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2026-09-10 13:50:56 +02:00
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+From: https://forums.atariage.com/topic/133686-please-explain-riot-timmers/#comment-1617207
+
+
+        NOTE about timer initial state from
+        (https://forums.atariage.com/topic/256802-two-questions-about-the-pia/):
+        - the increment seem to be always set to 1024T (unless you just played
+          a game and swap it with the testrom leaving the console unpowered for
+          a very short time)
+        - the actual timer value is random
+        - the interrupt flag is also random
+
+
+The documentation for the 6532 "RIOT" chip calls it the "interrupt flag register."
+The VCS.H file calls it "TIMINT" (for "timer interrupt"). It's located at address
+$5 of the RIOT chip. On the Atari 2600, the RIOT chip addresses begin at address
+$280, so that means the TIMINT register is at address $285 (and its many mirrors).
+
+This register holds two RIOT interrupt flags-- the timer interrupt flag (bit 7)
+and the PA7 edge-detect interrupt flag (bit 6). These flags will trigger IRQ
+interrupts, but they have no "real" effect on an Atari 2600, because the Atari
+2600 uses a 6507 CPU, and the 6507 chip doesn't have an IRQ line, therefore the
+Atari 2600 doesn't have IRQ interrupts.
+
+I don't know if the PA7 edge-detect interrupt flag has any significance for
+Atari 2600 programming, so I won't try to explain it. But the timer interrupt
+flag can be used to tell if the timer has counted down past 0.
+
+Whenever you set the interval timer by writing a value to TIM1T, TIM8T, TIM64T,
+or T1024T, the timer interrupt flag will be cleared automatically. The timer
+then counts down to 0, starting from the value you've set it to. The first
+decrement occurs right after you set the timer (i.e., 1 cycle later), but the
+subsequent decrements occur at the interval you've selected (i.e., after every
+1 cycle, 8 cycles, 64 cycles, or 1024 cycles). When the timer reaches 0, it
+will stay 0 for the indicated interval, and will then wrap around to 255. The
+moment it wraps around from 0 to 255, the timer interrupt flag will be set.
+Once the timer has wrapped around and the timer interrupt flag has been set,
+the timer will decrement every 1 cycle.
+
+Whenever you read the timer or the timer interrupt flag, the timer interrupt
+flag will be cleared. However, the flag will *not* be cleared if you read it
+(or the timer) at the same moment that the flag is set.
+
+Anyway, let's suppose you write a value of 5 to TIM8T, which will cause the
+timer to count down from 5 to 0 at an interval of 8 cycles. The timer and the
+timer interrupt flag will contain the following values:
+
+    Instruction: | Cycle:	  | Timer:	     | Flag:
+    -------------|------------|--------------|-------------
+    LDA #5	     | 00, 01	  | ???, ???	 | ???, ???
+    STA TIM8T	 | 02, ... 05 | ???, ... 005 | ???, ... 000
+    		     | 06, ... 13 | 004, ... 004 | 000, ... 000
+    		     | 14, ... 21 | 003, ... 003 | 000, ... 000
+    		     | 22, ... 29 | 002, ... 002 | 000, ... 000
+    		     | 30, ... 37 | 001, ... 001 | 000, ... 000
+    		     | 38, ... 45 | 000, ... 000 | 000, ... 000
+    		     | 46		  | 255		     | 128
+    		     | 47		  | 254		     | 128
+    		     | 48		  | 253		     | 128
+    		     | etc.	      | etc.		 | etc.
+
+In other words, "LDA #5" takes 2 cycles, and we don't know what the timer or
+the flag are set to. "STA TIM8T" takes 4 cycles. On the last cycle of that
+instruction, the timer will be set to 5, and the flag will be cleared. On the
+very next cycle, the timer will decrement from 5 to 4, but the flag will still
+be 0. The timer will stay at 4 for 8 cycles, then it will decrement to 3 and
+stay at 3 for 8 cycles, etc. When the timer reaches 0, it will stay at 0 for
+8 cycles.
+
+Then the timer will decrement from 0 to 255, and the flag will be set. The
+timer flag is bit 7, so it has a value of 128. The timer will continue
+decrementing, but it will decrement once each cycle, rather than every 8 cycles
+as before.
+
+One way to check when the timer has finished counting down is to read the timer
+(or INTIM), which tells us what value is in the timer at that moment. We could
+then test the value of the timer to see if it's equal to 0, not equal to 0,
+plus/positive (0 to 127), or minus/negative (128 to 255). But that method can
+present problems, depending on what value we set the timer to, and how we're
+testing its value. For example, if you set the timer to 255, and then test
+whether its value is minus, then it will be minus right away. Or if you set the
+timer to 64, and then test whether its value is plus, then it will be plus
+right away. A more generic test would be to see if its 0 yet-- but that would
+mean you aren't waiting until it's counted *past* 0.
+
+Another way to check when the timer has finished counting down is to read the
+timer flag (or TIMINT). As long as TIMINT is still 0, the timer hasn't finished
+counting down yet. But if TIMINT is 128, then the timer has finished counting
+down. If you aren't worried about the PA7 edge-detect interrupt flag (bit 6 of
+TIMINT), then you can just use "not equal 0."
+
+So let's say you want to wait for 20 scan lines before doing something. A scan
+line is 76 cycles long, so 20 scan lines would be 1520 cycles. Dividing that by
+64, we get 23.75. So we could do the following:
+
+        LDA #23
+        STA TIM64T
+    ; do some stuff here, then start checking the timer flag
+    LOOP
+        LDA TIMINT
+        BEQ LOOP; this will keep looping as long as the flag isn't set yet
+        STA WSYNC; finish off the scan line so we resume at the beginning of a scan line
+    ; now we've waited 20 scan lines
+
+Note that it takes time to set the timer, and it takes time to read the timer,
+so that needs to be factored into the amount of time we set the timer for. You
+might need to adjust the number you set the timer to in order to get the desired
+result. For example, we might need to use "LDA #22" instead of "LDA #23."