s21 interrupt and Internet of Thing board

there are four interrupts: int0, int1, int2, int3
interrupt vector at M[1000-1003]
only one interrupt is accepted and it runs to completion. ***
priority: int0 > int1 > int2 > int3

instruction to support interrupt

int #n       op x 31 24  software
      interrupt 

      ei #n        op x 31 25  enable
      interrupt

      di #n        op x 31 26  disable
      interrupt

      reti         op x 31 27 
      return from interrupt

      wfi          op x 31 28 
      sleep and wait for interrup

#n is in r2 field. It is 5-bit (0..31)

when interrupt occurs, it jumps to ISR with return address stored in r31. reti terminates ISR and restores PC from r31 and enable interrupt to let other ISR run. wfi puts the processor to sleep and wait for interrupt. When interrupt occurs, the processor jumps to ISR when it returns it will continue the next instruction followed wfi.

IoT board

timer0 ties to int0
timer1 ties to int1
when reset, only int0 is enable

stack pointer is r29
return value is r28
interrupt return is r31

trap r1 #13     set timer0 range
trap r1 #14     set timer1 range

trap r1 #18     read port, r1 is port number
trap r1 #19     write port, r1 is port number

port

10   analog input (sine wave)
11   digital input (square wave)
12   random number input
13   master clock input           the number of clock since the board reset

last update 31 Jan 2025