
CHAPTER 3 CPU ARCHITECTURE
2 8
3.2 Processor Registers
The
m
PD780228 subseries units incorporate the following processor registers.
3.2.1 Control registers
The control registers control the program sequence, statuses and stack memory. A program counter (PC), a
program status word (PSW) and a stack pointer (SP) are control registers.
(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 and register contents are set.
RESET input sets the reset vector table values at addresses 0000H and 0001H to the program counter.
Figure 3-7. Program Counter Configuration
(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 reset upon execution of the RETB, RETI and POP PSW
instructions.
RESET input sets the PSW to 02H.
Figure 3-8. Program Status Word Configuration
(a) Interrupt enable flag (IE)
This flag controls interrupt request acknowledge operations of CPU.
When IE = 0, all interrupts, except the non-maskable interrupt, are disabled (DI status).
When IE = 1, the interrupts are enabled (EI status). At this time, acknowledging interrupts is controlled
with an inservice priority flag (ISP) and an interrupt mask flag for various interrupt sources and a priority
specify flag.
The interrupt enable flag is also reset to 0 when the DI instruction or an interrupt request has been
acknowledged and is set to 1 when the EI instruction has been executed.
(b) Zero flag (Z)
When the operation result is zero, this flag is set (1). It is reset (0) in all other cases.
7
0
IE
Z
RBS1
AC
RBS0
0
ISP
CY
PSW
PC15 PC14 PC13 PC12
PC11 PC10
PC9
PC8
PC7
PC6
PC5
PC4
PC3
PC2
PC1
PC0
15
0
PC