80C186EC/80C188EC Microprocessor User's Manual

PERIPHERAL CONTROL BLOCK
4.4.3 F-Bus Operation
The F-Bus functions differently than the external data bus for byte and word accesses. All write
transfers on the F-Bus occur as words, regardless of how they are encoded. For example, the instruction
OUT DX, AL (DX is even) will write the entire AX register to the Peripheral Control
Block register at location [DX]. If DX were an odd location, AL would be placed in [DX] and
AH would be placed at [DX–1]. A word operation to an odd address would write [DX] and [DX–
1] with AL and AH, respectively. This differs from normal external bus operation where unaligned
word writes modify [DX] and [DX+1]. In summary, do not use odd-aligned byte or word
writes to the PCB.
Aligned word reads work normally. Unaligned word reads work differently. For example, IN AX,
DX (DX is odd) will transfer [DX] into AL and [DX–1] into AH. Byte reads from even or odd
addresses work normally, but only a byte will be read. For example, IN AL, DX will not transfer
[DX] into AX (only AL is modified).
No problems will arise if the following recommendations are adhered to.
Word reads
Aligned word reads of the PCB work normally. Access only evenaligned
words with IN AX, DX or MOV word register, even PCB
address.
Byte reads
Byte reads of the PCB work normally. Beware of reading word-wide
PCB registers that may change value between successive reads (e.g.,
timer count value).
Word writes
Always write even-aligned words to the PCB. Writing an oddaligned
word will give unexpected results.
For the 80C186 Modular Core, use either
– OUT DX, AX or
– OUT DX, AL or
– MOV even PCB address, word register.
For the 80C188 Modular Core, using OUT DX, AX will perform an
unnecessary bus cycle and is not recommended. Use either
– OUT DX, AL or
– MOV even-aligned byte PCB address, byte register low byte.
Byte writes
Always use even-aligned byte writes to the PCB. Even-aligned byte
writes will modify the entire word PCB location. Do not perform
unaligned byte writes to the PCB.
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