PDP-10 EvenOdd1./title></head><body> MIT PDP-10 'Info' file converted to Hypertext 'html' format by <a href="ftp://ftp.netcom.com/pub/hb/hbaker/home.html">Henry Baker</a><p> <a href="Even-Odd.html">Previous</a> <a href="pdp-10.html">Up</a> <a href="EvenOdd2.html">Next</a> <h1>Separate Even Characters, Odd Vowels, and Odd Consonants.</h1> The following program accepts a line of input (terminated by carriage return) from the terminal and outputs the even (i.e., every second) letters followed by those odd letters that are not vowels, followed by those odd letters that are vowels.<p> In the program of example 4, take the code <pre> L2: MOVEI A,0 ;Store a terminating character (code zero) IDPB A,B ;at the end of the string in BUF. ;Now it is an ASCIZ string. MOVEI A,BUF PUSHJ P,OUTSTR ;Call OUTSTR to output ASCIZ string ;starting at address in A. MOVEI A,[ASCIZ / /] PUSHJ P,OUTSTR ;Output a CRLF. JRST L </pre> and replace it with <pre> L2: MOVEI A,0 ;Deposit code zero to terminate string IDPB A,B ;of odd letters (i.e., make it ASCIZ). MOVE B,[440700,,BUF] ;Take pointer for odd letters MOVE C,B ;Put pointer for vowels L3: ILDB A,B ;Get one odd letter. JUMPE A,L4 ;Stop scanning when we reach the zero. PUSHJ P,ISVOW ;Is this a vowel? JRST L3A ;No. IDPB A,C ;Yes. Store for later. JRST L3 L3A: .IOT CHTTYO,A ;Not a vowel, so output now. JRST L3 L4: IDPB A,C ;Store code zero to end string of vowels. MOVEI A,BUF PUSHJ P,OUTSTR ;Output that string. MOVEI A,[ASCIZ / /] PUSHJ P,OUTSTR ;Output a CRLF. JRST L ;Subroutine to skip if the character in A is a vowel. ISVOW: CAIE A,"A ;If character is upper case A CAIN A,"a ;or if it is lower case A, JRST POPJ1 ;jump to a skip return. CAIE A,"E ;Same for E, etc. CAIN A,"e JRST POPJ1 CAIE A,"I CAIN A,"i JRST POPJ1 CAIE A,"O CAIN A,"o JRST POPJ1 CAIE A,"U CAIN A,"u JRST POPJ1 CAIE A,"Y CAIN A,"y JRST POPJ1 POPJ P, ;Not a vowel, so return with no skip. ;Standard address of single skip return. POPJ1: AOS (P) POPJ P, </pre> <h2>NOTES:</h2> <ul> <li>Skip return: A subroutine is said to skip return when it skips the instruction which follows the call. The subroutine call instruction therefore acts as a conditional skip instruction. Subroutines called with a PUSHJ skip return by incrementing the return address where it lives on the stack. Subroutines called in other ways implement skip returning in other ways. <li>POPJ1: This is the standard name for a place to jump to for a skip return. Since the procedure for a skip return is independent of the address of the subroutine, every subroutine can use the same POPJ1. If you see the label POPJ1 used, you can assume it is a skip return of the sort shown above. <li>Consecutive skip instructions appear in ISVOW. When this happens, each instruction that can be skipped is indented one more than the previous one.<p> If, on the other hand, you had a subroutine that could skip twice, and followed it by two non-skipping instructions, each of those instructions would be indented only once. <li>Re-using the string: as we scan the string of odd characters for vowels, we print out the consonants and store the vowels in a string again, reusing the same space. The reason why it works to be reading out one string and writing another string in the same memory space is that we write at most one character for each character we read. So we can never clobber a character that has not been read. Even if every character is a vowel, the characters are stored into the bytes they have just been read out of, so nothing is lost. <li>Lower case letters make a difference in a character constant (such as "a). </ul></body></html>