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CHAPTER 3 CPU ARCHITECTURE
User’s Manual U15104EJ2V0UD
3.2 Processor Registers
The
µ
PD178054 Subseries units incorporate the following processor registers.
3.2.1 Control registers
The control registers control the program sequence, statuses and stack memory. The control registers consist
of a program counter (PC), a program status word (PSW) and a stack pointer (SP).
(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 3-7. Configuration of Program Counter
PC15
PC
PC14 PC13 PC12 PC11
PC9
PC8
15
0
PC10
PC7
PC6
PC5
PC4
PC3
PC1
PC0
PC2
7
0
IE
Z
RBS1
AC
RBS0
0
ISP
CY
PSW
(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 restored upon execution of the RETB, RETI and POP PSW instructions.
Reset input sets the PSW to 02H.
Figure 3-8. Configuration of Program Status Word