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The 68901
handles a variety of input/output chores on the ST. It con
tains an 8-bit parallel I/O port, each bit of which is used for a
different purpose. The port is used for monochrome monitor
detection, RS-232 carrier detect, ring detect, and clear to send
(CTS), parallel port and blitter-busy detection a unit, and
data requests from the floppy drives, DMA port, intelligent
keyboard controller, and MIDI ports. The MFP chip's on
board Universal Synchronous/Asynchronous Receiver/Trans
mitter (USART) provides the hardware interface for the ST's
serial port. The chip also has four general-purpose timers.
They're used for the RS-232 baud rate generator, the 200 Hz
system clock, and as a horizontal blanking counter.
The serial port, the timers, and each bit of the parallel I/O
port are capable of generating an interrupt. The MFP chip
interrupts have an Interrupt Priority Level (IPL) of six, but
they are not auto vectored. This means that when an MFP in
terrupt occurs, the IPL 6 interrupt handler is not called. In
stead, the MFP chip directs exception processing through
one of its 16 exception vectors. These are vectors 64-80,
which are located at addresses 256 - 323 ($100 —$143). Al
though all MFP interrupts have an overall 68000 IPL of 6,
there are subpriorities that are handled by the MFP chip.
MFP Registers
The ST system communicates with the MFP chip through its
24 8-bit registers. On the current ST models, these registers
are found at the 24 odd addresses, starting at address
$FFFFFA01. Since this may change with future models, pro
grammers should, whenever possible, communicate with the
MFP chip through XBIOS routines like Xbtimer(), Rsconf(),
Multi-Function Peripheral (MFP) chip
301
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