MIT PDP-10 'Info' file converted to Hypertext 'html' format by Henry Baker

Reading and Writing Files.

As a preface to this example, several things need to be explained. In the foregoing examples, only terminal I/O has been discussed. Here we introduce disk file input. One important thing to emphasize is the concept of device independence: most of the system calls for disk i/o are the same as those for say, magtape i/o. Naturally, different devices have different phyical characteristics; therefore, some differences between devices will be apparent. Despite these differences, a programmer really needs to learn only one set of basic concepts for i/o.

When doing i/o it is necessary to specify a physical device, a mode (input vs. output, character vs. word), and possibly a file name to select among "files" on a particular physical device.

Each job can has sixteen i/o channels available, each of which can be opened individually to do i/o. The same OPEN system call used to open a tty channel is also used to open i/o channels for other devices. OPEN also specifies the mode and filenames.

You have already seen the use of modes .UAI (unit ascii input) and .UAO (unit ascii output). Modes .UII and .UIO are "image" i/o instead of ascii i/o. They transfer whole words instead of characters, and are used for binary files.

There are two filenames, and also a directory name. These are specified as words of SIXBIT just like the device name.

Once the channel is open, the .IOT UUO is used to transfer the characters in or out, just as it is with the terminal. In fact, the above examples could be adapted to read and write input using disk files only by changing the two OPEN system calls. Alternatively, filename translations could be used to translate device TTY: to a disk file.

Writing output to a file does require one thing that writing output to the terminal does not need: you must "close" the channel when you are finished to make the file appear properly on disk. Closing a terminal i/o channel is also allowed but nobody bothers since it is not necessary. Closing is done with the .CLOSE UUO, which takes the channel number in its AC field just like .IOT.

Here is an example which copies the file SYS;SYSTEM MAIL (the announcements printed when DDT or PWORD starts up) to the file FOO BAR on your own directory.

        TITLE   FILE COPY
A=1

CHDSKI==1
CHDSKO==2

FCOPY:  .CALL [ SETZ ? SIXBIT/OPEN/
                [.UAI,,CHDSKI] ? [SIXBIT/DSK/]  ;Mode, channel and device name
                [SIXBIT /SYSTEM/] ? [SIXBIT/MAIL/]      ;Two filenames.
                400000,,[SIXBIT /SYS/]]                 ;Directory name.
         .LOSE %LSFIL
        .CALL [ SETZ ? SIXBIT/OPEN/
                [.UAO,,CHDSKO] ? [SIXBIT/DSK/]
                [SIXBIT /FOO/] ? 400000,,[SIXBIT/BAR/]] ;Note no directory name
!
                                ;The default (your working directory) is used.
         .LOSE %LSFIL
LOOP:   .IOT CHDSKI,A           ;Read next input character.
        JUMPL A,EOF             ;Negative => there is none, it's eof.
        .IOT CHDSKO,A           ;Else write char to output file.
        JRST LOOP

EOF:    .CLOSE CHDSKO,          ;Make output file appear.
        .CLOSE CHDSKI,          ;Release input file (to be clean).
        .LOGOUT 1,

        END FCOPY

NOTES:

This example is so simple that no stack is needed, and no tty channels.
When reading from a file instead of the terminal, it is possible to reach th e end of the file. In ascii mode (.UAI), this is signalled by a negative value returned by the .IOT. The right halfword of that value contains the character ^C, which is the standard end-of-file padding character on ITS. That is, ^C is used to fill out the last word of a file of characters if that last word is not entirely used up. In image mode, .IOT causes an error interrupt at end of file . Usually binary files are read many words at a time using SIOT (which will be explained soon) or block-mode .IOT; these do provide a way to detect end of fil e without getting an error.
.LOGOUT 1, is a convenient way to exit a program and kill the job. It kills the job if it is running under DDT (the usual condition) and also kills the job if it is disowned. When a program running under DDT does a .LOGOUT 1, DDT just prints an asterisk.