Motorola M68000 User's Guide download pdf (Page 217)

CHAPTER 15

THE 68030

In the latter part of 1986 Motorola announced its latest member of

the 68000 family, the MC68030. This new super chip should be in full

production during 1987. This second-generation 32-bit microprocessor is

actually a combination of an enhanced MC68020 and a subset of the

MC68851 paged memory management unit. The combination really gives

awesome capability to one single tiny chip.

Programming the 68030 will be no different from programming the

68020 unless you are involved with the PMMU (paged memory manage

ment unit) portion of the chip. In that case, you would need to know the

details of the MC68851 coprocessor subset. I will not present all these

details; rather, I will give an introduction to the concepts of memory

management so that you can read the manufacturer’s documentation on

the 68030 or 68851 with less difficulty. At the time of this writing only

preliminary documentation on the 68030 was available. The information

presented here is as accurate as possible with this preliminary documen

tation.

The 68030 operates at 20 MHz, compared to a top speed of 16.7 MHz

for the 68020. However, the 68030 actually has an effective speed which

is twice that of the 68020. This is accomplished by a combination of

data and instruction caches and a pipelined architecture. You will recall

from Chapter 14 that the 68020 has an instruction cache. Its purpose is

to reduce the access time to instructions in memory. It is most effective

for program loops that are small enough to be entirely contained in the

cache. The operation of the instruction cache in the 68030 is similar to

that of the instruction cache in the 68020. However, the addition of a data

cache allows a greater improvement in speed. Both the instruction cache

and the data cache are 256 bytes. Figure 21 shows the block diagram of

the 68030. You will notice that there are two internal address and data

buses. This duplication allows simultaneous access to the instruction and

data caches.

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Motorola M68000 User's Guide Page 217