SIMSCRIPT II.5 Programming Language

SIMSCRIPT II.5 Programming Language
to a computer. A mechanism is also required to specify in detail the formats in which data should
be read and written, and a facility to transmit data in internal machine representation.
Although the input/output programming statements provided are similar from one SIMSCRIPT II.5
compiler implementation to another, the exact interpretation and additional parameters required by
these statements may vary from machine to machine. This variation is due to the differences in operating
system requirements and device characteristics of different computer systems. Individual
SIMSCRIPT II.5 user manuals describe the nature of these differences.
In general, three pieces of information must be specified when an input/output operation takes
place:
1. A physical device
2. A data or information list
3. The desired data format.
In the statements read, print, and list, a physical device is implied (some default input and
output device), the data list is stated explicitly, and the format is, in the first instance, "free," in the
second, a "picture," and in the third, standardized. The statements described here provide a more
flexible means of specifying this information.
3.4.1 Physical Device Specification
SIMSCRIPT II.5 programs may reference specific data streams using a logical unit number in certain
input and output (I/O) statements. Each SIMSCRIPT II.5 logical unit number may then be associated
with a specific device or data file through the computer system job control or command
language. This allows each SIMSCRIPT II.5 program to refer to a file or I/O device in a common
logical manner, postponing the detailed specification of individual file or data characteristics for
definition in the appropriate system command language. Each logical unit number, therefore, is
linked to an individual file or specific I/O device through the operating system.
Each file or I/O device that is to be referenced within a program is assigned a logical unit number.
Logical unit numbers in SIMSCRIPT II.5 are integer numbers within the range 1 to 99 inclusive,
units 98 and 99 being reserved for SIMSCRIPT II.5 system use. Some implementations may not
support 99 distinct units. Consult the appropriate user's manual for details. A specific I/O device
or file may be selected as the current input or output unit by executing a statement of the form:
device for input or
use unit device for input
and
use device for output or
use unit device for output
The word unit may be omitted. The value device may be any arithmetic expression that
evaluates to a logical unit number. The SIMSCRIPT II.5 logical unit number referenced will be
104