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processor A microprocessor from Motorola. It was the
successor to the Motorola 68010 and was followed by the
Motorola 68030. The 68020 has 32-bit internal and external
data and address buses and a 256-byte instruction buffer,
arranged as 64 direct-mapped 4-byte entries[?].
The 68020 added many improvements to the 68010 including a
32-bit ALU and external data bus and address bus, and
new instrucitons and addressing modes. The 68020 (and
68030) had a proper three-stage pipeline.
The new instructions included some minor improvements and
extensions to the supervisor state, some support for
high-level languages which didn't get used much (and was
removed from future 680x0 processors[?]), bigger (32 x 32-bit)
multiply and divide instructions, and bit field manipulations.
The new adderessing modes added another level of indirection
to many of the pre-existing modes, and added quite a bit of
flexibility to various indexing modes and operations.
The instruction buffer (an instruction cache) was 256
bytes, arranged as 64 direct-mapped 4-byte entries. Although
small, it made a significant difference in the performance of
many applications.
The 68881 and the faster 68882 FPU chips could be used with
the 68020.
The 68020 was used in many models of the Apple Macintosh II
series of personal computers and Sun 3 workstations.
(2001-03-07)
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