C164CI_02 datasheet(76/484 Pages) INFINEON

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C164CI/C164SI

Derivatives

Central Processing Unit (CPU)

User’s Manual

4-18

V3.1, 2002-02

Because logical ALU operations cannot produce an invalid result, the V-flag is cleared

by these operations.

The V-flag is also used as a ‘Sticky Bit’ for rotate right and shift right operations. With

only using the C-flag, a rounding error caused by a shift right operation can be estimated

up to a quantity of one half of the LSB of the result. In conjunction with the V-flag, the

C-flag allows evaluation of the rounding error with a finer resolution (see Table 4-2).

For Boolean bit operations with only one operand, the V-flag is always cleared. For

Boolean bit operations with two operands, the V-flag represents the logical ORing of the

two specified bits.

Z-Flag: The Z-flag is normally set to ‘1’ if the result of an ALU operation equals zero,

otherwise it is cleared.

For the addition and subtraction with carry, the Z-flag is only set to ‘1’, if the Z-flag

already contains a ‘1’ and the result of the current ALU operation additionally equals

zero. This mechanism is provided to support multiple precision calculations.

For Boolean bit operations with only one operand, the Z-flag represents the logical

negation of the previous state of the specified bit. For Boolean bit operations with two

operands, the Z-flag represents the logical NORing of the two specified bits. For the

prioritized ALU operation, the Z-flag indicates whether the second operand was zero.

E-Flag: The E-flag can be altered by instructions which perform ALU or data movement

operations. The E-flag is cleared by those instructions which cannot be reasonably used

for table search operations. In all other cases, the E-flag is set depending on the value

of the source operand to signify whether the end of a search table is reached or not. If

the value of the source operand of an instruction equals the lowest negative number

which is representable by the data format of the corresponding instruction (‘8000H’ for

the word data type, or ‘80H’ for the byte data type), the E-flag is set to ‘1’; otherwise, it is

cleared.

MULIP-Flag: The MULIP-flag will be set to ‘1’ by hardware upon entrance into an

interrupt service routine when a multiply or divide ALU operation was interrupted before

completion. Depending on the state of the MULIP bit, the hardware decides whether or

not a multiplication or division must be continued after the end of an interrupt service.

The MULIP bit is overwritten with the contents of the stacked MULIP-flag when the

return-from-interrupt-instruction (RETI) is executed. This normally means that the

MULIP-flag is cleared again afterwards.

Table 4-2

Shift Right Rounding Error Evaluation

C-Flag

V-Flag

Rounding Error Quantity

No rounding error

0 <

Rounding error

< 1/2 LSB

Rounding error

= 1/2 LSB

Rounding error

> 1/2 LSB

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