Atari ST series Technical Reference Manual page 310

Jdisint(), Jenabint() and Mfpint(). The MFP registers, and
their functions, are:
Register 1. General Purpose I/O Interrupt Port (GPIP).
This is the data register for the 8-bit parallel I/O port, where
the data is read and written.
Register 2. Active Edge Register (AER). For parallel port
input bits. This register specifies whether the interrupt will
occur on low-to-high transitions (1), or high-to-low transi­
tions (0).
Register 3. Data Direction Register (DDR). This register
specifies whether each bit of the parallel I/O port will be
used for input (0), or output (1).
Register 4. Interrupt Enable Register A (IERA). Each bit
of this register is used to determine whether the correspond­
ing interrupt will be enabled (1), or disabled (0). These are
detailed in Table F-l. The interrupts that have an asterisk
next to their priority level are initially disabled.
Table F -l. Interrupt Disable Register A (IERA)
Internal
Bit
Priority
imber
Level
7
15*
6 14*
5 13*
4 12
3 11
2
10
1 9
0 8*
Register 5. Interrupt Enable Register B (IERB). Each bit
of this register is used to determine whether the correspond­
ing interrupt will be enabled (1), or disabled (0). The inter­
rupts that have an asterisk next to their priority level are ini­
tially disabled.
Table F-2. Interrupt Enable Register B (IERB)
Internal
Bit Priority
Number Level
7
6
APPENDIX F
Interrupt
Source
I/O Port Bit 7 Monochrome monitor detect
I/O Port Bit 6 RS-232 Ring Indicator (RI)
Timer A
USART
USART
USART
USART
Timer B
Interrupt
Source
7* I/O Port Bit 5 Floppy drive/DMA port
6 I/O Port Bit 4 Keyboard and MIDI
Function
User-defined timer interrupt
RS-232 receive buffer full
RS-232 receive error
RS-232 transmit buffer empty
RS-232 transmit error
Horizontal blank counter
Function
data request
ACIA data request