Counter/Timers (8254)
Counter Use by STD DOS and STD ROM
The use of these counters by STD DOS is limited to the use of
counter/timer 0 as the DOS Timer 0 for its System Tick. This counter
is programmed into Mode 2 as a rate generator for a periodic
frequency of 18.2 ticks per second. The interrupt from this timer
generates a request on level 0 (IR0) at the 8259A Interrupt Controller
on board at this frequency. STD DOS maintains its software real-time
clock with this System Tick.
Counter 0 is dedicated for STD DOS use and should not be altered in
any way by user programs. The remaining counters 1 and 2 are free
for application use and may be selected to drive interrupt request
levels 4 and 2 (IR4 and IR2) at the 8259A, respectively. Since IR4 is
used by DOS for COM1, we recommend that you use counter 2, or
wire-wrap the interrupt to a level not used by STD DOS. Refer to
Chapter 12 for further information regarding the 8259A Interrupt
Controller.
The STD ROM Development System does not use the counter/timers
and leaves them uninitialized. They remain free for user programs.
11-25
Содержание ZT 8809A
Страница 15: ...Tables Table B 9 J5 Pin Assignments B 17 Table B 10 J6 Pin Assignments B 18 Table B 11 J7 Pin Assignments B 19...
Страница 334: ...Jumper Configurations W49 W48 A W47 B A B W37 W66 A W50 B C J7 W67 Figure A 11 W37 W47 50 W66 W67 Jumper Blocks A 34...
Страница 346: ...Jumper Configurations W61 W60 W62 W63 W64 A B A B W65 Figure A 13 W60 W65 Jumper Block A 46...