[RFC 07/10] hw/mos6522: Fix initial timer counter reload

[Finn Thain]

The first reload of timer 1 is early by half of a clock cycle as it gets
measured from a falling edge. By contrast, the succeeding reloads are
measured from rising edge to rising edge.

Neglecting that complication, the behaviour of the counter should be the
same from one reload to the next. The sequence is always:

N, N-1, N-2, ... 2, 1, 0, -1, N, N-1, N-2, ...

But at the first reload, the present driver does this instead:

N, N-1, N-2, ... 2, 1, 0, -1, N-1, N-2, ...

Fix this deviation for both timer 1 and timer 2, and allow for the fact
that on a real 6522 the timer 2 counter is not reloaded when it wraps.

Signed-off-by: Finn Thain <fthain@linux-m68k.org>
---
 hw/misc/mos6522.c | 19 +++++++++++--------
 1 file changed, 11 insertions(+), 8 deletions(-)

diff --git a/hw/misc/mos6522.c b/hw/misc/mos6522.c
index 5b1657ac0d..0a241fe9f8 100644
--- a/hw/misc/mos6522.c
+++ b/hw/misc/mos6522.c
@@ -63,15 +63,16 @@ static unsigned int get_counter(MOS6522State *s, 
MOS6522Timer *ti)
     if (ti->index == 0) {
         /* the timer goes down from latch to -1 (period of latch + 2) */
         if (d <= (ti->counter_value + 1)) {
-            counter = (ti->counter_value - d) & 0xffff;
+            counter = ti->counter_value - d;
         } else {
-            counter = (d - (ti->counter_value + 1)) % (ti->latch + 2);
-            counter = (ti->latch - counter) & 0xffff;
+            int64_t d_post_reload = d - (ti->counter_value + 2);
+            /* XXX this calculation assumes that ti->latch has not changed */
+            counter = ti->latch - (d_post_reload % (ti->latch + 2));
         }
     } else {
-        counter = (ti->counter_value - d) & 0xffff;
+        counter = ti->counter_value - d;
     }
-    return counter;
+    return counter & 0xffff;
 }
 
 static void set_counter(MOS6522State *s, MOS6522Timer *ti, unsigned int val)
@@ -103,11 +104,13 @@ static int64_t get_next_irq_time(MOS6522State *s, 
MOS6522Timer *ti,
 
     /* the timer goes down from latch to -1 (period of latch + 2) */
     if (d <= (ti->counter_value + 1)) {
-        counter = (ti->counter_value - d) & 0xffff;
+        counter = ti->counter_value - d;
     } else {
-        counter = (d - (ti->counter_value + 1)) % (ti->latch + 2);
-        counter = (ti->latch - counter) & 0xffff;
+        int64_t d_post_reload = d - (ti->counter_value + 2);
+        /* XXX this calculation assumes that ti->latch has not changed */
+        counter = ti->latch - (d_post_reload % (ti->latch + 2));
     }
+    counter &= 0xffff;
 
     /* Note: we consider the irq is raised on 0 */
     if (counter == 0xffff) {
-- 
2.26.3