Motorola 68012

The Motorola MC68012 processor is a 16/32-bit microprocessor from the early 1980s. It is an 84-pin PGA[1] version of the Motorola MC68010. The memory space was extended to 2 GB, and a read-modify-write cycle (RMC) pin, indicating that an indivisible read-modify-write cycle in progress, was added, in order to help the design of multiprocessor systems with virtual memory.[2][3] All other processors had to hold off memory accesses until the cycle was complete. All other features of the MC68010 were preserved.

Die of Motorola 68012.

The expansion of the memory space caused an issue for any programs that used the high byte of an address to store data, a programming trick that was successful with those processors that only have a 24-bit address bus (68000 and 68010). A similar problem affected the 68020.

Notes and references
  1. "Motorola 68012 (MC68012) microprocessor family".
  2. MC68010/MC68012 16-/32-Bit Virtual Memory Microprocessors (PDF). Motorola Semiconductor. May 1985.
  3. Avtar, Singh; Triebel, Walter A. (1991). 16-Bit and 32-Bit Microprocessors: Architecture, Software, and Interfacing Techniques. Englewood Cliffs, New Jersey: Prentice-Hall. ISBN 978-0138121570.
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