Previous Up Next The following program accepts a line of input (terminated by Carriage Return) from the terminal and outputs that line with the characters reversed.

        TITLE   REVERSE
A=1                             ;Symbolic AC names are defined
P=17

CHTTYO==1                       ;Channel for output
CHTTYI==2                       ;Channel for input

PDLLEN==100                     ;Length of push down stack
PDL:    BLOCK   PDLLEN          ;Storage for push down stack

BEG:    MOVE P,[-PDLLEN,,PDL-1] ;Initialize stack pointer.
                                ;Open TTY channels.
        .CALL [SETZ ? SIXBIT/OPEN/
                [.UAI,,CHTTYI] ? [SIXBIT/TTY/] ((SETZ))]
         .LOSE %LSFIL
        .CALL [SETZ ? SIXBIT/OPEN/
                [.UAO,,CHTTYO] ? [SIXBIT/TTY/] ((SETZ))]
         .LOSE %LSFIL
LOOP:   .IOT CHTTYO,["*]        ;Prompt for input.
        PUSHJ P,REVERS          ;do the work, once.
        .IOT CHTTYO,[^M]        ;Output CRLF to go to new line.
        .IOT CHTTYO,[^J]
        JRST LOOP               ;Jump back to repeat.

REVERS: .IOT CHTTYI,A           ;Read a character.
        CAIN A,^M               ;If it is a Carriage Return, the line is ended,
         POPJ P,                 ;so return.
        PUSH P,A                ;Else save this character on the stack,
        PUSHJ P,REVERS          ;call REVERS recursively,
        POP P,A                 ;get our character back
        .IOT CHTTYO,A           ;and print it.
        POPJ P,                 ;Return.

END BEG

NOTES:

• OPEN: Two tty channels are used, one for input and one for output. The input channel is opened in mode .UAI (unit ascii input) and the output channel in mode .UAO (unit ascii output). The mode is how the system knows whether to make an input channel or an output channel.
• .IOT: It works on input channels as well as on output channels. On an input channel, it stores the input character (or word) in the addressed memory location. The ITS system does not do any sort of input editing or "rubout processing". There is a library for that: SYSENG;RUBOUT >.
• To output "go to a new line", output a ^M followed by a ^J. ^M is the MIDAS syntax for the code for Control-M, which is Carriage Return. When the user types Carriage Return, which echoes as going to a new line, it is read by the program as only one character, a ^M.
• The subroutine REVERS is recursive. The first call to REVERS reads the first character, calls REVERS (to read the rest of the line and print it in reverse), and lastly prints the first character. Over the recursive call to REVERS, the first character is saved on the stack.
• P is the conventional name for the AC used as the stack pointer. The MOVE instruction is there to set it up with a pointer to the allocated area. Initializing a stack is one of the first things any real program should do.

  From then on, calling is done with PUSHJ and returning with POPJ. PUSH is used to save data on the stack, and POP to get it back.

• The POPJ P, which is skipped over by the CAIN is indented one extra column. If there is a sequence of skipping instructions, each instruction is indented one more than the previous one.

  It is reasonable to omit the address field of an instruction if it is unused (as it is in a POPJ). An omitted field assembles as zero, but you should never omit a field which is supposed to contain a zero which actually stands for something. For example, instructions which refer to AC zero should always have a zero (or better, a symbolic name) where the AC is referred to.