CHAPTER 3 CPU ARCHITECTURE
Preliminary User’s Manual U19014EJ1V0UD
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3.2 Processor Registers
The
µ
PD78F0730 incorporates 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, 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 signal generation sets the reset vector table values at addresses 0000H and 0001H to the program counter.
Figure 3-3. Format of Program Counter
15
PC PC15 PC14 PC13 PC12 PC11 PC10 PC9 PC8 PC7 PC6 PC5 PC4 PC3 PC2 PC1 PC0
0
(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 stored in the stack area upon interrupt request generation or PUSH PSW
instruction execution and are restored upon execution of the RETB, RETI and POP PSW instructions.
Reset signal generation sets PSW to 02H.
Figure 3-4. Format of Program Status Word
IE
Z
RBS1
AC
RBS0
ISP
CY
7
0
0
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 interrupt requests are 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.
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