UPD17068 datasheet(49/341 Pages) NEC

49 / 341 page

µPD17068

5.6

INDEX REGISTER (IX) AND DATA MEMORY ROW ADDRESS POINTER (MP: MEMORY POINTER)

5.6.1

Index Register (IX)

(1) Format of Index Register

Fig. 5-6 shows the format of the index register.

The index register consists of 11 bits: the lower 3 bits (IXH) of 7AH of the system register, 7BH (IXM), and

7CH (IXL). The lower 2 bits (bits 2 and 1 of 7AH) are always set to 0. In operation to access the VRAM, however,

only the higher 1 bit (bit 2 of 7AH) is always set to 0. For the method of VRAM access, see Section 16.5.7.

Fig. 5-6 Format of Index Register

(2) Index register functions

The index register is used to modify data memory addresses when a data memory manipulation instruction

is executed. That is, the data memory bank, row address, and column address specified by a data memory

manipulation instruction are ORed with the contents of the index register, and the instruction is executed for

the data memory location specified by the result of OR operation.

Note, however, that address modification is enabled only when the IXE flag (bit 0 of 7FH of the system

A dedicated instruction (INC IX) for incrementing the index register allows easy access to a data memory

location.

Address modification using the index register can be performed with all data memory manipulation

instructions.

With the instructions listed below, address modification using the index register is impossible.

Address

7AH

7BH

7CH

7EH

7FH

Index register (IX)

Memory pointer (MP)

IXH

IXM

MPH

MPL

IXL

Bit

Data memory

access

VRAM access

Power-on

Clock stop

Symbol

PSW

Program status word

(PSWORD)

Fixed

VRAMBANK

Row

address

Column

address

Column address

Row address

BANK

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