NEC PD754244 Manuel D’Utilisation

CHAPTER 4 INTERNAL CPU FUNCTION

User’s Manual U10676EJ3V0UM

4.8 Program Status Word (PSW) ... 8 Bits

The program status word (PSW) consists of flags closely related to the operations of the processor.

PSW is mapped to addresses FB0H and FB1H of the data memory space, and the 4 bits of address FB0H can

be manipulated by using a memory manipulation instruction.

Figure 4-13. Configuration of Program Status Word

RBE

MBE

IST0

IST1

(SK0)

Note

(SK1)

Note

(SK2)

Note

(CY)

Note

Cannot be
manipulated

Can be
manipulated

FB0H

FB1H

Can be manipulated
by dedicated instruction

Symbol

PSW

Address

FB0H

Note Not reserved as a reserved word.

Table 4-3. PSW Flags Saved/Restored to/from Stack

Flag Saved or Restored

Save

When CALL, CALLA, or CALLF instruction is executed

MBE and RBE are saved

When hardware interrupt occurs

All PSW bits are saved

Restore

When RET or RETS instruction is executed

MBE and RBE are restored

When RETI instruction is executed

All PSW bits are restored

(1) Carry flag (CY)

The carry flag records the occurrence of an overflow or underflow when an operation instruction with a carry

(ADDC or SUBC) is executed.

The carry flag also functions as a bit accumulator and can store the result of a Boolean operation performed

between a specified bit address and data memory.

The carry flag is manipulated by using a dedicated instruction and is independent of the other PSW bits.

The carry flag becomes undefined when the RESET signal is asserted.