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590 Part 6 Computer families Section 3 ÷ The IBM System/360-a series of planned machines which span a wide performance range

Fig. 2. Schematic of basic registers and data paths.

tional conditions, loads and relocates programs and data, manages storage, and supervises scheduling and execution of multiple programs. To a problem programmer, the supervisory program and the control equipment are indistinguishable.

The functional structure of SYSTEM/360, like that of most computers, is most concisely described by considering the data formats, the types of manipulations performed on them, and the instruction formats by which these manipulations are specified.

Information formats

The several SYSTEM/360 data formats are shown in Fig. 3. An 8-bit unit of information is fundamental to most of the formats. A consecutive group of n such units constitutes a field of length n. Fixed-length fields of length one, two, four, and eight are termed bytes, halfwords, words, and double words, respectively. In many instructions, the operation code implies one of these four fields as the length of the operands. On the other hand, the length is explicit in an instruction that refers to operands of variable length.

The location of a stored field is specified by the address of the leftmost byte of the field. Variable-length fields may start on any byte location, but a fixed-length field of two, four, or eight bytes must have an address that is a multiple of 2, 4, or 8, respectively. Some of the various alignment possibilities are apparent from Fig. 3.

Storage addresses are represented by binary integers in the system. Storage capacities are always expressed as numbers of bytes.

Processing operations

The SYSTEM/360 operations fall into four classes: fixed-point arithmetic, floating-point arithmetic, logical operations, and decimal arithmetic. These classes differ in the data formats used, the registers involved, the operations provided, and the way the field length is stated.

Fixed-point arithmetic

The basic arithmetic operand is the 32-bit fixed-point binary word. Halfword operands may be specified in most operations for the sake of improved speed or storage utilization. Some products and all dividends are 64 bits long, using an even-odd register pair.

Because the 32-bit words accommodate the 24-bit address, the entire fixed-point instruction set, including multiplication, division,

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