UPD7564 datasheet(17/56 Pages) NEC

17 / 56 page

µPD7564A, 7564A(A)

2.6

ACCUMULATOR (A): 4 BITS

The accumulator is a 4-bit register which plays a major role in many types of arithmetic operations. The

accumulator may be processed in 8-bit units when paired with the data memory addressed by the pair register HL.

Fig. 2-7 Accumulator Configuration

2.7

ARITHMETIC LOGIC UNIT (ALU): 4 BITS

The arithmetic logic unit is a 4-bit arithmetic circuit to perform arithmetic and bit processing such as binary

addition, logical operation, incrementation, decrementation, and comparison.

2.8

PROGRAM STATUS WORD (PSW): 4 BITS

The program status word consists of skip flags (SK1 and SK0) and a carry flag (C). Bit 1 of the PSW is fixed at 0.

Fig. 2-8 Program Status Word Configuration

SK1

SK0

PSW

(1) Skip flags (SK1 and SK0)

Skip flags store the following skip status:

• Stacking by the LAI instruction

• Stacking by the LHLI instruction

• Skip condition establishment by any instruction other than stack instructions

The skip flags are set and reset automatically when respective instructions are executed.

(2) Carry flag (C)

The carry flag is set to 1 when a carry from bit 3 of the ALU occurs when the add instruction (ACSC) is executed.

The flag is reset to 0 when the carry does not occur. The SC and RC instructions respectively set and reset the carry

flag. The SKC instruction tests the contents of the flag.

The content of the PSW are automatically stored in the stack area when the call instruction is executed. It cannot

be restored by the return inhstruction.

When the RESET signal is input, SK1 and SK0 are both cleared to zero and C becomes undefined.

UPD7564 Datasheet(PDF) 17 Page - NEC