442 Part 5 The PMS level

from a wide variety of external devices and in a wide variety of formats.

The specific functions of the major units can be described briefly as follows:

Arithmetic and processing unit. Using a 64-bit number word with algebraic sign, this unit carries out 7 different types of arithmetical operations, 5 types of choice (branch) operations, and 2 types of logical pattern-processing operations. See Table 2. Arithmetical operations can be performed in any of 16 possible formats. For example, arithmetic can be performed using either a pure binary or a binary-coded decimal number code, and in both fixed-point and floating-point notation. Fixed-point operations can also be carried out in a special half-word format in which two independently addressable half-words are stored in a single full-word storage location. These two half-words can be processed either separately, as independent words, or concurrently in duplex format. In duplex format, the respective lefthand and righthand halves of each double operand are processed simultaneously in a single instruction time, and the two independent half-word results are written back in the corresponding halves of the full-length result location.

Program control unit. The program control unit interprets and regulates the sequencing of instructions in the program. It operates with a 68-bit binary-coded 3-address instruction word. See Table 3. Each instruction word contains three 16-hit codes which specify the addresses of each of two operands, alpha and beta, and usually the address of the result of the operation, gamma, in the main memory. The memory location of the next instruction word is specified by a 16-bit address number contained in one of 16 possible base registers; a 4-hit code in the instruction word (d-digits) specifies which one of the base registers contains the desired word. Whenever a register is so used as a next-instruction address source, its contents are automatically increased by unity. Choice instructions, used for program branching, from time to time may cause a new alternative address number to be inserted in any one of the base registers. This register is then used as the source of the address number of the next instruction.