Where’s ARM4 and ARM5?
These were never made but ARM created a space for ARM4 and ARM5 in case they wanted to go to simpler products. As the engineering transitioned from Acorn to ARM Ltd., the number scheme for the processor was changed. As such the numbers 4 and 5 were skipped.
ARM’s First Chip
Although the ARM processor were developed as a cus- tom device for a highly specific purpose, the team designing it felt that the best way to produce a good custom solution was to produce a processor with good all-round performance. However, it’s interesting to note
that the ARM’s architectural fate was sealed accidentally. While most of the RISC processor vendors were designing relatively huge chips (SPARC RISC, Intel i860, AMD 29000, etc.), ARM opted to develop a small-scale processor. One of the reasons the ARM processor was designed as a small-scale solu- tion was that the resources to design it were not sufficient to allow the cre- ation of a large and complex device. While this is now a technical plus for the ARM processor, it began as a necessity for a processor designed by a team of talented, but inexperienced designers (outside of university projects, most team members were programmers and board-level circuit designers) using new tools, some of which were far from state-of-the-art.