Address Generation

Base Address is a 24-bit number contained in a general register specified by the program in a four-bit field, called the B field, in the instruction. Base addresses can be used as a means of independently addressing each program and data area. in array type calculations, it can specify the location of an array, and, in record-type processing, it can identify the record. The base address provides for addressing the entire main storage. The base address may also be used for indexing purposes.

Index is a 24-bit number contained in a general register designated by the program in a four-bit field, called the X field, in the instruction. It is included only in the address specified by the RX instruction format. The RX format instructions permit double indexing; that is, the index can be used to provide the address of an element within an array.

Displacement is a 12-bit number contained in a field, called the D field, in the instruction. The displacement provides for relative addressing of up to 4,095 bytes beyond the location designated by the base address. in array-type calculations, the displacement can be used to specify one of many items associated with an element. In the processing of records, the displacement can be used to identify items within a record.

In forming the address, the base address and index are treated as unsigned 24-bit positive binary integers. The displacement is similarly treated as a 12-bit positive binary integer, and 12 high-order zeros are appended. The three are added as 24-bit binary numbers, ignoring overflow. The sum is always 24 bits long. The bits of the generated address are numbered 8-31 , corresponding to the numbering of the base-address and index bits in the general register.

A zero in any of the X2, B1, or B2 fields indicates the absence of the corresponding address component. For the absent component, a zero is used in forming the address, regardless of the contents of General register 0. A displacement of zero has no special significance.

An instruction can designate the same general register both for address computation and as the location of an operand. Address computation is completed prior to the execution of the operation.

Unless otherwise indicated in the individual instruction definition, the computed operand address designates an operand in main storage. When a main-storage operand is designated, the address designates the leftmost byte of the operand. For branching instructions, the second-operand address is used as the branch address. For shifting instructions, the second-operand address is not used as an address but specifies the shift amount.