; FRISC CT io unit example
; ========================
;
; A FRISC CT unit is mapped to 4 word-length
; locations starting from 0xFFFF0000 and is
; attached to INT0.
; 
; The program counts 400 periods of the
; FRISC CT unit and after every 400 periods
; a in-memory counter is incremented.

CTLR `EQU 0FFFF0000
CTCR `EQU 0FFFF0004
CTIACK `EQU 0FFFF0008
CTIEND `EQU 0FFFF000C

; Initialize stack pointer to 0x10000

 `ORG 0
 MOVE 10000, R7
 JP MAIN
 
; Initialize interrupt vector
; Interrupt handler will be on 0x1000

 `ORG 8
 DW 1000

; Main program loop

MAIN 

; Initialize FRISC CT unit

; Count 400 times 

 MOVE %D 400, R0
 STORE R0, (CTLR) 

; Enable interrupts and start counting
 
 MOVE %B 11, R0
 STORE R0, (CTCR)

; Enable interrupts in CPU status register

 MOVE %B 10010000, SR
 
; Simulate work with an infinite loop

LOOP 
 JP LOOP

; Interrupt handler

 `ORG 1000

; Store context 
 
 PUSH R0
 MOVE SR, R0
 PUSH R0

; Set interrupt acknowledge

 STORE R0, (CTIACK)

; Increase counter

 LOAD R0, (COUNTER)
 ADD R0, 1, R0
 STORE R0, (COUNTER)

; Restore context
 
 POP R0
 MOVE R0, SR
 POP R0 

; Signal end of interrupt handling to IO unit
 
 STORE R0, (CTIEND)
 RETI 

; Counter state

COUNTER DW 0