UPD78F0138 NEC, UPD78F0138 Datasheet - Page 68

• Manufacturer Part Number: UPD78F0138
• Description: (UPD78xxxx) 8-Bit Single-Chip Microcontrollers
• Manufacturer: NEC

3.2 Processor Registers
3.2.1 Control registers
program counter (PC), a program status word (PSW) and a stack pointer (SP).
(1) Program counter (PC)
(2) Program status word (PSW)
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The 78K0/KE1 products incorporate the following processor registers.
The control registers control the program sequence, statuses and stack memory. The control registers consist of a
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 and register contents are set.
RESET input sets the reset vector table values at addresses 0000H and 0001H to the program counter.
The program status word is an 8-bit register consisting of various flags set/reset by instruction execution.
Program status word contents are automatically stacked upon interrupt request generation or PUSH PSW
instruction execution and are restored upon execution of the RETB, RETI and POP PSW instructions.
RESET input sets the PSW to 02H.
(a) Interrupt enable flag (IE)
PC
This flag controls the interrupt request acknowledge operations of the CPU.
When 0, the IE flag is set to the interrupt disabled (DI) state, and all maskable interrupt requests are
disabled. Other interrupt requests are all disabled.
When 1, the IE flag is set to the interrupt enabled (EI) state and interrupt request acknowledgement is
controlled with an in-service priority flag (ISP), an interrupt mask flag for various interrupt sources, and a
priority specification flag.
The IE flag is reset (0) upon DI instruction execution or interrupt acknowledgement and is set (1) upon EI
instruction execution.
PC15 PC14 PC13 PC12 PC11 PC10 PC9
15
PSW
Figure 3-18. Format of Program Status Word
IE
7
Figure 3-17. Format of Program Counter
CHAPTER 3 CPU ARCHITECTURE
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User’s Manual U16228EJ2V0UD
RBS1
PC8
AC
PC7
RBS0
PC6
0
PC5
ISP
PC4
PC3
CY
0
PC2
PC1 PC0
0