24 Interactive Terminals

Our remaining task, to be completed in this and the following chapter, is to consider the code which controls interactive terminals (or “terminals”, for short).

A wide variety of terminals is available and several different types may be simultaneously attached to a single computer. Distinguishing characteristics for different classes of terminal include (besides such non-essential features as shape, size and colour):

(a)

transmission speed, e.g. 110 baud for an ASR33 teletype, 300 baud for a DECwriter, 2400 baud or 9600 baud for a Visual Display unit (“VD”);

(b)

graphic character set, notably the full ASCII graphic set and the 64 graphic subset;

(c)

transmission parity: odd, even, none or inoperatlve;

(d)

output technique: serial printer or visual display;

(e)

miscellaneous: combined carriage return/line feed character, half duplex terminal (input characters do not need echoing); recognition of tab characters;

(f)

characteristic delays for certain control functions, e.g. carriage returns may not be completed within a single character transmission time, etc.

As well as the wide variety of terminals which are available and in use, there is also a variety of hardware devices which may be used to interface a terminal to a PDP 11 computer. For example:

DL11/KL11

single line, asynchronous
 

interface; 13 standard
 

transmission rates between
 

40 and 9600 baud;

DJ11

16 line, asynchronous, buffered
 

serial line multiplexer; 11
 

speeds between 75 and 9600 baud,
 

selectable in four line groups;

DH11

16 line, asynchronous, buffered,
 

serial line multiplexer; 14 speeds,
 

individually selectable; DMA

 

transmission

Each of the above interfaces will work in full or half duplex mode; handle 5, 6, 7 or 8 level codes; generate odd, even or no parity; and generate a stop code of 1, 1.5 or 2 bits.

In addition to the above asynchronous interfaces, there are a number of synchronous interfaces, e.g. DQ11.

Each interface has its own control characteristics and it requires a separate operating system device driver. The common code which can be shared between these is gathered into a single file “tty.c”, to be found on Sheets 81 to 85. A set of common definitions is gathered in the file “tty.h” on Sheet 79.

By way of example, Sheet 80 contains the file “kl.c”, which constitutes the device driver for a set of DL11/KL11 interfaces. This device driver always needs to be present, since one KL11 interface is invariably included in a system for the the operator’s console terminal.

24.1 The ’tty’ Structure (7926)

An instance of “tty” is associated with every terminal port to the system (no matter what type of hardware interface is used). A “port” in this context is a place to attach a terminal line. Hence a DL11 supplies only one port, whereas a DJ11 supplies up to sixteen ports.

The “tty” structure consists of sixteen words and includes:

A.

t_dev

fixed for a particular

 

t_addr

terminal port;

B.

t_speeds

fixed for a particular

 

t_erase

terminal. These values may

 

t_kill

be set by “stty” and

 

t_flags

interrogated by “gtty”;

C.

t_rawq

list heads for three

 

t_canq

character queues: the

 

t_outq

so-called “raw” input,

   

“cooked” input and the

   

output queues;

D.

t_state

status information which

 

t_delct

changes frequently during

 

t_col

normal processing;

 

t_char

 
Table 24.1

24.2 Interactive Terminals

The reader should study the information on Sheet 79 carefully. Certain items listed below are not referenced in any essential way in the selection of code examined here. 

   t_char    (7940)    NLDELAY   (7974)
   t_speeds  (7941)    TBDELAY   (7975)
   HUPCL     (7966)    CRDELAY   (7976)
   ODDP      (7972)    WOPEN     (7985)
   EVENP     (7973)    ASLEEP    (7993)

24.3 Initialisation

Initialisation of the “tty” structures is the responsibility of the various “open” routines in the device drivers, for example, “klopen” (8023).

The items in Group B of Table 24.1 may be changed by a “stty” system call. The current values may be interrogated by a “gtty” system call.

A description of these is contained in the sections, “STTY(II)” and “GTTY(II)” of the UPM. These calls are invoked by the “stty” shell command which is described in the section “STTY(I)”.

Since the “stty” and “gtty” system calls require a file descriptor as a parameter, they can only be applied to an “open” character special file.

The two system calls share a good deal of common code. We will trace the progress of an execution of “stty” below and leave the tracing of a similar execution of “gtty” to the reader.

24.4 stty (8183)

This procedure implements the “stty” system call. It copies three words of user parameter information into “u.u_arg[..]” using the parameter supplied as a pointer, and then calls “sgtty”.

24.5 sgtty (8201)

8206:

Get a validated pointer to a “file” array entry;

8209:

Check that the file is a “character special”;

8213:

Call the appropriate “d_sgtty” routine for the device type. (See Sheet 46.)

Note that the “d_sgtty” routine is “nodev” for the line printer and paper tape reader/punch.

24.6 klsgtty (8090)

This is an example of a “d_sgtty” routine. It calls “ttystty” passing a pointer to the appropriate “tty” structure as a parameter.

24.7 tysty (8577)

A call originating from “stty” will have a second parameter of zero.

8589:

Empty all the queues associated with the terminal forthwith. They quite likely contain nonsense;

8591:

Reset the speed information (useful in the case of a DH11 interface, but of little interest for the present selection of code);

Reset the “erase” character and the “kill” character. (“kill” here denotes “throw away the current input line”.) Note that if these characters are changed away from their normal values of “#” and “@” respectively, no corresponding changes are made to “maptab”. Nor should they!),

8593:

Reset the “flags” defining some relevant terminal characteristics (see Sheet 79):

flag

bit

if set ...

XTABS

1

the terminal will not interpret

   

horizontal tab characters
   

correctly;

LCASE

2

the terminal supports only the

   

64 character ASCII subset;

ECHO

3

the terminal is operating in

   

full duplex mode, and input

   

characters must be echoed

   

back;

CRMOD

4

upon input, a “carriage

   

return” is replaced by a

   

“line feed”; upon output, a
   

“line feed” is replaced by a
   

“carriage return” and a “line
   

feed”;

RAW

5

input characters are to be

   

sent to the program exactly

   

as received, without “erase”
   

or“kill” processing, or
   

adjustment for backslash
   

characters.

In addition, the following bits are interrogated by “ttyoutput” (8373) in choosing the delay which should ensue after the character indicated is sent, before sending the next character:

8,9

line feed;

10,11

horizontal tab;

12,13

carriage return;

14

vertical tab or form feed.

24.8 The DL11/KL11 Terminal Device Handler

The file “kl.c” constitutes the device handler for terminals connected to the system via DL11/KL11 interfaces. This group always has at least one member – the operator’s console terminal. Hence this device handler will always be present.

Each DL11/KL11 hardware controller provldes an asynchronous, serial interface to connect a single terminal to a PDP 11 system. For more complete details regarding this interface the reader should consult the “PDP11 Peripherals Handbook”.

24.9 Device Registers

Each DL11/RL11 unit has a group of four registers occupying four consecutive words on the UNIBS. UNIX maps a structure of type “klregs” (8016) onto each register group.

Receiver Status Register (klrcsr)

bit 7

Receiver Done. (A character has
 

been transferred into the
 

Receiver Data Buffer Register.);

bit 6

Receiver Interrupt Enable.
 

(When set, an interrupt is
 

caused every time bit 7 is set.);

bit 1

Data terminal ready;

bit 0

Reader Enable. Write only.
 

(When set, bit 7 is cleared.).

Receiver Data Buffer Register (klrbuf)

bit 15

Error indication, when set.

bits 7-0

Received character, Read
 

only.

Transmitter Status Register (kltcsr)

bit 7

Transmitter ready. This is
 

cleared when data is loaded
 

into the Transmitter Data
 

Buffer, and is set when the
 

latter is ready to receive
 

another chatacter;

bit 6

Transmitter Interrupt Enable.
 

(when set, causes an
 

interrupt to be generated
 

whenever bit 7 is set.)

Transmitter Data Buffer Register (kltbuf)

bits 7-0

Transmitted data. Write only.

24.10 UNIBUS Addresses

The Receiver Status Register always has its lowest address starting on a four word boundary. (The addresses which follow are all 18 bit octal addresses.)

 

Receiver

Transmitter
 

Status

Status

Operator’s console

777560 \(\rightarrow \)

777566

Group Two

776500 \(\rightarrow \)

776506
 

776510 \(\rightarrow \)

77656

 

776670 \(\rightarrow \)

776676

Group Three

775610 \(\rightarrow \)

775616
 

775620 \(\rightarrow \)

775626

 

776170 \(\rightarrow \)

776176

Apart from the operator’s console interface which has its own standard UNIBUS location, the interfaces are gathered into two groups (for reasons which are irrelevant here). Within each group, by convention, registers are allocated in consecutive locations starting at the lowest address.

24.11 Software Considerations

“NKL11” (8011) must be set to define, for a particular installation, the number of interfaces in the first two groups, and “NDL11” (8012), the nmber in the third group. Any hardware alterations which changed the actual number of interfaces would have to be reflected in the software by changing and recomiling “kl.c”, and relinking the operaing system.

It will be seen that “klopen” calculates the correct kernel mode address (16 bits) for the Receiver Status Register for each interface, and this is stored (8044) into the the “t_addr” element of the appropriate “tty” structure.

24.12 Interrupt Vector Addresses

The vector addresses for the first interface are 060 and 064 (for receiver and transmitter interrupts, respectively). Additional DL11/KL11 interfaces have vector addresses which are always at least 0300, and which are assigned according to rules which take into consideration other interfaces which may be present.

The second word of an interrupt doubiet is the “new processor status” word. The five low order bits of this word may be chosen arbitrarily, and are in fact used to define the minor device number (cf. a similar use to distinguish the various kinds of “traps” – see Sheet 05). A masked version of the new processor status word is provided to the interrupt handling routines as the parameter “dev” (see e.g. line 8070).

24.13 Source Code

We can now turn to a detailed study of the code in the files “kl.c” (Sheet 80) and “tty.c” (Sheets 81 to 85). We shall look first at “opening” and “closing” terminals as character special files and the handling of interrupts. Then in the next chapter we shall look at the receipt of data from the terminal, and finally transmission of data to the terminal.

“klread” (8062), “klwrite” (8066) and “klsgtty” (8090) have already been discussed above.

24.14 klopen (8023)

This procedure is called to “open” a terminal as a character special file. This call is usually made by the program “/etc/init” for each terminal which is to be active in the system. Since child processes inherit the open files of their parents, it is not usually necessary for other processes to “open” the device again. It will be noted that the there is no attempt to stop two unrelated processes having the terminal as an open file simultaneously.

8026:

Check the minor device number;

8030:

Locate the appropriate “tty” structure;

8031:

If the process opening the file has no associated controlling terminal designate the current terminal for this role. (Note that the reference stored is the address of a “tty” structure.):

8033:

Store the terminal device number in the “tty” structure

8039:

Calculate the address of the appropriate set of device registers for the terminal and store

8045:

If the terminal is not already “open”, do some initialisation of the “tty” structure ..

8046:

“t_state” is set to show the file is “open”, so that the next three lines will not be executed if the file is opened a second time, possibly undoing the effect of a “stty” system call:

“t_state” is also set to show “CARR_ON” (“carrier on”). This is a software flag which shows that the terminal is logically enabled, regardless of the true hardware status of the terminal. If “CARR_ON” is reset for a terminal, the system should ignore all input from the terminal.

(This does not seem to be entirely true, and this point will be taken up again later.);

8047:

The standard terminal is assumed to be unable to interpret horizontal tabs, to support only the 64 character ASCII subset, to run in full duplex mode and to require both “carriage return” and “line feed” characters to provide normal “new line” processing. (Could this be a Model 33 teletype?);

8048:

The “erase” and “kill” characters are set according to the UNIX convention;

8051:

The Receiver Control Status register is initialised with the pattern “0103” so that the terminal is made ready, reading is enabled and receiver interrupts are enabled;

8052:

The Transmitter Control Status register is initialised so that an interrupt will be generated whenever the interface is ready to receive another character.

Note that the “open” routine does not distinguish between the cases where the file is opened for reading only, or writing only, or for both reading and writing.

24.15 klclose (8055)

8057:

Find the address of the appropriate “tty” structure in the array of such structures, “kl11” (8015). (This operation may be observed in all the procedures in the second column of Sheet 80, and its relevance should be noted.)

8058:

“wflushtty” (8217) allows the output queue for the terminal to “drain” and then flushes the input queue

8059:

“t_state” is reset so that “ISOPEN” and
“CARR_ON” are no longer true.

24.16 klxint (8070)

This procedure is executed in response to a transmitter interrupt. It should be compared with “pcpint” (8739) and “lpint” (8976). Note that the parameter “dev” is a masked version (low order five bits preserved) of the “new processor status” word in the interrupt vector. Provided the vector was properly initialised, the minor device number will be properly identified.

The second part of the test on line 8074 will be discussed at the end of the next chapter.

24.17 klrint (8078)

This procedure is executed in response to a receiver interrupt. It is not so readily compared with “pcrint” (8719) although similarities certainly exist.

8083:

Read the input character from the Receiver Data Buffer register;

8084:

Enable the receiver for the next character;

8085:

The comment says “hardware botch”. Better believe it;

8086:

Pass the character to “ttyinput” to insert it into the appropriate “raw” input queue.