Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 37)

Microcomputer Architecture 23

instruction is. Many instructions are actually represented by several dif

ferent opcodes, each specifying a different version of the instruction.

Virtually all systems will have two kinds of memory: read-only mem

ory (ROM) and random-access memory (RAM). Read-only memory, as

its name implies, can be read but not written. How then is it possible

to use it? Actually, ROM chips can be written, but not by a program.

Certain types of ROM chips have data stored when the chips are manu

factured. These ROM chips can never be changed; the data is part of the

mask used to create the chips. Other types of ROM chips can be erased,

either electrically or using ultraviolet light, and then “reprogrammed.”

There are special ROM programming devices to do this. ROM’s can be

used to store a program that will never change. A good example of this

is an operating system. All or part of your operating system is more than

likely in ROM.

RAM memory is something of a misnomer, since ROM is in fact also

a random-access type of memory. By random access we mean that any

location in the memory can be accessed in any order, without restriction

to a sequential order. Read/write memory actually is what is normally

meant by RAM. This is the memory that holds your program and data,

as well as data that must be maintained by the operating system. The

amount of RAM memory your system has will vary, but some amount

of RAM is required with any system. The more RAM memory available,

the larger your program and data can be if it is all to fit into memory at

the same time.

User and Supervisor Modes

The 68000 executes programs in one of two modes, user or supervisor.

If a program is running in the user mode, it is most likely a normal

everyday program. You will more than likely be writing mostly user

mode programs. The supervisor mode is used by programs that require

complete control over all aspects of the hardware. Your operating system

is a prime example of a program that would run in supervisor mode.

If a program is running in the user mode, it is restricted in a number

of ways. Some instructions are designated as privileged. One of these is

the STOP instruction. A program in user mode cannot execute any of

the privileged instructions. This helps to prevent a program with bugs

from crashing the system it is running on. In a multiple user system it is

important that one user not be able to do damage to another user. If a

user crashes the system, or otherwise performs a privileged instruction,

it could affect all users.

While not built into the 68000 CPU chip, many machines have im

plemented various forms of memory protection. The 68000 provides the

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Motorola M68000 Betriebsanweisung Seite 37