Processor Registers; Control Registers; Program Counter Configuration; Program Status Word Configuration - NEC UPD789426 Series User Manual

3.2 Processor Registers

µ
The PD789426, 789436, 789446, and 789456 Subseries provide 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. The
program counter, program status word, and stack pointer are control registers.
(1) Program counter (PC)
The program counter is a 16-bit register that 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 reset vector table values at addresses 0000H and 0001H to the program counter.
15
PC PC15 PC14 PC13 PC12 PC11 PC10 PC9
(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.
The 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 PSW to 02H.
PSW
60
CHAPTER 3 CPU ARCHITECTURE
Figure 3-13. Program Counter Configuration
PC8
Figure 3-14. Program Status Word Configuration
7
IE Z 0 AC
User's Manual U15075EJ2V1UD
PC7 PC6 PC5 PC4 PC3
0
0 0 1 CY
0
PC2 PC1 PC0