August 27, 1998
GENERAL RELEASE SPECIFICATION
Normally, the address in the program counter increments to the next sequential
memory location every time an instruction or operand is fetched. Jump, branch,
and interrupt operations load the program counter with an address other than that
of the next sequential location.
3.6
CONDITION CODE REGISTER (CCR)
The CCR shown in Figure 3-1 is a 5-bit register in which four bits are used to
indicate the results of the instruction just executed. The fifth bit is the interrupt
mask. These bits can be individually tested by a program, and specific actions can
be taken as a result of their states. The condition code register should be thought
of as having three additional upper bits that are always ones. Only the interrupt
mask is affected by a reset of the device. The following paragraphs explain the
functions of the lower five bits of the condition code register.
3.6.1 Half Carry Bit (H-Bit)
When the half-carry bit is set, it means that a carry occurred between bits 3 and 4
of the accumulator during the last ADD or ADC (add with carry) operation. The
half-carry bit is required for binary-coded decimal (BCD) arithmetic operations.
3.6.2 Interrupt Mask (I-Bit)
When the interrupt mask is set, the internal and external interrupts are disabled.
Interrupts are enabled when the interrupt mask is cleared. When an interrupt
occurs, the interrupt mask is automatically set after the CPU registers are saved
on the stack, but before the interrupt vector is fetched. If an interrupt request
occurs while the interrupt mask is set, the interrupt request is latched. Normally,
the interrupt is processed as soon as the interrupt mask is cleared.
A return from interrupt (RTI) instruction pulls the CPU registers from the stack,
restoring the interrupt mask to its state before the interrupt was encountered. After
any reset, the interrupt mask is set and can only be cleared by the Clear I-Bit
(CLI), or WAIT instructions.
3.6.3 Negative Bit (N-Bit)
The negative bit is set when the result of the last arithmetic operation, logical
operation, or data manipulation was negative. (Bit 7 of the result was a logical
one.)
The negative bit can also be used to check an often tested flag by assigning the
flag to bit 7 of a register or memory location. Loading the accumulator with the
contents of that register or location then sets or clears the negative bit according
to the state of the flag.
3.6.4 Zero Bit (Z-Bit)
The zero bit is set when the result of the last arithmetic operation, logical
operation, data manipulation, or data load operation was zero.
MC68HC05SB7
REV 2.1
CENTRAL PROCESSING UNIT
MOTOROLA
3-3
FREESCALE [ Freescale ]
