; Program to show clock interrupts of Z80 System
; Written for Z80 Emulator

; interrupt handling routine must reside at address 0008
; to correspond with RST 8 instruction for a clock interrupt in
; the Z80 system

; this interrupt service routine shows some typical aspects of
; Z80 interrupt programming:
; - the use of EXX to quickly make available an alternate register set
;   without the overheads of stacking to main store
; - the use of B as a counter for a looping instruction
; - the use of C (the unused half of BC) to store the port address

INTERRUPT EQU 1000            ; define the start for the main routine

          ORG #8
SERV_INT: EXX                 ; exchange register sets
          LD HL,INT_MESS      ; point to interrupt message
          LD B,25             ; message length
          LD C,1              ; port for output (console screen)
          OTIR                ; output the message
          EXX                 ; get back our first set of registers
          EI                  ; re-enable interrupts that were disabled
          RETI                ; return from interrupt service routine

; this following routine is a simple loop to continuously display a message
; we'll use an alternate format of the OUT instruction just to be different

          ORG INTERRUPT
DISP_MESS:LD HL,STD_MESS      ; point to standard message
          LD B,20             ; message length
LOOP:     LD A,(HL)           ; get next character
          OUT (1),A           ; output character to console screen
          INC HL              ; point to next character
          DJNZ LOOP           ; repeat until all have been printed
          LD A,R              ; get value of refresh register
          ADD A,32            ; offset to get normal ASCII character
          OUT (1),A           ; display this value
          LD A,13             ; now define a carriage return
          OUT (1),A           ; and display it
          JR DISP_MESS        ; just continue forever

INT_MESS: DEFM **
          DEFB 32
          DEFM Servicing
          DEFB 32
          DEFM interrupt
          DEFB 32
          DEFM **
STD_MESS: DEFM Normal
          DEFB 32
          DEFM processing...