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26 Assembly Language Programming for the 68000 Family

Input/Output

It may come as a surprise to you that the 68000 does not have

any input/output instructions. How then, is I/O performed? The 68000

family uses a technique known as memory mapped I/O. This means that

input/output devices are connected to the system via interface chips that

are connected to the CPU as if they were areas of memory. A small part

of the huge amount of memory we are allowed must be sacrificed; it is

now used for I/O and can’t be used for main memory at the same time.

The real advantage to the memory-mapped I/O technique is that rather

than being restricted to a small number of special I/O instructions, we

can use aU of the 68000 memory reference instructions with I/O devices.

A large variety of I/O interface chips are available for the 68000

family. A number of these were formerly used with the M6800 family.

The 68000 allows the use of these 8-bit chips as well as the newer

16-bit I/O devices specifically designed for the 68000. In Chapter 12

we will discuss the programming of a typical I/O chip. We will use an

asynchronous serial I/O device or UART chip.

Exercises

1. What are the three main parts of a microcomputer?

2. What is the difference between the MC68000 and the MC68008?

3. What is the newest member of the M68000 family?

4. Is the M68000 family an extension of the M6800 architecture?

5. How many bits is the internal architecture of the 68000?

6. What are the data bus sizes for the M68000 family?

7. Is the M68000 data bus synchronous or asynchronous?

8. What is the difference between RAM and ROM?

9. What is the purpose of supervisor mode?

10. Are registers faster or slower than memory?

11. How much memory is allowed with the MC68000?

12. What are the 32-bit general-purpose registers?

13. Is multi-byte data stored with the high order byte in the lowest or

highest address?

14. Are all instructions the same number of words?

15. What is the purpose of the program counter?

16. What register is used for the USP and SSP?

17. How many input/output instructions does the 68000 have?

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Motorola M68000 Betriebsanweisung Seite 40