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

addition, subtraction, multiplication, division, and comparison. The decimal digits 0 through 9 are represented in the 4-bit

binary-coded-decimal form by 0000 through 1001, respectively. The patterns 1010 through 1111 are not valid as digits and are interpreted as sign codes: 1011 and 1101 represent a minus, the other four a plus. The sign patterns generated in decimal arithmetic depend upon the character set preferred. For EBCDIC, the patterns are 1100 and 1101; for the code of Fig. 5, they are 1010 and 1011. The choice between the two codes is determined by a mode bit.

Decimal digits, packed two to a byte, appear in fields of variable length (from 1 to 16 bytes) and are accompanied by a sign in the rightmost four bits of the low-order byte. Operand fields can be located on any byte boundary, and can have lengths up to 31 digits and sign. Operands participating in an operation have independent lengths. Negative numbers are carried in true form. Instructions are provided for packing and unpacking decimal numbers. Packing of digits leads to efficient use of storage, increased arithmetic performance, and improved rates of data transmission. For purely decimal fields, for example, a 90,000-byte/second tape drive reads and writes 180,000 digits/second.

Instruction formats contain one, two, or three halfwords, depending upon the number of storage addresses necessary for the operation. If no storage address is required of an instruction, one half- word suffices. A two-halfword instruction specifies one address; a three-halfword instruction specifies two addresses. All instructions must be aligned on halfword boundaries.

The five basic instruction formats, denoted by the format mnemonics RR, RX, RS, SI, and SS are shown in Fig. 6. RR denotes a register-to-register operation, RX a register and indexed-storage operation, RS a register and storage operation, SI a storage and immediate-operand operation, and SS a storage-to-storage operation.

In each format, the first instruction halfword consists of two parts. The first byte contains the operation code. The length and format of an instruction are indicated by the first two bits of the operation code.

The second byte is used either as two 4-bit fields or as a single 8-bit field. This byte is specified from among the following:

The second and third halfwords each specify a 4-bit base register designator (B), followed by a 12-bit displacement (D).

An effective storage address E is a 24-bit binary integer given, in the typical case, by

where B and X are 24-bit integers from general registers identified by fields B and X, respectively, and the displacement D is a 12-bit integer contained in every instruction that references storage.

The base B can be used for static relocation of programs and data. In record processing, the base can identify a record; in array calculations, it can specify the location of an array. The index X can provide the relative address of an element within an array. Together, B and X permit double indexing in array processing.

The displacement provides for relative addressing of up to 4095 bytes beyond the element or base address. In array calculations, the displacement can identify one of many items associated with an element. Thus, multiple arrays whose indices move together are best stored in an interleaved manner. In the processing of records, the displacement can identify items within a record.

In forming an effective address, the base and index are treated as unsigned 24-bit positive binary integers and the displacement as a 12-bit positive binary integer. The three are added as 24-bit binary numbers, ignoring overflow. Since every address is formed with the aid of a base, programs can be readily and generally relocated by changing the contents of base registers.

A zero base or index designator implies that a zero quantity must be used in forming the address, regardless of the contents of general register 0. A displacement of zero has no special significance. Initialization, modification, and testing of bases and indices can be carried out by fixed-point instructions, or by BRANCH AND LINK, BRANCH ON COUNT, or BRANCH ON INDEX instructions. LOAD EFFECTIVE ADDRESS provides not only a convenient housekeeping operation, but also, when the same register is specified for result and operand, an immediate register-incrementing operation.

Normally, the CPU takes instructions in sequence. After an instruction is fetched from a location specified by the instruction