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eeguru
March 27th, 2011, 07:59 PM
Anyone have any insight as to if or how a RTC add-on ISA card is able to provided IRQ 8 every second? Or is it just not compatible in that respect and only provides the 0x70-71 I/O register interface?

Chuck(G)
March 27th, 2011, 09:46 PM
Are you talking about interrupt 8/IRQ 0? A 5150 doesn't have an IRQ 8 line.

eeguru
March 27th, 2011, 11:00 PM
Right, yes. My bad it wasn't until the AT IRQ 8 was added (and RTC). I guess my question becomes then - in "PC standard architecture" (whatever that means), do any programs ever use the periodic or alarm function in the RTC (tied to IRQ 8 ) do real-time book-keeping? If the yes, the same software could be running on a PC with a RTC adapter with no way of interrupt forwarding. So I'm guessing any software that did anything RTC related outside of looking at the first set of CMOS RTC registers would just break.

To simplify, what did early RTC add-on cards do with the IRQ line, nothing?

modem7
March 28th, 2011, 12:59 AM
To simplify, what did early RTC add-on cards do with the IRQ line, nothing?
Correct.

The ongoing ticking of the clock in a RTC card is something that happens within the card (via hardware). Pull the card out of the computer and it still keeps ticking (via battery power).
An RTC program runs at machine boot, reads the date/time from the RTC card (at that instant), uses that to set the DOS date/time, then terminates.

modem7
March 28th, 2011, 01:25 AM
Right, yes. My bad it wasn't until the AT IRQ 8 was added (and RTC). I guess my question becomes then - in "PC standard architecture" (whatever that means), do any programs ever use the periodic or alarm function in the RTC (tied to IRQ 8 ) do real-time book-keeping? If the yes, the same software could be running on a PC with a RTC adapter with no way of interrupt forwarding. So I'm guessing any software that did anything RTC related outside of looking at the first set of CMOS RTC registers would just break.
IBM never released an RTC card for the 5150 (PC) or 5160 (XT). Thus, third parties producing RTC cards had no 'standard' to adhere to for IBM compatibility purposes. Thus different RTC cards can be sitting at different I/O address ranges and be accessed/programmed via different commands.

No 'IBM standard' appeared until the 5170 (AT) was introduced. A programmer using the various TIME-OF-DAY functions (introduced with the 5170) in a program, or directly accessing the MC146818A RTC chip on the motherboard, will most likely add a check to the program to ensure that the program is running on an AT class one.

Chuck(G)
March 28th, 2011, 10:19 AM
Exactly so. All of the RTC stuff is present in AT+ BIOS extensions, but not the basic 5150/5160 BIOS. Interrupt 70h, IRQ 8h is used for the MC146818 clock chip. Support of all of the Int 15h, 8xh functions depends on presence of cascaded PICs and the RTC chip.

I used to have a 8-bit ISA card with a 146818 on it, but it used only a standard DOS CLOCK$ driver to provide time-of-day functions. The NVRAM was unused, as was the RTC interrupt function.

XT RTC multifunction cards that used the Motorola chip were in the minority--most often, other, smaller, less expensive chips were used.