Freescale Semiconductor, Inc.
tions. Shift and rotate instructions operate with and through the carry bit to facilitate
multiple-word shift operations.
3.1.6.2 Overflow (V)
The overflow bit is set if an operation causes an arithmetic overflow. Otherwise, the V
bit is cleared.
3.1.6.3 Zero (Z)
The Z bit is set if the result of an arithmetic, logic, or data manipulation operation is
zero. Otherwise, the Z bit is cleared. Compare instructions do an internal implied sub-
traction and the condition codes, including Z, reflect the results of that subtraction. A
few operations (INX, DEX, INY, and DEY) affect the Z bit and no other condition flags.
For these operations, only = and - conditions can be determined.
Freescale Semiconductor, Inc...
3.1.6.4 Negative (N)
The N bit is set if the result of an arithmetic, logic, or data manipulation operation is
negative (MSB = 1). Otherwise, the N bit is cleared. A result is said to be negative if its
most significant bit (MSB) is a one. A quick way to test whether the contents of a mem-
ory location has the MSB set is to load it into an accumulator and then check the status
of the N bit.
3.1.6.5 Interrupt Mask (I)
The interrupt request (IRQ) mask (I bit) is a global mask that disables all maskable in-
terrupt sources. While the I bit is set, interrupts can become pending, but the operation
of the CPU continues uninterrupted until the I bit is cleared. After any reset, the I bit is
set by default and can only be cleared by a software instruction. When an interrupt is
recognized, the I bit is set after the registers are stacked, but before the interrupt vector
is fetched. After the interrupt has been serviced, a return from interrupt instruction is
normally executed, restoring the registers to the values that were present before the
interrupt occurred. Normally, the I bit is zero after a return from interrupt is executed.
Although the I bit can be cleared within an interrupt service routine, “nesting” interrupts
in this way should only be done when there is a clear understanding of latency and of
the arbitration mechanism. Refer to
3.1.6.6 Half Carry (H)
The H bit is set when a carry occurs between bits 3 and 4 of the arithmetic logic unit
during an ADD, ABA, or ADC instruction. Otherwise, the H bit is cleared. Half carry is
used during BCD operations.
3.1.6.7 X Interrupt Mask (X)
The XIRQ mask (X) bit disables interrupts from the pin. After any reset, X is set by de-
fault and must be cleared by a software instruction. When an interrupt is recognized,
the X and I bits are set after the registers are stacked, but before the interrupt vector
is fetched. After the interrupt has been serviced, an RTI instruction is normally execut-
ed, causing the registers to be restored to the values that were present before the in-
CENTRAL PROCESSING UNIT
TECHNICAL DATA
For More Information On This Product,
Go to: www.freescale.com
3-5
FREESCALE [ FREESCALE SEMICONDUCTOR, INC ]
