Processor Registers; Control Registers - NEC 78K0 User Manual

3.2

Processor Registers

The 78K0/KF1 products 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 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.
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 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.
PSW
(a) Interrupt enable flag (IE)
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 interrupts are disabled.
When 1, the IE flag is set to the interrupt enabled (EI) state and interrupt request acknowledgment 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 acknowledgment and is set (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.
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CHAPTER 3 CPU ARCHITECTURE
Figure 3-11. Format of Program Counter
PC8
Figure 3-12. Format of Program Status Word
7
IE Z RBS1 AC
User's Manual U15947EJ2V0UD
PC7 PC6 PC5 PC4 PC3
*
0
RBS0 0 ISP CY
0
PC2 PC1 PC0