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

A Trivial Complete MIDAS Program.

This program stores, in each word of TABLE, the index of that word. Thus, the 0th word gets 0, the next gets 1, etc.

        TITLE   COUNT

A=1                             ;Define a name for an accumulator.

START:  MOVSI A,-100            ;initialize loop counter.
                                ;A contains -100,,0
LOOP:   HRRZM A,TABLE(A)        ;Use right half of A to index.
        AOBJN A,LOOP            ;Add 1 to both halves (-77,,1 -76,,2 etc.)
                                ;Jump if still negative.
        .VALUE                  ;Halt program.

TABLE:  BLOCK 100               ;Assemble space to fill up.

END START                       ;End the assembly.

NOTES:

TITLE: Every MIDAS program should begin with a TITLE statement which contains the program's name.
A: It is best to give names to the accumulators you use. Single character names are good except for an AC which is used generally for the same thing throughout the program. Define a symbolic name for an AC with the assembler operator =.
Comments: Everything after a semicolon is a comment, until the end of the line. Actually, this is only true if the semicolon is not inside of a text string of some sort.
START: This is a label, which marks a location in the program. The label START is used to name the instruction at which the program should start running. The name START itself is nothing special. MIDAS knows to use the value of START as the starting address because START is the operand of the END statement.
.VALUE: This is a system call instruction. It halts the program, and makes DDT print out the program counter. All system calls have names starting with a period.
BLOCK: The BLOCK n statement reserves n words of storage, which are not initialized.
END: The end statement tells MIDAS that the assembly input is finished, and its argument says what address to start the program at. In this case the argument is the value of the label START.