UPD78323LP datasheet(20/86 Pages) NEC

20 / 86 page

µPD78323, 78324

2.2.1

Control Register

The control registers carry out dedicated functions such as control of the program sequence, status and stack memory,

and modification of operand addressing. They consist of three 16-bit registers and one 8-bit register.

(1)

Program counter (PC)

This is a 16-bit register which holds the address information of the next program to be executed. It is normally incremented

according to the number of bytes of the instruction to be fetched. If an instruction with data branch is executed, immediate

data and the register content are set. RESET input sets and branches the data of 0000H and 0001H reset vector tables

in the PC.

(2)

Program status word (PSW)

This is a 16-bit register consisting of various flags which are set or reset by the result of instruction execution. Read/

write access is carried out in units of the higher 8 bits (PSWH) or lower 8 bits (PSWL). Each flag can be operated using

the bit operation instruction. If an interrupt request is made or BRK instruction is executed, data is automatically saved in

the stack and is recovered by RETI or RETB instruction.

All bits are reset to 0 by RESET input.

Figure 2-3. PSW Format

(a)

Interrupt priority level transition flag (LT)

65432

RBS2 RBS1 RBS0

7654

3210

RSS

P/V

This flag is used to control the interrupt priority. For normal operation of the interrupt control circuit, this bit must

not be operated by a program.

(b) Carry flag (CY)

If a carry is generated out of bit 7 or 15 as a result of the execution of an operation instruction or a borrow is generated

into bit 7 or 15, this flag is set to 1. In all other cases, this flag is reset to 0. This flag can be tested by the conditional

branch instruction.

When a bit control instruction is executed, this flag functions as a bit accumulator.

When the operation result is zero, this flag is set to 1. In all other cases, this flag is reset to 0. This flag can be tested

by the conditional branch instruction.

(d) Sign flag (S)

When MSB of the operation result is “1”, this flag is set to 1. When the MSB is “0”, this flag is reset to 0. This flag

can be tested by the conditional branch instruction.

(e)

Parity/overflow flag (P/V)

Only when an overflow or underflow occurs as two’s complement during execution of an arithmetic operation

instruction, this flag is set to 1. In all other cases, it is reset to 0 (overflow flag operation).

If the bit number of the operation result set to 1 is even during execution of an logic operation instruction, this flag

is set to 1. If the bit number is odd, this flag is reset to 0 (parity flag operation).

This flag can be tested by the conditional branch instruction.

PSWH

PSWL

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