Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 33)

Microcomputer Architecture 19

be modified by hand to allow for the differences in architectures. This

scheme did have the advantage that it allowed software vendors to get

their products to market quickly. However, the transposed code did not

run as well as if it had been written for the target machine in the first

place.

Motorola’s MACSS (advanced computer system on silicon) project

abandoned both object and source code compatibility with the older

MC6800 line. While this decision forced a slower introduction of software

for the M68000 system family, it allowed a completely unconstrained

design. The only concession Motorola made was at the bus interface level:

special pinouts are provided to accommodate the large number of 8-bit

peripheral chips already in existence. It should be noted that this is a plus,

and in no way affects the architecture or, for that matter, the M68000 bus

interface.

The question always arises, is a chip 8, 16, 32, or some other number

of bits? To properly answer this question requires setting a base of

comparison; we must compare apples with apples and oranges with

oranges. One basic metric that can be used is the internal register size. If

16-bit registers support 16-bit operations with the majority of arithmetic

and logical instructions, the chip can be classed as internally a 16-bit

architecture. If only a few of the registers and/or instructions are 16-bit,

and the remainder are 8-bit, the chip should be classified as an 8-bit chip.

The 8080 family is a good example of an 8-bit chip. Another perspective

is the width of the data path to and from memory. Contrary to popular

belief, the internal size does not have to be the same as the data path; the

data path can be larger or smaller. The only restriction is that the data

path always be a multiple of a byte (8 bits). The very popular 8088 is an

8-bit data bus version of the 16-bit data bus 8086. This is the chip found

in the original IBM PC.

The M68000 family uses a 32-bit architecture internally. It fully sup

ports its 32-bit registers with a rich instruction set performing 32-bit oper

ations. The MC68000 and MC68010 have a 16-bit data bus. The MC68020

and MC68030 have full 32-bit buses. The MC68008 is an 8-bit bus version

of the MC68000. Its position is similar to the Intel 8088 in that it allows

interfacing to 8-bit buses and memory components.

The astute reader may be asking the question, what effect does the

data bus width have on the microprocessor’s speed? This is not a simple

question to answer. A 16-bit bus does not necessarily allow a CPU to

operate twice as fast as an 8-bit bus. It is true, however, that if the CPU

desires to fetch a 16-bit value it will require two accesses to memory if an

8-bit bus is used. But even if the 16-bit bus is operating at twice the byte

transfer rate of the 8-bit bus, there are many other factors that control

the CPU speed.

A CPU requires a clock. The speed of this clock determines the inter

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