Atari ST series - Page 63

XBIOS Device and System Functions
int bitnum;
Offgibit(bitnum);
int bitnum;
Ongibit(bitnum);
where bitnum is the number of the bit (0-7) to change. Offgi-
bit changes the specified bit number to a 0, while Ongibit
changes the bit to a one.
The 68901 Multi-Function Peripheral (MFP) Chip is an
other important I/O chip on the ST. It contains an 8-bit paral
lel I/O port, a Universal Synchronous/Asynchronous Re
ceiver/Transmitter (USART) for serial I/O, and four general-
purpose timers. 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 6, but they are not autovectored. This means that
when an MFP interrupt occurs, the IPL 6 interrupt handler is
not called. Instead, the MFP chip directs execution to one of
its own interrupt handlers. The 16 MFP interrupts (in order
of priority), and their functions on the ST, are
Table 3-9. The 16 MFP Interrupts
Bit
Number
Interrupt Source
ST Function
0*
I/O Port Bit 0
Parallel port busy
1*
I/O Port Bit 1
RS-232 Data Carrier Detect (DCD)
2
I/O Port Bit 2
RS-232 Clear To Send (CTS)
3*
I/O Port Bit 3
Graphics blitter chip done
4*
Timer D
RS-232 baud rate generator
5 Timer C
System Clock (200 Hz)
6
I/O Port Bit 4
Keyboard and MIDI ACIA data
request
7*
I/O Port Bit 5 Floppy drive/DMA port data request
8* Timer B
Horizontal blank counter
9 USART
RS-232 transmit error
10 USART
RS-232 transmit buffer empty
11
USART
RS-232 receive error
12
USART
RS-232 receive buffer full
13* Timer A
User-defined timer interrupt
14*
I/O Port Bit 6
RS-232 Ring Indicator (RI)
15*
I/O Port Bit 7
Monochrome monitor detect
* See text below.
The interrupts that have an asterisk next to their bit
number are initially disabled, but the user may enable or dis-
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