Editing IBM PALM processor (section)

== Interrupt Architecture ==

Four hardware interrupt levels, in priority order:

{| class="wikitable styled-table" style="width:100%; text-align:center;"
|+'''PALM interrupt levels'''
! Level !! Priority !! Source(s)
|-
| '''0''' || Lowest (normal execution) || User program / interpreter
|-
| '''1''' || Higher || BSCA / Asynchronous Communications adapter
|-
| '''2''' || Higher || Tape drive, diskette drive, printer, Serial I/O
|-
| '''3''' || Highest || Keyboard
|}

The keyboard is the highest-priority interrupt source — appropriate for an interactive APL workstation where user input must never be missed.

Switching to a higher level changes the register bank in one cycle; no state is saved because each level has its own complete bank.