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7906 Group User’s Manual Rev.2.0

2.1 Central processing unit (CPU)

CENTRAL PROCESSING UNIT (CPU)

2-3

2.1.1 Accumulator (Acc)

Accumulators A and B are available. Also, accumulators A and B can be connected in series in order to

be used as a 32-bit accumulator (accumulator E).

(1)

Accumulator A (A)

Accumulator A is the main register of the microcomputer. The transaction of data such as calculation,

data transfer, and input/output are performed mainly through accumulator A. It consists of 16 bits,

and the low-order 8 bits can also be used separately. The data length flag (m) determines whether

the register is used as a 16-bit register or as an 8-bit register. Flag m is a part of the processor status

accumulator A are used, and the contents of the high-order 8 bits is unchanged.

(2)

Accumulator B (B)

Accumulator B is a 16-bit register with the same function as accumulator A. Accumulator B can be

used instead of accumulator A. The use of accumulator B, however except for some instructions,

requires more instruction bytes and execution cycles than those of accumulator A. Accumulator B is

also affected by flag m just as in accumulator A.

(3)

Accumulator E (E)

This 32-bit accumulator consists of accumulator A located in the low-order 16 bits and accumulator

B located in the high-order 16 bits. This accumulator is used by an instruction that handles 32-bit

data. It is not affected by flag m.

2.1.2 Index register X (X)

Index register X consists of 16 bits and the low-order 8 bits can also be used separately. The index register

length flag (x) determines whether the register is used as a 16-bit register or as an 8-bit register. Flag x

is a part of the processor status register, which is described later. When an 8-bit register is selected, only

the low-order 8 bits of index register X are used, and the contents of the high-order 8 bits are not

unchanged.

In an addressing mode in which index register X is used as an index register, the address obtained by

adding the contents of this register to the operand’s contents is accessed.

Also, each of the MVP, MVN and RMPA instructions uses index register X.

❈ Refer to “7900 Series Software Manual” for addressing modes and instructions.

2.1.3 Index register Y (Y)

Index register Y is a 16-bit register with the same function as index register X. Just as in index register

X, this register is affected by flag X.

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