History
The even numbers (68000, 68020, 68040, 68060) were reserved for major revisions to the 680x0 core architecture. The odd numbers (68010, 68030, 68050, 68070) were reserved for upgrades to the architecture of the previous chip. No 68050 or 68070 was ever produced by Motorola.
For example, the Motorola 68010 (and the obscure 68012) was a 68000 with improvements to the loop instruction and the ability to suspend then continue an instruction in the event of a page fault, enabling the use of virtual memory with the appropriate MMU hardware. There were, however, no major overhauls of the core architecture. Similarly, the Motorola 68030 was a process improvement on the 68020 with the MMU and a small data cache (256 bytes) moved on-chip. The 68030 was available in speed ratings up to 50 MHz.
The jump from the 68000/68010 to the 68020/68030, however, represented a major overhaul, with too many individual changes to list here.
By the time the 68060 was in production, Motorola had abandoned development of the 680x0-type chips in favor of the PowerPC. The 68060 was the last 680x0-type processor from Motorola.
There was a so-called 68070 processor, produced by Signetics (Philips), and was a modestly improved 68000 series processor, with a simple, on-chip MMU and I²C bus support. It came out long before the 68060, and was used mostly as an embedded processor in some consumer electronics items.
Read more about this topic: Motorola 68060
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