CPU-to-Host Communication
to this register automatically sets Bit 0 (OBF) in
the Status register. See Table 49.
The FDC37B77x CPU can write to the Output
Data register
via
register DBB. A write
Table 49 - Host Interface Flags
8042 INSTRUCTION
FLAG
OUT DBB
Set OBF, and, if enabled, the KIRQ output signal goes high
Host-to-CPU Communication
MIRQ
The host system can send both commands and
data to the Input Data register. The CPU
differentiates between commands and data by
reading the value of Bit 3 of the Status register.
When bit 3 is "1", the CPU interprets the register
contents as a command. When bit 3 is "0", the
CPU interprets the register contents as data.
During a host write operation, bit 3 is set to "1" if
SA2 = 1 or reset to "0" if SA2 = 0.
If "EN FLAGS" has been executed and P25 is
set to a one:; IBF is inverted and gated onto
MIRQ. The MIRQ signal can be connected to
system interrupt to signify that the FDC37B77x
CPU has read the DBB register.
If "EN FLAGS” has not been executed, MIRQ is
controlled by P25, Writing a zero to P25 forces
MIRQ low, a high forces MIRQ high. (MIRQ is
normally selected as IRQ12 for mouse support).
KIRQ
If "EN FLAGS" has been executed and P24 is
set to a one: the OBF flag is gated onto KIRQ.
The KIRQ signal can be connected to system
interrupt to signify that the FDC37B77x CPU has
written to the output data register via "OUT
DBB,A". If P24 is set to a zero, KIRQ is forced
low. On power-up, after a valid RST pulse has
been delivered to the device, KIRQ is reset to 0.
KIRQ will normally reflects the status of writes
"DBB". (KIRQ is normally selected as IRQ1 for
keyboard support.)
Gate A20
A general purpose P21 is used as a software
controlled Gate A20 or user defined output.
EXTERNAL
INTERFACE
KEYBOARD
AND
MOUSE
Industry-standard PC-AT-compatible keyboards
employ a two-wire, bidirectional TTL interface
for data transmission. Several sources also
supply PS/2 mouse products that employ the
same type of interface. To facilitate system
expansion, the FDC37B77x provides four signal
pins that may be used to implement this
interface directly for an external keyboard and
mouse.
If "EN FLAGS” has not been executed: KIRQ
can be controlled by writing to P24. Writing a
zero to P24 forces KIRQ low; a high forces
KIRQ high.
122
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