86:Ports:03

Synopsis

Port Number: 03h

Function: Interrupt Control Port

This port controls all devices that can generate interrupts.

Bit 0: Set if the ON key generated an interrupt.
Bit 1: Set if the LCD controller generated an interrupt (after DMA for rows 17 & 49). Used by the OS as a periodic interrupt source (approximately 200 Hz).
Bit 2: Set if the /INT pin generated an interrupt (unused on the TI-86).
Bit 3: Reset if the ON key is being pressed; set otherwise.
Bit 4-7: Unused.

Bit 0: Set 0 to disable ON key interrupts and to acknowledge an ON key interrupt. Set 1 to enable ON key interrupts.
Bit 1: Set 0 to disable LCD controller interrupts & to acknowledge an interrupt from the LCD controller. Set 1 to enable interrupts from the first timer.
Bit 2: Set 0 to disable external interrupts & to acknowledge an external interrupt. Set 1 to enable external interrupts.
Bit 3: Set 0 to turn off the LCD. Set 1 to turn on the LCD.
Bit 4-7: Unused.

You need to acknowledge interrupts. Otherwise a new interrupt will be generated directly after you enable interrupts again with EI. Because that disables that particular interrupt at the same time, you may have to write a second value to this port to re-enable it.
The OS shuts down by turning off the LCD & HALTing with the ON key interrupt enabled.