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

Alternative Implementation of Previous Example.

The following program performs the same function as the program written for example 5. Some different ideas are shown here.

        TITLE   EVEN ODD #2
A=1                                     ;SYMBOLIC AC NAMES ARE DEFINED
B=2
C=3
D=4
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

BUF:    BLOCK 30                ;Storage for 79 characters at 5 per word.
LINBUF: BLOCK 30                ;Read a line into here.
LINBFE::

START:  MOVE P,[-PDLLEN,,PDL-1]
                                ;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
L:      PUSHJ P,GETLIN          ;Read in the line.
        MOVE C,[440700,,LINBUF] ;Initialize byte pointer to fetch from the line
.
        MOVE B,[440700,,BUF]    ;Initialize byte pointer to store into BUF.
        SETZ D,0                ;Low bit of D is even-oddness of character.
L1:     ILDB A,C                ;Get next character.  Jump if end of line.
        JUMPE A,L2
        XORI D,1                ;Complement low bit -- count mod 2.
                                ;D is 1, then 0, then 1, then 0.
        XCT XTBL(D)             ;Execute an instruction to dispose of char.
        JRST L1

XTBL:   .IOT CHTTYO,A           ;Even character         non-vowel
        IDPB A,B                ;Odd character          vowel


L2:     MOVEI A,0               ;Deposit a null byte to make string
        IDPB A,B                ;   of odd letters ASCIZ.
        MOVE C,[440700,,BUF]    ;Fetch the string.
        MOVE B,C                ;Store back only the vowels.
L3:     ILDB A,C                ;Get one odd letter.
        JUMPE A,L4              ;Check for end of string.
        PUSHJ P,ISVOW           ;Test character, skip if vowel.
         TDZA D,D               ;Not a vowel, set D=0.
          MOVEI D,1             ;Vowel.  D=-1
        XCT XTBL(D)             ;Dispose of char depending.
        JRST L3

L4:     IDPB A,B                ;Store null to make string of odd vowels ASCIZ.
        MOVEI A,BUF
        PUSHJ P,OUTSTR          ;Type out that string.
        MOVEI A,[ASCIZ /
/]
        PUSHJ P,OUTSTR
        JRST L

ISVOW:
IRPC ZZ,,[AEIOUY]
        CAIE A,"ZZ
         CAIN A,"ZZ+40
          JRST POPJ1
TERMIN
        POPJ    P,

;Copy the GETLIN and OUTSTR subroutines from above into here.

        END     START

NOTES:

IRPC is an assembler macro operation which generates repetitive code with small variations. In this case, the three instructions CAIE, CAIN and JRST are repeated once for each vowel. The first time, ZZ is replaced by A. The second, ZZ is replaced by Z. The sixth time, ZZ is replaced by Y. As a result, the subroutine ISVOW in this example is actually identical to the ISVOW in the previous example, though it takes much fewer lines of source code.