Chapter 32 ½ The IBM System/38 535

Support for Common Programming Functions

The System/38 instruction interface is designed to provide direct support for a wide variety of functions common to control programming, compilers, and utilities. This increased level of machine function eliminates the need to implement these common functions in multiple programming components, increases consistency across all programming components, and supports programming approaches conducive to providing integrity and reliability.

There are two basic modes of addressing in the System/38 interface. The first is pointers, which allow varying addressability to all objects and bytes within space objects. The second, dictionary addressing, deals with program references to values within a space object.

Operands referenced in program instructions are defined in a dictionary portion of the program separate from the instructions themselves. Instruction operands are index references to these dictionary entries which define operand characteristics such as data type and length. Binary, zoned decimal, packed decimal, character, and pointer data types are examples of operand characteristics that may be defined. The dictionary entries do not contain the operand values; the specific location of the operands is not apparent to or required by programs. However, the user can control the general type of location characteristics: for example, relative to the area addressed by a pointer or relative to the storage area allocated for program variables within the executing process.

This approach of having instructions reference dictionary entries describing the operand characteristics allows additional capability over low-level instruction interfaces. For example, the following high-level capabilities are provided:

• Computational instructions are generic with respect to data type and length. For example, there is only one numeric add instruction in the System/38 instruction interface: it operates on whatever data is defined in the operand definition dictionary. This enables the use of source and receiver operands of varying type, length, and decimal positioning with all conversions and scaling being performed by the machine.
• Arrays may be defined in the interface and instruction operands support array indexing to locate specific elements of the array.
• Since applications often allow operations on multiple formats of data, some instructions (for example, the copy instructions) support late-binding of data definition where the data (type, length, and decimal positioning) need not be defined until the instruction is executed.

Figure 3 illustrates this System/38 program structure and the general relationship between a high-level language program and the corresponding System/38 constructs.
 

Support for Key System Functions

The System/38 machine was designed to support a usage environment characterized by a dynamically changing application load consisting of a wide variety of application types-all utilizing advanced functions such as data base. For example, batch, interactive, and transaction processing, along with program development activities, may all be executing concurrently with dynamically changing workloads and priorities. One of the key requirements for the System/38 instruction interface was to provide efficient support in this type of environment for application requirements such as multiprogramming and data base operations. This centralization of function in the machine simplifies the user programming task and reduces overhead in a dynamic multi-user environment.

Two examples of this system function support will be described here-multiprogramming and data base. Similar high levels of