NEC switch - Manual (Page 40)

CHAPTER 3 CPU ARCHITECTURE
User’s Manual U12978EJ3V0UD
40
3.2 Processor Registers
The µPD789800 Subseries provides the following on-chip processor registers.
3.2.1 Control registers
The control registers contain special functions to control the program sequence, statuses and stack memory. A
program counter, a program status word, and a stack pointer are the control registers.
(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 or register contents are set.
RESET input sets the program counter to the reset vector table values at addresses 0000H and 0001H.
Figure 3-5. Configuration of Program Counter
PC15 PC PC14 PC13 PC12 PC11 PC10 PC9 PC8 PC7 PC6 PC5 PC4 PC3 PC2 PC1 PC0
15 0
(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 RETI and POP PSW instructions.
RESET input sets the PSW to 02H.
Figure 3-6. Configuration of Program Status Word
7 0
IE Z 0 AC 0 0 1 CY