Motorola M68000 Betriebsanweisung PDF herunterladen (Seite 72)

58 Assembly Language Programming for the 68000 Family

way as the other registers. We don’t treat the contents of the CCR as a

numeric quantity. Instead, we indirectly use the values of individual bits

in the CCR. Each of these special bits, or condition codes, has a specific

meaning when set or reset. If a bit is set it has a value of binary 1. If it is

reset it has a value of binary 0. The CCR is an 8-bit register, but not all

of the 8 bits are used. It is organized as follows:

CONDITION CODE REGISTER

7 4 0

X(extend) Transparent to data movement. When affected, it is set the

same as the C bit.

N (negative) Set if the most significant bit of the result is set. Cleared

otherwise.

Z(zero) Set if the result equals zero. Cleared otherwise.

V(overflow) Set if there was an arithmetic overflow. Cleared otherwise.

C(carry) Set if a carry is generated out of the most significant bit of

the operands for an addition. Also set if a borrow is

generated in a subtraction. Cleared otherwise.

After the execution of a 68000 instruction, some of the condition codes

may be affected. Not all instructions affect the CCR. The majority of

CCR usage is related to arithmetic operations such as addition, subtrac

tion, and the comparison of two numeric quantities.

The representation of numbers in two’s complement binary was dis

cussed in Chapter 1. Recall that numbers can be interpreted as signed or

unsigned depending on the interpretation of the most significant bit. This

bit is referred to as the sign bit for signed numbers. Many times we will

desire to treat a number as a positive value only. By allowing the use of

the most significant bit as a normal bit of the number, and not a sign,

we can double the magnitude of the numbers that can be represented.

The ADD and SUB instructions operate identically for both signed and

unsigned numbers. The interpretation is up to the programmer. How

ever, by the use of particular bits in the CCR we can test the outcome

of arithmetic operations for both signed

and unsigned numbers.

The Carry Bit

One of the most important checks that must be made on an arithmetic

operation is whether or not the result has exceeded the size of the

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Motorola M68000 Betriebsanweisung Seite 72