8x300 design guide - Al Kossow's Bitsavers - Trailing-Edge

This method of fast I/o port selection is suitable for use in small
systems. It can be used in the same system with normal select I/o
(such as 8X32s), but when using fast I/O select the programmer must
ensure that a programmable I/O port has not been left selected on
the bank in use.
On large systems extra delays may be encountered, with the result
that it may be necessary to program the I/O port enable address in
the instruction preceding its usage. A double set of D-latches are
used as address hold latches to insure that the address appears
sufficiently early in the instruction cycle. Refer to Figure 2-25.
TO IV BYTES{
MASTER
ENABLE -a BIT
BIT N
1
}
EXTRA BITS
ADDED FOR
FAST SelECT
ROM
BANK SelECT
1/0 BUS
16
INSTRUCTION ..... +----11
8X300
Figure 2-25. Fast I/O Select For Large Systems
In both examples, Figures 2-24 and 2-25, the program memory
extension is usually made in increments of four or eight bits. If only
one I/O port is to be enabled, then the remaining extension bits may
be used for other control or display drive functions. If, on the other
hand, a maximum of eight extension bits are decoded, this allows
I/O port selection from up to 256 ports.
2-41