Atari ST series Technical Reference Manual 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
0*
I/O Port Bit 0
1*
I/O Port Bit 1
2
I/O Port Bit 2
3*
I/O Port Bit 3
4*
Timer D
5 Timer C
6
I/O Port Bit 4
7*
I/O Port Bit 5
8* Timer B
9 USART
10 USART
11
USART
12
USART
13* Timer A
14*
I/O Port Bit 6
15* I/O Port Bit 7
* See text below.
The interrupts that have an asterisk next to their bit
number are initially disabled, but the user may enable or dis-
ST Function
Parallel port busy
RS-232 Data Carrier Detect (DCD)
RS-232 Clear To Send (CTS)
Graphics blitter chip done
RS-232 baud rate generator
System Clock (200 Hz)
Keyboard and MIDI ACIA data
request
Floppy drive/DMA port data request
Horizontal blank counter
RS-232 transmit error
RS-232 transmit buffer empty
RS-232 receive error
RS-232 receive buffer full
User-defined timer interrupt
RS-232 Ring Indicator (RI)
Monochrome monitor detect
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