CHAPTER 4 CPU ARCHITECTURE
User’s Manual U14826EJ5V0UD
44
4.2 Processor Registers
The
µ
PD789860, 789861 Subseries provide the following on-chip processor registers:
4.2.1 Control registers
The control registers have special functions to control the program sequence statuses and stack memory. The
control registers include a program counter, a program status word, and a stack pointer.
(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 reset vector table values at addresses 0000H and 0001H to the program counter.
Figure 4-5. Program Counter Configuration
0
15
PC14
PC15
PC
PC13 PC12 PC11 PC10
PC9
PC8
PC7
PC6
PC5
PC4
PC3
PC2
PC1
PC0
(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 PSW to 02H.
Figure 4-6. Program Status Word Configuration
7
0
IE
Z
0
AC
0
0
1
CY
PSW
(a) Interrupt enable flag (IE)
This flag controls interrupt request acknowledge operations of the CPU.
When IE = 0, the interrupt disabled (DI) status is set. All interrupt requests except non-maskable interrupt
are disabled.
When IE = 1, the interrupt enabled (EI) status is set. Interrupt request acknowledgment is controlled with
an interrupt mask flag for various interrupt sources.
This flag is reset to 0 upon DI instruction execution or interrupt acknowledgment and is set to 1 upon EI
instruction execution.
(b) Zero flag (Z)
When the operation result is zero, this flag is set to 1. It is reset to 0 in all other cases.
(c) Auxiliary carry flag (AC)
If the operation result has a carry from bit 3 or a borrow at bit 3, this flag is set to 1. It is reset to 0 in all
other cases.
Содержание PD789860
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