A memory control unit for determining the actual physical location of an instruction address in one of a plurality of memory units in a digital computer wherein the instruction address and the memory addresses are both in a block, track and word hierarchy, where the instruction address may not correspond to the memory address due to differences in the number of available blocks for recording program instructions from memory unit to memory unit, utilizing: a block address register for storing the block portion of the instruction address; a shift register associated with each memory unit wherein the number of available blocks for each memory unit is stored; a flip flop for bit by bit comparison of the contents of the block address register with the contents of selected shift registers; a counter for shifting from one to another of the shift registers when the contents of the block register exceeds the contents of the selected shift register, and an adder for decrementing the contents of the block address register by an amount equal to the number contained in the previously selected shift register. The memory control unit also includes another flip flop responding to the bit by bit serial comparison of the block address register and the selected shift register for detecting whether the instruction address starts in the last available block in a memory unit, and a circuit for evaluating the track and word portion of the address of each word of the instruction being executed for determining when the last word of an instruction has been executed by the computer.