupt and uP 201 transfers the received byte to the tail of a queue in HRXQ 241. The tail of the queue is indicated by HRXW pointer 417. What happens next depends on whether TERM 103 or AD 109 has been selected as the destination for the byte. In the former case, when the byte reaches the head of the queue, indicated by HRXR pointer 415, uP 201 moves the byte from HRXQ 241 to DATA 409, a variable in CTLD 239 which is used to store the byte while it is processed. uP 201 then processes the byte as determined by the byte's value and the state of TERM 103. If AD 109 has been selected as the destination, uP 201 moves the byte from HRXQ 241 to the tail of ATXQ 247, indicated by ATXW pointer 425, and when the byte reaches the head of ATXQ 247, indicated by ATXR pointer 423, uP 201 moves the byte from there to auxiliary output byte register (AOBR) 411, one of the registers of AUXPRS 229. In response thereto, AUXP 227 transmits the data byte to AD 109. Continuing with path 403 to HC 101, when the source of data on that path is AD 109, AUXP 227 receives the data a byte at a time from AD 109. Each byte is received in auxiliary input byte register (AIBR) 413 and when AUXP 227 receives the byte, it generates an interrupt to uP 201. uP 201 moves the byte to the tail of the queue in ARXQ 245, indicated by the pointer ARXW 429. When the byte reaches the head of the queue, indicated by the pointer ARXR 429, uP 201 moves the byte to the tail of the queue in HTXQ 243, indicated by the pointer HTXW 421. When the byte reaches the head of the queue, indicated by the pointer HTXR 419, uP 201 moves it to host output byte register (HOBR) of HOSTPRS 225 together with a transmit command, and HOSTP 223 responds to the command by transmitting the byte to HC 101. When the source of data is the terminal, uP 201 has placed the data byte in terminal receive queue (TRXQ) 240. Bytes placed in TRXQ 240 go to the tail of the queue, marked by the pointer TRXW 433, and uP 201 reads bytes from the head of the queue, marked by TRXR 431, to the tail of the queue in HTXQ 243, from whence they are transmitted as described above. Bytes on path 401 whose destination is AD 109 are interpreted in TERM 103 only when they affect the path. The only codes which fall into this category are INT and N ULL, the flow control codes XON and XOFF, and the command CSI 4 i, which changes the destination of path 401 to TERM 103. These codes are detected and processed in a preferred embodiment by the interrupt code which responds to the interrupt from HOSTP 223 when the byte is received. 5. Control of Data Paths 401 and 403: Figs. 5 -7 Data paths 401 and 403 are controlled by means of protocols sent between HC 101 and TERM 103. In response to these protocols, uP 201 sets certain variables in CTLD 239 and controls TERM 103 as determined by the values of the variables. The protocols and the variables set by them control the source and destination of data being transferred, the flow of data on a path, and access to a path by the TERM 103 and AD 109 respectively. The variables are shown in Figure 5 and the protocols and modes are shown in Figures 6 and 7. The discussion begins with an overview of protocols in TERM 103. The mode protocols, shown in Figure 6, are typical command protocols for TERM 103. As can be seen from protocols 601 and 603, all command protocols begin with a command sequence indicator (CSI) byte, which has the hexidecimal (X) value X′9B′. The mode protocols are private extensions of the ANSI standard, and this fact is indicated in these protocols by the next byte, which contains the ASCII code for the ">" character, indicating a private extension. In other command protocols, a private extension is indicated by a private command specifier. Then come the parameters, in this case, bytes containing ASCII codes for decimal representations of numbers indicating the modes which are to be set or reset by the command. Each parameter is separated from the next by the ASCII code for the ";" character. The last byte in the command contains the command specifier, in the case of protocol 601, the ASCII code for "h" and in the case of protocol 603, the ASCII code for "1". Further protocols are shown in Fig. 7. Data transfer control codes 702 are simply byte codes with the hexidecimal values shown; data transfer commands are ANSI standard commands, and therefore lack either the byte containing the code for ">" or a private command specifier. Except as previously described, uP 201 responds to the protocols only when TERM 103 is the destination of data from HC 101. In the case of the control codes, uP 201 responds directly to the control code to carry out the action indicated by the control code. In the case of the commands, uP 201 collects the parameters and command specifier in COMB 251 and then executes the specified command using the parameters. Beginning with the source and destination of data being transferred, the destination for data on data path 401 may be controlled by means of the data transfer commands CSI 4 i 713 and CSI 5 i 715, shown in Figure 7. CSI 4 i establishes TERM 103 as the destination of data on path 401, while CSI 5 i establishes AD 109 as the destination. Within CTLD 239, the variable RXDEST 519 indicates the last of commands 713 and 715 received in TERM 103. In response to CSI 4 i 713, uP 201 sets RXDEST 519 to 0; in response to CSI 5 i 715, uP 201 sets RXDEST 519 to 1. When RXDEST 519 has the value 0, P 201 loads the byte at HRXR 415 in HRXQ 241 into DATA 409 for processing; when RXDEST 519 has the value 1, uP 201 loads that byte at ATXW 425 in ATXQ 247. The source of data received by HC 101 is controlled by the setting of the transmit source (TS) mode. When the TS mode is reset, TERM 103 is the source; when the mode is set, AD 109 is the source. The TS mode may be set or reset either by means of the mode protocols shown in Fig. 6 or as a side effect of CSI 4 i 713 and CSI 5 i 715. The latter occurs only when the link routing mode (LR) is set. When that is the case, the source and destination routes are linked so that they are both to either TERM 103 or AD 109 and both change together in response to CSI 4 i 713 and CSI 5 i 715. When the LR mode is reset, CSI 4 i 713 and CSI 5 i 715 affect only path 401. The TS and LR modes are controlled by bits in mode flags 505 in CTLD 239. The TS mode is controlled by TS bit 517 in the variable mode flags 2 (MF2) 512 of flags 505; the LR mode is controlled by LR bit 507 in the variable mode flags 1 (MF1) 506; both are set and reset by uP 201 in response to the mode protocols shown in Fig. 6. Thus, when uP 201 receives set mode protocol 601 with parameter 33 for the link routing mode, it responds thereto by setting LR bit 507, and when it receives protocol 601 with parameter 32 for the transmit source mode, it sets TS bit 517. uP 201 resets the bits when it receives parameter 32 or 33 in reset mode protocol 603. When LR bit 507 is set and uP 201 receives a CSI 4 i protocol, uP 201 further responds to LR bit 507 by resetting TS 517 and to CSI 5 i by setting TS 517. When TS 517 is reset, uP 201 responds to the presence of a byte in TRXQ 240 by moving it to HTXQ 243 for transmittal to HC 101; when TS 517 is set, uP 201 responds to the presence of a byte in ARXQ 245 by moving it to HTXQ 243. Thus, by means of set mode protocol 601, reset mode protocol 603, CSI 4 i protocol 713, and CSI 5 i protocol 715, a program executing on HC 101 can specify any of four possibilities for paths 401 and 403: AD 109 as both source of and destination for data transferred between TERM 103 and HC 101; TERM 103 as both source of and destination for data transferred between TERM 103 and HC 101; AD 109 as the source and TERM 103 as the destination for data transferred between TERM 103 and HC 101; and TERM 103 as the source and AD 109 as the destination for data transferred between TERM 103 and HC 101. To ensure rapid switching of path 403 from AD 109 to TERM 103 response to a change in TS 517, ARXQ 245 may transfer bytes to HTXQ 243 only when HTXQ 243 contains less than 32 characters and the transfer will stop when the number of bytes in HTXQ 243 reaches 32. Consequently, HTXQ 243 will never contain more than 32 bytes of data from AD 109. 6. Flow Control on Paths 401 and 403: Figs. 5 - 7 The queues in paths 401 and 403 have only a limited capacity. In TERM 103, HRXQ 241, HTXQ 243, ATXQ 247, and ARXQ 245 each have a maximum capacity of 256 bytes. Similarly, HC 101 or AD 109 may have limited buffer space for receiving transmissions from TERM 103. Consequently, it is necessary to control the flow of data between HC 101 and TERM 103 and between TERM 103 and AD 109 to ensure that HC 101 does not send data to TERM 103 when there is no room in HRXQ 241 or ATXQ 247 and AD 109 does not send data to TERM 103 when there is no room in ARXQ 245 or HTXQ 243 and that TERM 103 does not send data to HC 101 or AD 109 when they have no room for it. As shown in Fig. 7, the ANSI standard protocols include two flow control protocols sent by the device receiving data to the device providing data to indicate whether the device providing data should start or stop transmitting. XON protocol 703, which has the value X′11′, indicates that the device providing the data may start transmitting. XOFF protocol 705, which has the value X′13′, indicates that the providing device must cease transmitting. TERM 103 has a set up mode which permits a user of the terminal to specify matters including whether TERM 103 will provide XON/XOFF protocols to HC 101 and respond to those protocols from HC 101 and whether TERM 103 will provide XON/XOFF protocols to AD 109 and respond to those protocols from AD 109. In the set up mode, uP 201 executes a program which provides menus indicating the set up options to the users and which then sets variables in CTLD 239 according to the options chosen by the users. The variables set in set up mode may be copied to NVRAM 311, where they are retained during periods when TERM 103 is turned off. In a preferred embodiment of TERM 103, the variables governing generation of and response to XON/XOFF are HCOF 529, which indicates whether TERM 103 is to respond to XON/XOFF protocols from HC 101 and provide them to HC 101, and ACOF 530, which indicates whether TERM 103 is to respond to XON/XOFF protocols from AD 109 and provide them to AD 109. When these variables specify automatic XON/XOFF for either port, the effect on incoming XON/XOFF protocols is the following: the interrupt code which responds to interrupts from the port checks for and responds to XON/XOFF codes; otherwise, it does not. When an XOFF code is encountered in HIBR 405, the interrupt code sets a variable to indicate that fact in HOFSTATE 531; as long as that variable indicates XOFF, no bytes are transmitted from HTXQ 243; when an XON protocol is encountered, the variable is reset and transmission resumes. XOFF and XON protocols received in ARXQ 245 are handled the same way, except that the variable set is in AOFSTATE 533 and the queue affected is ATXQ 247. Since checking for XON/XOFF is done by the interrupt code, TERM 103 responds to the protocols even though ARXQ 245 or HRXQ 241 is full. The effect regarding outgoing XON/XOFF protocols is the following: the code which manages HRXQ 241, ATXQ 247, ARXQ 245, and HTXQ 243 constantly checks the state of these queues to determine whether an XOFF protocol should be generated and if one has been, whether an XON protocol should be generated. Once an XOFF protocol has been generated to HC 101 or AD 109, TERM 103 keeps on receiving data from the device concerned until the queue which is receiving the data is actually full. At that point, TERM 103 discards further received characters; however, as mentioned above, the interrupt handling code continues to respond to XON, XOFF, INT, and NULL protocols and, as will be explained below, TERM 103 also always responds to a CSI 4 i command. When the queue management code determines that HRXQ 241 or ARXQ 245 can no longer receive data it checks the variables to determine whether automatic XON/XOFF is specified for the port; if it is, the code transmits an XOFF protocol by the relevant port; similarly, the code which uP 201 executes when HRXQ 241 or ARXQ 245 can again receive data checks the variables and transmits an XON protocol by the relevant port if automatic XON/XOFF is specified for the port. If TERM 103 was set up with automatic XON/XOFF specified for transfers between HC 101 and AD 109, HC 101 may nevertheless override the set up. HC 101 does so by means of a set mode protocol 601 which has the parameter 39, indicating that flow control protocols and signals from AD 109 are to be ignored by TERM 103. uP 201 responds to parameter 39 in set mode protocol 601 by setting ignore auxiliary flow control (IAF) bit 514 in MF 2 512. When that bit is set, the interrupt code which responds to interrupts in AUXP 227 ignores XON/XOFF and any flow control signals from AD 109 regardless of the setting of ACOF 530. If there is data in ATXQ 247 awaiting an XON protocol from AD 109, that data is sent as soon as the set mode protocol 601 sets IAF bit 514. When the bit is reset by means of reset mode protocol 603, TERM 103 again responds as determined by ACOF 530. Continuing with the generation of XON/XOFF protocols in TERM 103 when automatic XON/XOFF is set, the manner in which these protocols are generated depends on which path and which destination is involved. With from host path 401, when the destination is TERM 103, generation of XON/XOFF protocols depends only on the state of HRXQ 241. The code which manages HRXQ 241 transmits an XOFF protocol via HOSTP 223 to HC 101 when the values of queue pointers HRXR 415 and HRXW 417 (in QPTRS 503 in Fig. 5) indicate that there are less than 64 empty bytes in HRXQ 241. Once the code has transmitted an XOFF protocol, it waits until the queue pointers indicate that there are fewer than 32 bytes containing data in HRXQ 241 before it transmits an XON protocol to HC 101. Variables indicating the last flow control protocol to be transmitted are contained in HOFSTATE 531. If the destination of data on path 401 is AD 109, generation of XON/XOFF protocols to HC 101 depends on the state of both HRXQ 241 and ATXQ 247. uP 201 issues an XOFF protocol to HC 101 when either queues lack sufficient space and an XON protocol to HC 101 only when both queues have sufficient space. The code which manages HRXQ 241 and that which manages ATXQ 247 examine queue pointers HRXR 415, HRXW 417, ATXR 423, and ATXW 425 and issue an XOFF protocol if there are either fewer than 64 free bytes in HRXQ 241 or fewer than 96 free bytes in ATXQ 247. Once the code has issued an XOFF protocol, it issues an XON protocol only when ATXQ 247 and HRXQ 241 each contain fewer than 32 characters. Thus, when HC 101 receives an XOFF, there is either room in HRXQ 241 or room in ATXQ 247. After sending the XOFF, TERM 103 continues receiving bytes in HRXQ 241 and processing the bytes, either consuming them in TERM 103 or sending them to ATXQ 247, depending on whether TERM 103 or AD 109 is the destination. The bytes are further processed significantly faster than they are sent from HC 101 to HRXQ 240. Thus, if HC 101 responds to the XOFF by immediately sending a CSI 4 i 713 protocol to TERM 103, there should be room in HRXQ 241 to receive the protocol. Once received, the protocol is processed by TERM 103 and TERM 103 becomes the destination. XOFF is generated, there will always be room in HRXQ 241 to receive and process a CSI 4 i 713 protocol making TERM 103 the destination. Emergency control of TERM 103 by HC 101 is achieved in a preferred embodiment by means of a novel response to the ANSI INT protocol. The response is enabled when the enable interrupt control mode is set. The mode has the parameter 40. As previously indicated, the INT protocol is responded to by the code which responds to the interrupt from HOSTP 223 when a byte is received from HC 101. TERM 103 consequently will respond to INT even when HRXQ 241 is full. When TERM 103 receives the INT protocol and the enable interrupt control mode is set, uP 201 responds to the protocol by immediately clearing HRXQ 241 and ATXQ 247, immediately terminating processing of any command protocol currently being processed, reenabling HRXQ 241 for receipt of data, clearing HTXQ 243 and ARXQ 245, enabling TERM 103 as the destination on path 401 and the source on path 403, and if flow control is enabled, sending XON to HC 101 and a flow control signal to AD 109 to indicate that HRXQ 241 and ARXQ 245 can receive data. With to host path 403, generation of XON/XOFF protocols to AD 109 is determined by the state of ARXQ 245, as determined from the values of ARXR 427 and ARXW 429. XOFF is generated when ARXQ 245 has fewer than 64 bytes free; XON is generated when ARXQ 245 next contains fewer than 32 bytes of data. State indicating which was last generated is contained in AOFSTATE 533. As previously mentioned, ARXQ 245 transfers data to HTXQ 243 only when HTXQ 243 contains fewer than 32 bytes and transfers no more than 32 bytes at a time. 7. Control of Access to Path 403: Figs. 5-7 A difficulty with placing control of the source of data for to-host path 403 with HC 101 is that the device which is presently not a source for HC 101 may need to communicate with HC 101. For example, if AD 109 is presently the source and a user of TERM 103 inputs data to TERM 103 to which HC 101 must respond, HC 101 will not know that it needs to respond until it issues a command which resets TS 517. TERM 103 uses two approaches to solve this problem: a message waiting enable mode which permits TERM 103 to transmit message waiting control code (MW) X′95′ to HC 101 when the source which is not presently transmitting to HC 101 has data to transmit and an interleaving mode which permits TERM 103 to interleave data from the terminal with data from AD 109 when TS 517 specifies AD 109 as the source of the data being transmitted. The two modes are mutually exclusive, i.e., no MW protocol will be sent when TERM 103 is in interleaving mode. The modes are controlled by bits in MF1 506 and MF2 512 and are set and reset by set mode protocol 601 and reset mode protocol 603. The message waiting enable mode is controlled by MW bit 511 of MF1 506; the parameter used to set or reset the mode is 35; the interleaving mode is controlled by IL bit 509 in MF1 506; its parameter is 34. A third mode, the tag mode, is used together with the interleaving mode to specify that a tag code be inserted in front of a sequence of bytes to indicate whether the bytes are from TERM 103 or AD 109. The mode is controlled by TA bit 515 in MF2 512 and its parameter is 31. Beginning with the message waiting enable mode and taking the situation in which AD 109 is the source of data in path 403, when MW bit 511 is set, the code which manages TRXQ 240 branches when the pointers in the queue indicate that it contains a byte. The code to which it branches examines MW bit 511, and if it is set, further examines the variable TMWS 523, which is set when the MW protocol is sent, thus preventing repeated sending of the protocol. If TMWS 523 is not sent, the code outputs the MW code to HTXQ 243. When HC 101 receives the MW code and responds with a command which resets TS bit 517 so that the terminal is the source, the code which executes the command resets TMWS 523. When TERM 103 is the source of data on path 403 and a byte from AD 109 appears in ATXQ 247, the procedure outlined above is followed, except that the variable which is checked before sending the MW protocol, set after it is sent, and reset when TS 517 is set to specify AD 109 as the source is AMWS 525. Continuing with the interleaved mode, that mode is effective only when TS bit 517 specifies that AD 109 is the source. When IL mode 509 is set, data from TERM 103 is interleaved with data from AD 109; when it is reset, the data comes solely from AD 109. Interleaving is managed as follows: when the source which is not presently transmitting has data, the transmitting source is permitted to transfer only a limited number of bytes more from its queue to HTXQ 243. When AD 109 is the transmitting source, it is permitted to transfer bytes from ARXQ 245 to HTXQ 243 until it has transmitted 20 more bytes or until ARXQ 245 is empty, whichever happens first. When TERM 103 is the transmitting source, it is permitted to transfer bytes from TRXQ 240 to HTXQ 243 until TRXQ 240 is empty when TRXQ 240 contains less than 20 characters, until it has transmitted 20 bytes, or if the 20 bytes include the beginning of a protocol, until the protocol is finished. Transmitting complete protocols is permitted in order to simplify handling of protocols in HC 101. If TA 515 indicates that tags are to precede series of bytes from TERM 103 and AD 109, an X′12′ code is placed in HTXQ 243 ahead of a series from TERM 103 and an X′14′ code ahead of a series from AD 109. In TERM 103, switching from one source to another is controlled by a counter maintained in one of the registers of uP 201 and the variable TAGS 521, whose value indicates the source of the data presently being transmitted to HC 101. When IL 509 is set and a switch has been made, TAGS 521 is set to indicate the new source and the counter begins counting. When 20 bytes have been output from the source queue to HTXQ 243, when the source queue is empty, or in the case of TRXQ 240, the protocol has been finished, TAGS 521 is examined to determine the current source, the source is switched, TAGS 521 is set to indicate the new source, the counter is reset, and transfer from the new source queue to HTXQ 243 begins. If TA 515 indicates that tags are in use, the tag for the source indicated by the new value of TAGS 521 is put at the tail of HTXQ 243 before transmission from the new source to HTXQ 243 begins. 8. Transparency Mode Single-byte ANSI codes may contain 7 or 8 significant bits. Since ANSI codes may be employed in environments in which only 7 significant bits can be transmitted and in environments in which devices employ 8-bit non-ANSI codes, there is an ANSI code consisting of a prefix and a code value corresponding to each possible 8-bit code. A transparency mode in TERM 103 automatically converts 8-bit codes to their prefix and code equivalents and vice-versa. The conversion consists of subtracting a value from the 8-bit code to produce a new code and then adding a prefix. The kind of prefix depends on whether the data link between HC 101 and TERM 103 transfers 7 or 8 bit codes. In the former case, the prefix is two 7-bit codes; in the latter case, the prefix is a single 8-bit code. The value of the prefix and the value subtracted from the original code depend on the value of the 8-bit code being converted. The rule is the following: With the 8-bit data link, PREF2 has the value X′8E′ and PREF3 has the value X′8F′; with the 7-bit data link, PREF2 has the value ESC X′4E′ and PREF3 has the value ESC X′4F′. Transparency mode is controlled by transparency mode parameter 30 in set mode protocol 601 and reset mode protocol 603; the current setting is retained in TR bit 513 of MF2 512. The value of the variable PREF 527 indicates whether PREF2 or PREF 3 is required. When data is being transferred from HC 101 to AD 109 and TR bit 513 indicates that the transparency mode is set, uP 201 watches for PREF 2 and PREF 3 when it transfers codes from HRXQ 241 to ATXQ 247. If it encounters a prefix, it does not transfer the prefix to ATXQ 247, but instead waits for the code following the prefix, performs the operation on the code specified by the prefix, and then transfers the resulting code to ATXQ 247. uP 201 detects the two-byte prefixes by watching for ESC as it transfers codes from HRXQ 241 to ATXQ 247. If one is received, uP 201 watches for either X′4E′, to which it responds by setting PREF 527 to indicate PREF2, or X′4F′, to which it responds by setting PREF 527 to indicate PREF3. uP 201 detects the single-byte prefixes by watching for X′8E′ and X′8F′. uP 201 responds to X′8E′ by setting PREF 527 to indicate PREF2 and to X′8F′ by setting PREF 527 to indicate PREF3. The prefixes are not transferred to ATXQ 247, and when uP 201 transfers the next code, it determines from the value of PREF 527 and the value of the code whether it should add X′5E′ or X′BC′ to the code's value. uP 201 then transfers the result of that operation to ATXQ 247. When data is being transferred from AD 109 to HC 101 and transparency mode is enabled, uP 201 performs the reverse conversion as it transfers codes from ARXQ 245 to HTXQ 243. If the code's value is in the range X′20′ to X′7E′, uP 201 simply transfers the code; if the code's value is in the range X′7F′ to X′DC′, uP 201 sets PREF 527 to indicate PREF2; if it is in the range X′DD′ through X′1F′, uP 201 sets PREF 527 to indicate PREF3. If PREF2 is indicated and there is a 7-bit link between HC 101 and TERM 103, uP 201 places ESC and H′4E′ in HTXQ 243, subtracts X′5E′ from the code, and then places the result in HTXQ 243. If PREF3 is indicated and there is a 7-bit link, uP 201 proceeds as described, except that it places ESC and H′4F′ in HTXQ and subtracts X′BC′. If there is an 8-bit link, uP 201 places X′8E′ and the code minus X′5E′ in the buffer when PREF2 is indicated and X′8F′ and the code minus X′BC′ when PREF3 is indicated.

Claims: Display apparatus (103,105) connected to a host processor (101) and to an auxiliary device (109), comprising: a first host path (403) for selectively transferring data from either the display apparatus (103, 105) or the auxiliary device (109) to the host processor (101); and a second host path (401) for selectively transferring data from the host processor (101) to either the display apparatus (103, 105) or the auxiliary device (109); characterized by: data transfer control means (201, 235, 239) for controlling logic means (223, 227, 240, 243, 245) connected to the first host path (403) and logic means (223, 227, 241, 247) connected to the second host path (401) such that, the auxiliary device (109) functions as both a source of and a destination for data transferred between the display apparatus (103, 105) and the host processor (101); or the display apparatus (103, 105) functions as both a source of and a destination for data transferred between the display apparatus (103, 105) and the host processor (101); or the auxiliary device (109) functions as a source of data and the display apparatus (103, 105) functions as a destination for data transferred between the display apparatus (103, 105) and the host processor (101); or the display apparatus (103, 105) functions as a source of data and the auxiliary device (109) functions as a destination for data transferred between the display apparatus (103, 105) and the host processor (101). The apparatus of claim 1, wherein the display apparatus (103, 105) is responsive to protocols from the host processor (101); characterized in that: the protocols include transfer control protocols (601, 701); and the data transfer control means (201, 235, 239) includes means (201) for controlling the first host path (403) and the second host path (401) responsive to the transfer control protocols (601, 701). The apparatus of claim 2, further characterized in that the transfer control protocols (601, 701) include: a protocol (611) for specifying the source of data being transferred to the host processor (101); and a protocol (713, 715) for specifying the destination of data being transferred from the host processor (101). The apparatus of claim 3, wherein: the host processor (101) is responsive to protocols from the display apparatus (103, 105); the transfer control protocols include a transmission termination request protocol (705) which is sent from the display apparatus (103, 105) to the host processor (101) and to which the host processor (101) responds by suspending the current transmission to the display apparatus (103, 105); and the second host path (401) includes a receiving buffer (241, 247) for receiving data from the host processor (101); characterized in that: the data transfer control means (201, 235, 239) includes means (201) for monitoring the receiving buffer (241), sending a termination request protocol (705) to the host processor (101) when there is still sufficient room in the receiving buffer (241) to contain a protocol (713, 715) specifying the destination of data being transferred from the host processor (101), and causing the second host path (401) to continue receiving data from the host processor (101); and the display apparatus (103, 105) includes means (201) which will respond to a destination specification protocol (713, 715) sent by the host processor (101) on transmission of the transmission termination request protocol (705). The apparatus of claim 2 wherein the host processor (101) is responsive to protocols from the display apparatus (103, 105); characterized in that: the transfer control protocols include: a first tag protocol (707) sent by the display apparatus (103, 105) to the host processor (101) indicating that the following data is from the display apparatus (103, 105); and a second tag protocol (709) sent by the display apparatus (103, 105) to the host processor (101) indicating that the following data is from the auxiliary device (109); and the data transfer control means (201, 235, 239) includes means (201) for alternately and for a fixed period of time transferring data from the auxiliary device (109) to the host processor (101) and from the display apparatus (103, 105) to the host processor (101) and preceding the data from the display apparatus (103, 105) with the first tag protocol (707) and the data from the auxiliary device (109) with the second tag protocol (709). The apparatus of claim 2 wherein the host processor (101) is responsive to protocols from the display apparatus (103, 105); characterized in that: the transfer protocols include a message waiting protocol (711) sent from the display apparatus (103, 105) to the host processor (101) which indicates that the one of the display apparatus (103, 105) and the auxiliary device (109) which is not currently transferring data to the host processor (101) has data to transfer; and the data transfer control means (201, 235, 239) includes means (201) for monitoring the first host path (403) to determine whether the one of the display apparatus (103, 105) and the auxiliary device (109) not currently transferring data has data to transfer, and when that is the case, inserting the message waiting protocol (711) in the data being transferred to the host processor (101).

Inventors: MOONEY ROBERT C, PEIRENT RICHARD J, MOONEY, ROBERT C., PEIRENT, RICHARD J.
Applicants: WANG LABORATORIES, WANG LABORATORIES INC.
Application year: 1987
Main classification: G06F3
IPC codes: G06F13, G09G5, G06F3