Integer Condition Codes - Motorola MC68030 User Manual

3.3 INTEGER CONDITION CODES
3-14
The CCR portion of the SR contains five bits which indicate the results of
many integer instructions. Program and system control instructions use cer-
tain combinations of these bits to control program and system flow.
The first four bits represent a condition resulting from a processor operation.
The X bit is an operand for multiprecision computations; when it is used, it
is set to the value of the C bit. The carry bit and the multiprecision extend
bit are separate in the M68000 Family to simplify programming techniques
that use them (refer to Table 3-8 as an example).
The condition codes were developed to meet two criteria:
• Consistency - across instructions, uses, and instances
• Meaningful Results - no change unless it provides useful information
Consistency across instructions means that all instructions that are special
cases of more general instructions affect the condition codes in the same
way. Consistency across instances means that all instances of an instruction
affect the condition codes in the same way. Consistency across uses means
that conditional instructions test the condition codes similarly and provide
the same results, regardless of whether the condition codes are set by a
compare, test, or move instruction.
In the instruction set definitions, the CCR is shown as follows:
x
N
v
where:
X (extend)
Set to the value of the C bit for arithmetic operations. Otherwise not
affected or set to a specified result.
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 arithmetic overflow occurs. This implies that the result cannot be
represented in the operand size. Cleared otherwise.
C (carry)
Set if a carry out of the most significant bit of the operand occurs for an
addition. Also set if a borrow occurs in a subtraction. Cleared otherwise.
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