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--- base:efficient_tod_initialisation [2019-05-12 20:34] – silverdr
+++ base:efficient_tod_initialisation [2020-01-31 17:49] – [Proper solution] silverdr
@@ Line 45: / Line 45: @@
 which correspond to the four possible C64 hardware setup combinations. Comparing to $51, which falls more or less in the middle, gives us the expected answer. If the HI byte of the timer has value higher than $51 we have 60Hz supplied to the TOD pin. If OTOH it has lower value, we have 50Hz supplied.
-Please also note that we use CIA #2 and not CIA #1. CIA #2 is chosen because changing the timer values there does not affect regular IRQ timings. Moreover, KERNAL re-initialises those timers whenever it wants to use them. This saves as a few bytes, which would otherwise be needed to save and restore timer registers' original values. Last and least we do not disable NMIs as it is assumed that checking/setting the params will be done as part of application initialisation, before setting up IRQ/NMI handlers.
+Advantages:
+  * Does not break on Super-CPU and similar
+  * No screen side-effects
+  * Short and fast
+Please also note that we use CIA #2 and not CIA #1. CIA #2 is chosen because it timers are by default not used by the system. Moreover KERNAL initialises those timers according to its needs, whenever it wants to use them. This saves us a few bytes, which would otherwise be needed to preserve original settings. Last and least we do not disable NMIs as it is assumed that checking/setting the params will be done as part of application initialisation, before any IRQ/NMI handlers are set.
 ===== Dual purpose =====
 I mentioned before that this routine can serve also another purpose and give us potentially even more savings. Yes, with addition of only a few bytes:
@@ Line 62: / Line 67: @@
     ; we run on PAL machine with 60Hz TOD clock
 </code>
-we can determine not only the TOD frequency but also the video standard. This is possible because – unlike the TOD frequency – the CPU clock frequency is directly related to the computer's video standard. PAL computers have their CPU (and CIAs) clocked at 985248.444Hz while NTSC ones run faster and have their CPU/CIAs clocked at 1022727.14Hz. This is a side-effect of deriving all (except TOD) required clock frequencies from a single crystal of either 14318180Hz (for NTSC machines) or 17734472Hz (for PAL ones).
+we can determine not only the TOD frequency but also the video norm. This is possible because – unlike TOD frequency – the CPU clock frequency is directly related to the computer's video standard. PAL computers have their CPU (and CIAs) clocked at 985248.444Hz while NTSC ones run faster and have their CPU/CIAs clocked at 1022727.14Hz. This is a side-effect of deriving all (except TOD) required clock frequencies from a single crystal of either 14318180Hz (for NTSC machines) or 17734472Hz (for PAL ones).