
6 7
CHAPTER 5 CPU ARCHITECTURE
5.2 Processor Registers
The
m
PD78064 and 78064Y subseries units incorporate the following processor registers.
5.2.1 Control registers
The control registers control the program sequence, statuses and stack memory. The control registers consist
of a program counter, a program status word and a stack pointer.
(1) Program counter (PC)
The program counter is a 16-bit register which holds the address information of the next program to be
executed.
In normal operation, the PC is automatically incremented according to the number of bytes of the instruction
to be fetched. When a branch instruction is executed, immediate data and register contents are set.
RESET input sets the reset vector table values at addresses 0000H and 0001H to the program counter.
Figure 5-9. Program Counter Configuration
(2) Program status word (PSW)
The program status word is an 8-bit register consisting of various flags to be set/reset by instruction execution.
Program status word contents are automatically stacked upon interrupt request generation or PUSH PSW
instruction execution and are automatically reset upon execution of the RETB, RETI and POP PSW
instructions.
RESET input sets the PSW to 02H.
Figure 5-10. Program Status Word Configuration
(a) Interrupt enable flag (IE)
This flag controls the interrupt request acknowledge operations of the CPU.
When 0, the IE is set to DI, and only non-maskable interrupt request becomes acknowledgeable. Other
interrupt requests are all disabled. When 1, the IE is set to EI and interrupt request acknowledge enable
is controlled with an inservice priority flag (ISP), an interrupt mask flag for various interrupt sources and
a priority specification flag.
The IE is reset to (0) upon DI instruction execution or interrupt acknowledgement and is set to (1) upon
EI instruction execution.
(b) Zero flag (Z)
When the operation result is zero, this flag is set (1). It is reset (0) in all other cases.
(c) Register bank select flags (RBS0 and RBS1)
These are 2-bit flags to select one of the four register banks.
In these flags, the 2-bit information which indicates the register bank selected by SEL RBn instruction
execution is stored.
PC
15
0
7
0
IE
Z
RBS1
AC
RBS0
0
ISP
CY