Operand alignment controls for VFL instructions

Operand controls are provided in an I-unit which includes a plurality of address operand pairs (AOP's). Each AOP has an operand request register and an operand buffer. The AOP's are used to fetch the subline (e.g. doubleword) of operands in variable length field (VLF) instructions (such as LM, MVC, CLC, XC, etc.). Each AOP is capable of requesting and receiving a single subline fetched by a storage control. Each AOP buffers its received subline until needed for executing the instruction. The bytes of VLF operands are not aligned on subline boundaries. The AOP's are dynamically selected to fetch the sublines of current operand(s). The AOP's selected for a single operand are sequenced by a chain of back pointers held in latches, which respectively represent the AOP's. Each latch receives the identifier (ID) of the previous AOP in the chain for the operand. An associative search through the previous ID's in all latches obtains the forward order of AOP ID's. The AOP's are selected, started, and released dynamically so that the same AOP may be used more than once for different sublines of a single operand. Two AOP's receive the same first fetch request for an operand when required for operand alignment.