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CONNECTING EXTERNAL DEVICES

7733 Group User’s Manual

12–10

12.1.10 Operation of bus interface unit (BIU)

Figures 12.1.6 and 12.1.7 show operating waveform examples of signals which are input to or output from

the external when accessing external devices. These waveforms are described in relation to the basic

operating waveforms. (Refer to section “2.2.3 Operation of bus interface unit (BIU).”)

(1) When fetching an instruction into an instruction queue buffer

When an instruction which is next fetched resides at an even address

When the external data bus is 16 bits wide, the BIU fetches two bytes of the instruction at a time

with waveform (a). When the external data bus is 8 bits wide, the BIU fetches only one byte of the

instruction with the first half of waveform (e).

When an instruction which is next fetched resides at an odd address

When the external data bus is 16 bits wide, the BIU fetches only one byte of the instruction with

waveform (d). When the external data bus is 8 bits wide, the BIU fetches only one byte of the

instruction with the first half of waveform (f).

When branched to an odd address by executing a branch instruction or others with the 16-bit external

data bus, at first, the BIU fetches one byte of an instruction with waveform (d) and then fetches

instructions by the two bytes with waveform (a).

(2) When reading or writing data from or to memory • I/O

When accessing 16-bit data which starts from an even address, waveform (a) or (e) is applied.

When accessing 16-bit data which starts from an odd address, waveform (b) or (f) is applied.

When accessing 8-bit data which resides at an even address, waveform (c) or the first half of

waveform (e) is applied.

When accessing 8-bit data which resides at an odd address, waveform (d) or the first half of

waveform (f) is applied.

For instructions which are affected by data length flag (m) and index register length flag (x), an

operation is applied as follows:

•When “m” or “x” = “0,” operation or is applied.

•When “m” or “x” = “1,” operation or is applied.

Settings of flags “m” and “x” and selection of the external data bus width do not affect each other.

12.1 Signals required for accessing external devices

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