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24 Assembly Language Programming for the 68000 Family

user/supervisor mode information on every reference to memory. If the

system is so designed, certain areas of memory can be restricted to ref

erences only when in the supervisor mode. If a user mode instruction

were to try to access this “protected” area of memory, a special condi

tion or “exception” would occur and the operating system could then take

control before any damage is done.

The CPU Registers

In one sense, a register is a type of memory location. However, it is

located right on the CPU chip itself. Registers differ from conventional

memory locations in that they operate at a higher speed. In other words, if

we use registers in a calculation, it will be faster to perform than if mem

ory locations were used. Additionally, registers are specified by special

names rather than just by numbers. The existence of a good register set

is a major asset to the architecture of a particular microprocessor. The 68000

family is a good example of a microprocessor with a rich register set.

Depending on whether a program is running in user or supervisor

mode, there is a slightly different view of the CPU registers. This view

is known as the programmer s model. The following paragraphs discuss

the programmers model for the user mode.

The 68000 has sixteen 32-bit general-purpose registers. These are di

vided into two groups of eight. The eight data registers, DO through D7,

are the registers you would normally use to perform arithmetic oper

ations. These can be used as bytes, words, or longwords. The second

group of eight general-purpose registers are the address registers, A0

through A7. These registers can be used for arithmetic operations, but

are primarily designed for use in the special addressing modes discussed

in subsequent chapters. The address registers can be used as words or

longwords, but not as bytes. In the next chapter you will learn more of

the details concerning the use of the sixteen general-purpose registers.

Address register A7, also known as the user stack pointer (USP), has a

special interpretation by the 68000. Some instructions affect this register

without its being explicitly specified. You will learn all about stacks and

the use of the USP in Chapter 7. For now, consider it as just one of the

eight address registers.

Another very important register is the program counter, or PC. This

32-bit register is used to hold the memory address of the next instruction

that the CPU will execute. The programmer never explicitly references

this register; its contents are always updated by the CPU. Normally, the

PC advances as the program executes sequential instructions that are in

1 2 ... 33 34 35 36 37 38 39 40 41 42 43 ... 255 256

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