The 68000 Instruction Set

Abstract

Although the 68000 bears some family resemblance to the 6800, its designers took full advantage of the benefits of large-scale integration and 16-bit architecture to provide many more internal registers. This allows many variables to be held in the processor itself rather than in external storage, so avoiding some store read and write cycles and speeding computation. Also, the use of a 16-bit word to encode the op. code of each instruction enables a much richer and more numerous instruction set to be provided, together with many more addressing modes. The address bus comprises 23 lines, so that a total of 8 megawords or 16 megabytes of program and data storage can be attached to the CPU chip and addressed directly. This is 256 times more than can be attached to the 6800 and more than is normally needed by any single program. The 68000 is well suited to multi-user systems in which a number of users each have access to the system for a brief period in turn.