NAME
timer_simulate_retrace
SYNOPSIS
#include <allegro.h>
void timer_simulate_retrace(int enable);
DESCRIPTION
The DOS timer handler can be used to simulate vertical retrace interrupts. A retrace interrupt can be extremely useful for implementing smooth animation, but unfortunately the VGA hardware doesn´t support it. The EGA did, and some SVGA chipsets do, but not enough, and not in a sufficiently standardised way, for it to be useful. Allegro works around this by programming the timer to generate an interrupt when it thinks a retrace is next likely to occur, and polling the VGA inside the interrupt handler to make sure it stays in sync with the monitor refresh. This works quite well in some situations, but there are a lot of caveats:
You can´t use the retrace simulator in SVGA modes. It will work with some chipsets, but not others, and it conflicts with most VESA implementations. Retrace simulation is only reliable in VGA mode 13h and mode-X.
Retrace simulation doesn´t work under win95, because win95 returns garbage when I try to read the elapsed time from the PIT. If anyone knows how I can make this work, please tell me!
Retrace simulation involves a lot of waiting around in the timer handler with interrupts disabled. This will significantly slow down your entire system, and may also cause static when playing samples on SB 1.0 cards (because they don´t support auto-initialised DMA: SB 2.0 and above will be fine).
Bearing all those problems in mind, I´d strongly advise against relying on the retrace simulator. If you are coding in mode-X, and don´t care about your program working under win95, it is great, but it would be a good idea to give the user an option to disable it.
Retrace simulation must be enabled before you use the triple buffering functions in a mode-X resolution. It can also be useful for simple retrace detection, because the polling vsync() function can occasionally miss retraces if a soundcard or timer interrupt occurs at exactly the same time as the retrace. When retrace interrupt simulation is enabled, vsync() will check the retrace_count variable rather than polling the VGA, so it won´t miss retraces even if they are masked by other interrupts.
SEE ALSO
install_timer(3alleg), retrace_count(3alleg), retrace_proc(3alleg), request_scroll(3alleg), vsync(3alleg), timer_can_simulate_retrace(3alleg), timer_is_using_retrace(3alleg), enable_triple_buffer(3alleg), ex3buf(3alleg)