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

Sample Routine to do Buffered Input.

So far, the i/o in the examples has always transferred a single character at a time. When a large file is to be read or written, it is worth while to go to the extra trouble of transferring many words or characters at a time. There ar e two ways of doing this. The old way is to open the i/o channel in "block" mode instead of "unit" mode: use mode .BAI, .BAO, .BII or .BIO instead of .UAI, .UAO , .UII or .UIO. Then the .IOT UUO itself transfers a block of consecutive words instead of a single word or character. The newer way is to open the channel in unit mode as usual, and use the system call SIOT to transfer any number of consecutive bytes. SIOT can be interspersed with individual .IOTs.

We assume that INCHAN has a file open in mode .UAI, unit ascii input. We use the SIOT system call to read many characters at a time into a buffer, and then get them out of the buffer one by one.

SIOT expects three arguments: a channel number, a byte pointer and a number of characters. The byte pointer argument should not be a literal, because the SIOT increments it past the characters that are transferred. The count is also modified; it is decremented for all the characters transferred (decremented down to zero if the transfer is completed). On input, if end of file is encountered before the requested number of characters is obtained, then the byte pointer and count indicate the characters actually transferred.

INBFR:  BLOCK INBFL             ;This is the buffer.  40 to 200 is
                                ;good for INBFL (160. to 640. characters).

INCNT:  0                       ;Number of characters left in buffer
                                ;not fetched yet.
INPTR:  0                       ;Byte pointer used for fetching characters

;Read another input character into A.
;Skip if successful.  No skip if EOF.
GETCHR: SOSL INCNT              ;Anything left in the buffer?
         JRST GETCH1
        PUSH P,A                ;No - must read another load.
        PUSH P,B
        MOVE A,[440700,,INBFR]
        MOVEM A,INPTR
        MOVEI B,INBFL*5
        ;; Do the input.  A says where to put it and B says how many chars.
        .CALL [ SETZ ? SIXBIT /SIOT/ ? %CLIMM,,INCHAN
                        A ? 400000,,B]
                                ;Note "immediate" argument:
                                ;%CLIMM,,INCHAN is the same as [INCHAN].
         .LOSE %LSFIL
        MOVNS B                 ;B is left with number of characters
        ADDI B,INBFL*5          ;We wanted but didn't get.
        MOVEM B,INCNT           ;Compute how many chars we did get.
        POP P,B
        POP P,A
        SOSG INCNT              ;If we got none, it's eof.
         POPJ P,
GETCH1: ILDB A,INPTR
        AOS (P)
        POPJ P,

Note that this GETCHR routine is completely equivalent to

GETCHR: .IOT INCHAN,A
        SKIPGE A
         AOS (P)
        POPJ P,

but the buffered one is much faster because it does not have to do a system call for each character. If your program is supposed to process characters quickly, use buffered input. If characters will be read only infrequently and a few at a time, use simple input.