8 - • • • • • • • • • •
FEIED- • • • • • • • • • • • • • -• • • • • • • • • • • • • • • • •
4 -
I... .... .... . ... . .. . ... .
2 - • • • • • • • • • • • • • • • •
1 - • • • • • • • • • • • • • • • • • •
•• ••• •
: .. : .: ".: •
Fig. 1.2. < Character Coding for Eight-Channel Paper Tape.
To check that each character is recorded correctly, each column of the tape is punched with an odd number of holes. A check hole must be present in any column whose basic code (X, 0, 8, 4, 2, 1) consists of an even number of holes. Internal checking devices in the 1620 investigate each vertical array of punches to assure that this condition has been satisfied.
A punch in the ElL (end-of-line) channel is a specific function char-acter used to mark the end of a record on the tape. < The tape feed code consists of punches in the X, 0, C, 8, 4, 2, and 1 channels and is used to indicate blank character positions. A paper tape reader automatically skips over areas of tape punched with the tape feed code.
Figure 1.2 shows the 1~20 coding for all characters on an 8-channel paper tape.
COMPUTER CHARACTERISTICS
MACHINE CYCLES
All computer operations take place in fixed time intervals called machine cycles. These time intervals are measured by pulses from an electronic clock in the system. Within a machine cycle, ,the computer can perform
8 Introduction to Data Processing Systems a specific machine operation. The number of machine cycles required to execute a single instruction depends on the nature and function of the instruction.
SERIAL AND PARALLEL OPERATION
Computers are classified as either serial or parallel depending on the method the computer uses to perform arithmetic. Essentially, all arith.
metic is performed by addition.
In a serial computer, numbers to be added are considered one position at a time (the units position, tens position, hundreds, and so on) in the same way that addition is done with paper and pencil. Whenever a carry is developed, it is retained temporarily and then added to the sum of the next higher order position.
The time required for serial operation depends on the number of digits in the factors to be added. Serial addition is shown in Table 1.1.
Table 1.1
1ST STEP 2ND STEP 3RD STEP 4TH STEP
Augend 1234 1234 1234 1234
Addend 2459 2459 2459 2459
Carry 1 1
Sum 3 93 693 3693
In a parallel computer, addition is performed on complete data words (a "word" is made up of two or more storage positions). The words are combined in one operation, including carries. Any two data words, regardless of the magnitude of the numbers contained in the words, can be added in the same time. Table 1.2 shows parallel addition.
Augend Addend Carry Final Result
Table 1.2
00564213 00000824
1 00565037
Introduction to Data Processing Systems 9 FIXED AND VARIABLE WORD LENGTH
"Fixed word length" and "variable word length" are terms used to describe the unit of data that can be addressed (referenced) and proc-essed by a computer system.
In fixed word length operation, information is handled and addressed in units or words containing a predetermined number of positions. The size of a word is designed into the system and normally corresponds to the smallest unit of information that can be addressed for processing in the central processing unit. Records, fields, characters, or factors are all manipulated in parallel as words.
In variable word length operations, data-handling circuitry is designed to process information serially as single characters. Records, fields, or factors may be of any practical length within the capacity of the storage unit. Information is available by character instead of by word.
Operation within a given data processing system may be entirely of a fixed word nature, entirely variable, or a combination of both. In the 1620 Data Processing System, data are stored and processed as single characters. All arithmetic and data-handling operations are done se-rially, character by character.
Chapter 2
Introduction to the 1620 Data Processing System
The 1620 Data Processing System (Fig. 2.1) is an electronic digital computer designed for technological and commercial applications. The heart of the system is the 1620 Central Processing Unit (Fig. 2.2) which houses the arithmetic and logical units, the magnetic core storage (20,000 positions), and the console panel and typewriter. The central processing unit is augmented by the 1622 Card Read-Punch and/or the 1621 Paper Tape Reader and the 1624 Paper Tape Punch, depending on whether the system is to process punched cards, paper tape, or both.
Expansion of the basic system is possible by increasing the size of the magnetic core storage in increments of 20,000 positions until a maximum of 60,000 positions is reached (Fig. 2.3). A variety of special devices and additional instructions is available to increase the power and flex-ibility of the system.
Data and instructions entered into the system are placed in core storage as decimal digits. Each core storage position can be referred to individually and can store one digit of information. The addressing system provides for the selection of any digit or group of digits in stor-age. The 1620 can also process alphabetic characters and special char-acters such as $,
*, -, +,
etc.The arithmetic and logical section of the computer is directed by the stored program. The 1620 has more than 30 different operations in its
10
Fig. 2.1. The 1620 Data Processing System.
12 Introduction to the 1620 Data Processing System
Fig. 2.2. The 1620 Central Processing Unit.
Fig. 2.3. Additional Core Module with Housing.
Introduction to the 1620 Data Processing System 13 repertoire. Among these is a powerful set of branching instructions that make logical decisions based on the results of tests perfonned on a system of indicators and switches.
Addition, subtraction, and multiplication operations are perfonned by a table look-up method described in Chapter 4. Addition and multi-plication tables are stored in specified areas of storage and are auto-matically referred to when one of the arithmetic operations is being performed. Division is accomplished by a division simulating program or by an automatic division feature.
The 1620 is a variable field computer in the complete sense of the term. Not only can data fields be of different lengths, but these same variable length fields can also be factors in all arithmetic operations without editing for size or position. Accuracy of results is insured by automatic internal checking that operates when data is being entered, read out, or processed by the system.
The console of the 1620 consists of control keys, switches, indicator panel, and typewriter. The control keys and switches are used for man-ual or automatic operation of the system. The console panel provides a visual indication of the status of various registers and control circuitry . within the computer. The typewriter is used as an output device, for direct entry of data and instructions into core storage, and for pennanent logging of the operator's intervention during the execution of a program.
Information is entered into the system by the input devices: the 1621 Paper Tape Reader, the 1622 Card Read-Punch, and the typewriter.
Eighty-column cards are read at the rate of 250 cards per minute. The paper tape reader reads an 8-channel paper tape at the rate of 150 char-acters per second. Speed of typewriter infonnation entry depends upon the operator's ability.
The recording of processed information is accomplished by the output devices; the 1622 Card Read-Punch, the 1624 Paper Tape Punch, and the typewriter. Cards are punched at the rate of 125 80-column cards per minute; the tape punch punches information in an 8-channel. paper tape at the rate of 15 characters per second; and the typewriter types at the rate of 10 characters per second.
Program preparation is simplified by the use of two major program-ming systems. These are the Symbolic Programprogram-ming System (SPS) and Fortran (from "formula translation"), both of which will be discussed in detail.
SPS, which simplifies program writing by reducing the clerical work involved, assembles a program written in mnemonic or symbolic nota-tion by converting the symbols to machine language instrucnota-tions and assigning locations in core storage for both data and instructions.
Fortran translates a problem expressed as a series of algebraic
state-14 Introduction to the 1620 Data Processing System ments into a complete machine language program, generating the step-by-step instructions necessary to solve the problem. A program written in Fortran for the 1620, after minor modifications, can also be translated and executed on other computers such as the IBM 7090, 1401, and many others.
INTERNAL DATA REPRESENTATION
DIGITS
Each core storage position in the 1620 has a unique address and can store one digit of information. Each digit is in a binary coded decimal (BCD) form represented by a 6-bit numeric code. In this code, six posi-tions of binary notation (0 or 1) are used and each of these posiposi-tions is called a bit (binary digit). Each position has one of two conditions:
either a bit is present represented by a ''1'' or it is not present represented by a "0." The six positions are divided into three groups: one check bit (C bit), one flag bit (F bit), and four numeric bits with the assigned values of 8, 4, 2, and 1 (table 2.1).
Table 2.1
CHECK FLAG
BIT BIT NUMEmcAL BITS
C F 8 4 2 1
The value of a decimal digit is the sum of the bits present in the numeric portion of the 6-bit code. Only bit combinations whose sum is 9 or less are used. Using the notation that a "1" indicates the presence of a bit and a "0" indicates the absence of a hit, we would represent the decimal digit 6 as 0110 considering only the numeric positions. The digit 8 is represented as 1000.
The check bit is used for· parity checking purposes. A parity check is a built-in method of checking the validity of coded information. This code checking occurs automatically within the computer as the data processing operations are carried out. Each character in the computer represented in the 6-bit numeric code must consist of an odd number of bits. During processing, a character with an even number of bits causes the machine to signal a parity error. When a digit is read ipto the computer by an input device, it is automatically converted to the
Introduction to the 1620 Data Processing System 15
1. Field Definition: The high-order position of a numeric field is defined by the presence of a flag (the terms "flag" and "flag bit" are used number -537 would appear in storage as
537.
3. Carries: Flags present in certain digits of the addition table (see Chapter 4) are interpreted as carries in arithmetic operations.
A record mark character (:I:) is a nondecimal digit with C-8-2 or F-8-2 coding. It is primarily used in input and output operations and in record transmission within the computer. The novice programmer will find that a good portion of his errors occur in attempting to do arithmetic operations on record marks.
16 Introduction to the 1620 Data Processing System A numeric blank has C-8-4 coding. It is used for the control of blank columns when cards are being punched, and, like the record mark, can-not be used in arithmetic operations. Unlike the record mark, it may can-not even be present in an instruction. must always occupy an even-numbered core position.
Table 2.3 shows the double-digit representation of all the alphameric
Introduction to the 1620 Data Processing System 17 FIELDS
A field consists of a number of consecutive digits which are considered as a group in arithmetic and internal data transmission instructions. A field is always addressed (referenced) by its low-order digit, which oc-cupies the highest numbered core storage position of the field. A field is processed serially from right to left into successively lower core stor-age positions until a digit with a flag is sensed. The digit with the flag is treated as part of the field, but no more digits are processed.
The absence of a flag in the low-order position of a field (the addressed digit) is unconditionally interpreted as a positive field of data.
One-digit fields of data are not allowed. The smallest allowable data field is two digits.
Figure 2.4 illustrates the processing of a field.
- - - F i e l d - - - - l
X X X
t
Direction Processedt
Flag Bit Addressed Digit (End of Field) (low-Order Position of Field)
Fig. 2.4. Field Processing.
RECORDS
A record consists of a field or fields of data related to input-output operations. A record is addressed at its high-order digit, which occupies the lowest core storage position of the record. Records are processed
Record Mark Record Mark
~~x J,.1d 0
Xr 0J"1d 0
XI
X~fi'ld~ i
X X~---Record---""
•
Arrows Indicote Direction of Processing Fig. 2.5. Record Processing.
18 Introduction to the 1620 Data Processing System serially from left to right (high-order to low-order digits). A record mark (:I:) defInes the end of a record and is located in the highest num-bered core location.
Figure 2.5 illustrates the processing of a record as compared to the processing of a fIeld.