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7721 Group User’s Manual

CENTRAL PROCESSING UNIT (CPU)

2–16

2.3.1 Banks

The access space is divided in units of 64 Kbytes. This unit is called “bank.” The high-order 8 bits of

address (24 bits) indicate a bank, which is specified by the program bank register (PG) or data bank

If the program counter (PC) overflows at a bank boundary, the contents of the program bank register (PG)

is incremented by 1. If a borrow occurs in the program counter (PC) as a result of subtraction, the contents

of the program bank register (PG) is decremented by 1. Normally, accordingly, the user can program

without concern for bank boundaries.

SFR (Special Function Register) and internal RAM are assigned in bank 016. For details, refer to section

“2.4 Memory assignment.”

2.3.2 Direct page

A 256-byte space specified by the direct page register (DPR) is called “direct page.” A direct page is

specified by setting the base address (the lowest address) of the area to be specified as a direct page into

the direct page register (DPR).

By using a direct page addressing mode, a direct page can be accessed with less instruction cycles than

otherwise.

Note: Refer also to section “2.1 Central processing unit.”

2.3 Access space

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