Motorola 68020

The Motorola 68020, also referred to as the 68020 or '020, is a 32-bit CISC processor that was produced by Motorola's Semiconductor Products Sector, superseding the Motorola 68000 and 68010. The 68020 was only used in the Macintosh II and Macintosh LC as it was soon succeeded by the Motorola 68030.

Features
Introduced in 1984, processor efficiency of the 68020 was improved over the 68000 with a 256-byte instruction cache and a 3-stage pipeline that aimed to keep three sections of the CPU simultaneously occupied during each clock cycle. With a 32-bit address bus, the 68020 could theoretically access up to 4GB of RAM. However, logic board designs and classic Mac OS limitations kept the actual operating maximum much lower. The 32-bit data bus doubled the efficiency of memory throughput, though the 16-bit bus of the lower-end Macintosh LC only took advantage of half of it.

Variants

 * Motorola 68EC020 - a low-cost version with a 24-bit address bus that was limited to accessing 16MB of RAM. It was used only by the Amiga 1200 computer and some game consoles.

Related processors

 * Motorola 68881 - an optional floating-point unit that accelerates floating-point math instructions. This functionality was later built into high-end versions of the Motorola 68040 and PowerPC processors.
 * Motorola 68851 - an optional paged memory management unit that adds support for virtual memory. This functionality was later built directly into the 68030.
 * VLSI VI475 - a custom hardware memory management unit which added backwards compatibility for 24-bit addressing to the 68020 processor, but did not include paged memory management required for virtual memory support like the more expensive 68851.