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CENTRAL PROCESSING UNIT (CPU)
2.1 Central processing unit (CPU)
7700 FAMILY SOFTWARE MANUAL
2–2
2.1 Central processing unit (CPU)
The CPU of 7700 Series has the ten registers as shown in Figure 2.1.1. Each of these registers is described
below.
2.1.1 Accumulator (Acc)
Accumulators A and B are available and each can be used as 8-bit or 16-bit register as necessary.
(1) Accumulator A (A)
Accumulator A is the main register of the microcomputer. Data operations such as calculations, data
transfer, and input/output are executed mainly through accumulator A. It consists of 16 bits, and the
low-order 8 bits can be also used separately. The data length flag (m) determines whether the register
is used as a 16-bit register or as an 8-bit register. It is used as a 16-bit register when the flag m is
“0”, and as an 8-bit register when the flag m is “1”. The flag m is a part of the processor status register
(PS) which is described later. When an 8-bit register is selected, the low-order 8 bits of the accumu-
lator A are used and the contents of the high-order 8 bits are unchanged.
(2) Accumulator B (B)
Accumulator B is a 16-bit register with the same function as accumulator A. The instructions of 7700
Series can use accumulator B instead of accumulator A, but the use of accumulator B requires more
instruction bytes and execution cycles than accumulator A. Accumulator B is also controlled by the
data length flag m just as in accumulator A.
2.1.2 Index register X (X)
Index register X consists of 16 bits and the low-order 8 bits can be also 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. It is used
as a 16-bit register when the flag x is “0” and as an 8-bit register when the flag x is “1”. The flag x is a
part of the processor status register (PS) which is described later. When an 8-bit register is selected, the
low-order 8 bits of the index register X are used and the contents of the high-order 8 bits are unchanged.
In addressing mode in which the index register X is used as the index register, the contents of this register
is added to obtain the real address.
When executing the block transfer instruction
MVP
or
MVN
, the contents of the index register X indicate
the low-order 16 bits of the source data address. The third byte of the
MVP
or
MVN
instruction is the high-
order 8 bits of the source data address.
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, the index register length flag (x) determines whether this register is used as a 16-bit register or as an
8-bit register.
When executing the block transfer instruction
MVP
or
MVN
, the contents of the index register Y indicate
the low-order 16 bits of the destination data address. The second byte of the
MVP
or
MVN
instruction is
the high-order 8 bits of the destination data address.