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The CPU and the bus send or receive data via BIU because each operates based on different clocks
(Note). The BIU allows the CPU to operate at high speed without waiting for access to the memory • I/O
devices that require a long access time.
The BIU's functions are described bellow.
Note: The CPU operates based on φ
bus operates based on the
(1) Reading out instruction (Instruction prefetch)
When the CPU does not require to read or write data, that is, when the bus is not in use, the BIU
reads instructions from the memory and stores them in the instruction queue buffer. This is called
instruction prefetch.
The CPU reads instructions from the instruction queue buffer and executes them, so that the CPU
can operate at high speed without waiting for access to the memory which requires a long access
time.
When the instruction queue buffer becomes empty or contains only 1 byte of an instruction, the BIU
performs instruction prefetch. The instruction queue buffer can store instructions up to 3 bytes.
The contents of the instruction queue buffer is initialized when a branch or jump instruction is
executed, and the BIU reads a new instruction from the destination address.
When instructions in the instruction queue buffer are insufficient for the CPU's needs, the BIU
extends the pulse duration of clock φ
required number of instructions or more.
(2) Reading data from memory•I/O device
The CPU specifies the storage address of data to be read to the BIU's data address register, and
requires data. The CPU waits until data is ready in the BIU.
The BIU outputs the address received from the CPU onto the address bus, reads contents at the
specified address, and takes it into the data buffer.
The CPU continues processing, using data in the data buffer.
However, if the BIU uses the bus for instruction prefetch when the CPU requires to read data, the
BIU keeps the CPU waiting.
(3) Writing data to memory•I/O device
The CPU specifies the address of data to be written to the BIU's data address register. Then, the
CPU writes data into the data buffer. The BIU outputs the address received from the CPU onto the
address bus and writes data in the data buffer into the specified address.
The CPU advances to the next processing without waiting for completion of BIU's write operation.
However, if the BIU uses the bus for instruction prefetch when the CPU requires to write data, the
BIU keeps the CPU waiting.
CENTRAL PROCESSING UNIT (CPU)
. The period of φ
CPU
_
signal. The period of the
E
in order to keep the CPU waiting until the BIU fetches the
CPU
7751 Group User's Manual
2.2 Bus interface unit
is normally the same as that of φ . The internal
CPU
_
signal is twice that of φ at a minimum.
E
2–13
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