HOPL

Illustrative Symbolic Programs - I
Figure VII-3 is a symbolic program for processing data read in from magnetic tape in Single Instruction Mode. The program reads 50 signed words, none of which contain all ZEROes, from magnetic tape unit 3. As the words are read into core memory, a count is kept of those which are negative.
The ORC (origin) pseudo-operation specifies that the program is to begin in memory starting at decimal location 00500; the symbol START will be assigned the value 00500, and the 94AP location counter will begin there.
The operation CLA ZERO clears the Accumulator. 94AP interprets the code ZERO as a non-existent addressable register, with the result that the Accumulator is loaded with all ZEROes. Once the Accumulator is cleared, the STR COUNT instruction causes memory location COUNT to be cleared also. The RPT-STR sequence following clears 50 consecutive memory locations starting at location DATA. For details concerning the REPEAT function, see Central Processor Instructions in Section III.
The SNS instruction SETS the Interpret Sign (ISN) switch. The assembly program translates the mnemonic ISN into the binary designation for the Interpret Sign switch. In Single Instruction Mode, the ISN switch must be SET before each input-output instruction where interpret sign mode operation is desired.
The input-output instruction RAN (Read Alphanumeric) causes an In-Out Processor to be selected and attached to magnetic tape unit 3. MT3 is interpreted and translated by the assembly program. As soon as the selected process has been initiated, the Central Processor program proceeds to the next instruction, in this case LXS. When a Processor is immediately available, the Central Processor program is not held up at all.
While the magnetic tape unit is preparing to read, LXS causes index register 1 to be cleared (e.g. loaded with ZEROes) and index register 2 to be loaded with the binary equivalent of decimal 50. Immediately following initialization of the two index registers, a check is made to see if a word has arrived in memory from magnetic tape. On the first time through the program, for example, Index Register 1 contains all ZEROes. Thus the instruction CLA DATA, 1 causes the contents of location DATA to be loaded into the Accumulator. If a word from magnetic tape has not yet arrived in location DATA, that location will contain all ZEROes, since the RPT-STR sequence earlier in the program cleared it out. Consequently, the TRZ (Transfer on Accumulator ZERO) instruction will succeed and control will be transferred to *-I, or back to the CLA DATA, 1 instruction. In this manner, the program is held in a loop until data arrives from magnetic tape. As soon as a word arrives, the TRZ instruction fails and control is passed on to the next instruction in sequence, namely TRN (Transfer on Accumulator Negative).
If all 50 of the words read from magnetic tape were positive, the TRN instruction would consistently fail and the program would always proceed directly to the TRX (Transfer on Index) instruction. Each time TRX is executed, the contents of Index Register 1 arc incremented by ONE and the contents of Index Register 2 are decremented by ONE, except at the end of the Loop, when the contents of Index Register 2 have been counted down to ONE. Thus, the major 'loop in the program is between the TRX instruction and location LOOP. Each time CLA DATA, P is executed, a new word in memory is loaded into the Accumulator. Once TRX has counted the contents of Index Register 2 completely down, the Central Processor halts (HLT).
If the program encounters a negative word, the
TRN instruction succeeds and control is transferred to location NEGA below. The ADB (Add Modifier) instruction causes the contents of location COUNT (originally ZERO) to be incremented by O m each time a negative word is found. The Unconditional Transfer (TRU) instruction returns the program to the TRX instruction, and the loop continues.
Two sets of data locations are reserved for the program.
DATA BSS 50 saves 50 locations for the incoming data. COUNT BSS 1 saves one location for the negative-word counter. The END operation signals the assembly program that the end of the symbolic program has been reached. ORG, BSS, and END are pseudo-operations and do not appear in the machine- language program.

Figure VII-4 is an Order Sequence Mode program involving both an input operation and error print out. The program searches the first file on magnetic tape unit 1 for a key of decimal 5. When the key is located, the following 200 signed words are read into memory. Parity errors, if encountered during input, are not allowed to stop the operation. If the first file does not contain a key of 5, the message "no key on mtl" is printed out on the electric typewriter.

The program begins in memory location 00750. The first operation is a rewind instruction, which causes magnetic tape unit 1 to be rewound to the load point on the tape. The second operation is the Order Sequence Instruction SS, exercising several options.
The third S causes the ISN switch to be SET for all operations in the following Order Sequence. The suffix N causes the No Halt on Processor Error switch (NHP) to be SET, so that possible parity errors will not stop the operation. The integer 1 specifies that the Order Sequence program is to use Processor 1. The variable field of SSSNl indicates that the Order Sequence program beginning in location READ and that magnetic tape unit 1 is to be the medium.
The instruction following SSSNl is sense (SEN).
The variable field causes the Central Processor to wait at the SEN' instruction until the Order Sequence initiated by SSSNl is completed, i.e. as long as MTl is still connected. As soon as the sequence is complete, the Central Processor proceeds to the next instruction (HLT) and stops.
The first Order Sequence program begins at location READ. A11 orders in the sequence refer to magnetic tape unit 1. The first order, the Search Key, order (SKF), causes the Processor to search magnetic tape unit 1 for a key word containing the binary equivalent of a decimal IS in its a portion. As soon as the key word is located, control is transferred to the next order in sequence, namely SCWDP. If the key word is not located by the time an End-of-Pile (EOF) condition is reached on magnetic tape unit 1, No-Hey (NKY) program interrupt occurs, the Order Sequence is disrupted, and Central Processor control is transferred to memory location 00001. Interrupt may be caused either when a Block-End marker (BLE) is reached or an End-Of-File marker (EOF) is reached by making the SK -, order suffix either B (block) or F (file), respectively.
If the key word is located, the order SCWDP is initiated. The SC portion of the order specifies alphanumeric, read-in. The option W indicates that words (as opposed to blocks) are to be read. The D restricts the type of words read in to data words (as opposed to key words). Non-data words are merely skipped over. The final suffix P causes the Order Sequence to proceed to the next order as soon as the read operation has been completed. In this manner, 200 data words are read into memory starting with location DATA.
The order STD is a variation of the End Sequence order (ES). STD causes the Order Sequence to be terminated and the device and the Processor to be logically disconnected from the central computer. In the program illustrated, if the key is found, the Central Processor program ultimately halts at memory location 00752 (decimal). The second Order Sequence program (starting at location PRINT) is not executed.
If the key word is not found by the time an End- Of-File is reached, program interrupt occurs. Processor 1 and magnetic tape unit 1 are logically disconnected from the central computer. Central Processor control is transferred to memory location 00001. The second ORG (origin) pseudo-operation in the symbolic program causes the 94AP location counter to begin counting at location 00001. The Unconditional Transfer instruction (TRU) is placed in location 00001, so that when program interrupt occurs, the transfer instruction will transfer control of the Central Processor to location READ minus 2, which is the WAN instruction for initiating the Order Sequence program for error printout.
The second Start Order Sequence instruction, SS2, specifies that Processor 2 be used with the electric typewriter (FLX). It indicates that the Order Sequence program begins in location PRINP.
The first Order Sequence order at location PRINT is WA (Write Alphanumeric). Four alphanumeric words, beginning in location OUTPUT, are to be written on the electric typewriter. The first word contains the alphanumeric codes for two functions; lower case (octal 02) and carriage return (octal 04), to initialize the typewriter. The pseudo-operation BCI (binary-coded information) converts the following three words into the binary equivalents of the alphanumeric text in the variable field. Thus, the WA order causes the sentence "no key on ml' ' to be typed.
As soon as WAN has been executed, the electric typewriter is logically disconnected. The Central Processor halts at memory location 00754.

94AP(ID:7933/)

Sylviania symbolic assembler