Differences

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--- hardware:interrupts [2016/06/05 15:12]
ex_writer [User-Defined Interrupts]
+++ hardware:interrupts [2016/06/11 22:17] (current)
ex_writer [Automatic Interrupts (ACTINT)]
@@ Line 48: / Line 48: @@
   * Enable screen interrupts and set its mode in INMOD ($E)
-  * Designate the target interrupt scanline in INLIN ($F)
+  * Designate the target interrupt scanline in INLIN ($F)+++
   * Set the interrupt handler address...
     * low byte in INFBK ($D)
@@ Line 55: / Line 55: @@
   * Enable interrupts with the EI opcode
++++<wrap alert>necessary only with multiple interrupts? screen interrupts seem to function fine when INLIN isn't set </wrap>
 === Interrupt Enable and Mode (INMOD) ===
@@ Line 113: / Line 114: @@
 ===== Automatic Interrupts (ACTINT) =====
-<WRAP alert>To be written...</WRAP>
+<WRAP todo>Test and clarify</WRAP>
+The Bally Arcade system ROM provides an "automatic" interrupt service via the [[system routines:interrupt scheduler#ACTINT]] system routine.
+In the system ROM source, ACTINT is listed as follows:
+<code z80>
+MACTIN:
+        DI                      ; MAKE SURE INTERRUPT IS DISABLED
+        PUSH    AF
+        PUSH    BC
+        PUSH    DE
+        PUSH    HL
+        IM      2
+        LD      A,ITAB SHR 8
+        LD      I,A
+        LD      A,$C8
+        OUT     (INLIN),A
+        LD      A,ITAB & 0FFH
+        OUT     (INFBK),A
+        CALL    TIMEZ           ; UPDATE TIMEOUT,MUSIC AND SECONDS
+        LD      C,$0F           ; USE CT0-3
+        CALL    TIMEY           ; DEC CT0-3
+        POP     HL
+        POP     DE
+        POP     BC
+        POP     AF
+        EI
+        RET
+</code>
+ACTIN does the following:
+  * Sets Z80 interrupt mode 2 (IM2)
+  * Sets I register and INFBK to vector to ITAB on interrupt (which calls MACTIN)
+  * Sets INLIN to $C8 (i.e., line 100)
+  * Calls TIMEZ (updates game timer, blackout timer, and music processor)
+  * Calls TIMEY (decrements counter-timers CT0–CT3)
 ===== Screen Interrupt Examples =====