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Text File | 1994-08-13 | 12.9 KB | 272 lines

OSINITER.FAX INFORMATION REGARDING INTERRUPTS AND OS/2 2.0 IN AT BUS SYSTEMS --------------------------------------------------------------- INTERRUPT PROBLEMS ON AN ISA SYSTEM On an ISA system, having a shared interrupt-request line can cause problems. ISA systems have what are called "edge triggered" interrupts whereas Micro Channel and EISA systems use "level sensitive" interrupts. "Edge triggered" interrupts can only be sensed for a very short period of time. If a second interrupt arrives from another adapter while the first interrupt is still being processed, the second interrupt will be lost. In your computer system, this situation can lead to various difficulties such as printers that do not seem to print smoothly or reliably, or communications sessions where some characters are getting lost. However, with single-tasking systems such as DOS, the two adapters that are sharing the interrupt might never cause any real problems because they might never be in use at the same time. OS/2 2.0, however, presents a different set of problems. If you have multiple serial communications adapters, there is a greater probability that you might try to use two or more of them at the same time. If some of them have previously been set up using shared interrupts, problems can occur that probably didn't happen in DOS. OS/2 2.0 can detect that an interrupt line is shared and will not allow simultaneous use. Assume that COM1 and COM3 are sharing Interrupt Request line 4 (IRQ4). If you try to use both COM ports at the same time, the OS/2 operating system will not allow the second one to start. A well-written OS/2 communications program will recognize that the port cannot be opened and an error message will be displayed. A DOS application, however, is unprepared to respond to this unfamiliar situation. It will probably suspend, waiting for the port that will not open. Another potential source of trouble is having multiple hardware adapters that are sharing the same I/O address. The various hardware adapters in your computer must have their own addresses. Consider what might happen, for example, if the commands that were meant for your printer were instead routed to your disk drive. The solution for all of these problems is to ensure that all your hardware adapters have their own unique I/O addresses and IRQ assignments. COM3 OR COM4 SUPPORT ON AN ISA SYSTEM The original ISA machine (the IBM PC-AT) allowed for the definition of up to four serial communication ports. However, there has never been any hardware architectural standard that defined the I/O port addresses or Interrupt Request (IRQ) lines associated with communication ports 3 or 4. Over the years, a convention has developed that places the port addresses for COM3 and COM4 at 03E8 and 02E8 respectively. This is a generally accepted convention, but not a standard. Check the documentation and the settings of the adapters in your system to verify your hardware environment. After you have checked and set the I/O and IRQ values on your COM ports or internal modems, you must add this information to the communications device-driver (COM.SYS) statement in the CONFIG.SYS file. You might also need to tell your communications application software where the COM ports are. ProComm software, for example, has a configuration screen that enables you to specify these settings. If the application, operating system, and hardware are not in agreement, then the application will not run. OS/2 COM ports do not need to be defined in sequence. It is acceptable to have a COM4 without having a COM3. DOS, however, might have difficulty if there is a gap in the port definition. To avoid confusion for DOS, you can define COM ports that do not have any physical adapters attached in the COM.SYS statement. These substitute definitions will serve as placeholders. COM1 and COM2 are assumed to have standard values and do not need to be explicitly set up unless you want to set some non-standard values to accommodate your particular configuration. To enable COM3 or COM4 on an ISA system, place the following in the CONFIG.SYS file: DEVICE=X:\OS2\COM.SYS (n,a,i) (n,a,i) where X = the drive where OS/2 is installed n = the COM port that you are attempting to access a = communications port I/O address (03E8, 02E8, for example) i = IRQ level, which is usually a jumper setting on the I/O adapter For example, to specify that COM3 is at address 03E8 on IRQ5 and that COM4 is at address 02E8 on IRQ10, use the following statement (assuming that OS/2 is installed on drive C): DEVICE=C:\OS2\COM.SYS (3,03E8,5) (4,02E8,10) The I/O address and IRQ level should be noted in the documentation that came with your adapter. Either or both might be fixed values or can be set to a range of values via jumpers or switches. In some cases you might find that the values are fixed or that the range of settings available to you is insufficient to avoid the sharing conflict. In that case, you must purchase a different, more versatile adapter or accept that you cannot use both adapters at the same time. SETTING THE INTERRUPT REQUEST (IRQ) LEVEL ON AN ISA SYSTEM The following information will help you determine what IRQ settings you can use for COM3 or COM4 port adapters to avoid shared interrupts. On an ISA machine there are a total of 15 IRQ levels available. Many of these are already being used. Most are already in use because they are the the standard settings for the more common devices. These standard settings are as follows: IRQ LEVEL DEVICE ASSOCIATED 0 System Timer 1 Keyboard 2 Secondary Interrupt Controller (see note) 3 COM2 (Serial Communications Port 2) 4 COM1 (Serial Communications Port 1) 5 LPT2 (Parallel Port 2) 6 Diskette 7 LPT1 (Parallel Port 1) 8 Realtime Clock 9 open 10 open 11 open 12 open 13 Math Coprocessor 14 Hard Disk 15 open NOTE: On the IBM-AT (ISA bus), the IRQ9 pin is identical with the IRQ2 pin on the original IBM-PC. If you have an older, 8-bit adapter whose documentation states that it uses IRQ2, be aware that this will actually be interpreted as IRQ9 when plugged into the 16-bit ISA bus. The IRQ levels shown as "open" have no established, standardized use. When setting the IRQ values on your COM3 or COM4 ports, you are likely to find these levels available to use without conflict with some other adapter. Furthermore, if you don't have two parallel ports installed, IRQ5 might be usable for some other purpose, such as COM3 or COM4. Be cautious about doing this because it might cause a problem later if you decide to install a second parallel port. In addition, some other non-standard device might already be using IRQ5. When trying to manage the IRQ levels of your various hardware adapters to avoid conflicts, you may find that your 8-bit adapters cause problems. Except for IRQ9, only 16-bit adapters are configurable to use IRQ levels higher than 7. A glance at the IRQ table will also show that the low-numbered IRQ lines already have some standard function assigned. It might be that your only alternative for avoiding some IRQ conflicts is to purchase a more versatile 16-bit adapter. If you have non-standard 8-bit adapters, be especially careful of interrupt conflicts. For example, the SoundBlaster adapter is configured at the factory to use IRQ7. IRQ7, however, is the standard assignment for LPT1, the first printer port. This conflict might not be apparent with DOS because DOS printing typically does not use the interrupt line. OS/2 2.0, however, requires it, and the hidden conflict can become the source of printing problems. It is also fairly common to discover that the interrupt feature on your parallel port adapter does not work. In DOS, this might not have any effect. In OS/2 2.0, however, your printer might be very erratic or not work at all. Under OS/2 interrupts can not be shared. Results are unpredictable if interrupts are shared. Sharing interrupts is not a problem under DOS. It is possible for devices that are sharing interrupts to work perfectly under DOS and have problems under OS/2. (Note: On the IBM-AT (ISA bus) the IRQ9 pin is identical with the IRQ22 pin on the original IBM-PC. If you have an older, 8-bit adapter whose documentation states that it uses IRQ2 then be aware that this will actually be seen as IRQ9 when plugged into the 16-bit ISA bus.) If multiple hardware adapters of any kind (not just communications) are using the same IRQ level then the effect on your computer will be unpredictable. However, with single tasking systems like DOS, the two adapters which are sharing the interrupt may never cause any problems since they may never be in use at the same time. OS/2, however, presents a different set of problems. If we have two, three or four adapters, the probability is now high that they are used at the same time. If some of adapters had been set up using shared interrupts then the scene is set for mysterious things to occur in OS/2. OS/2 can, however, detect that an interrupt line is shared and will disallow the simultaneous use. Assume that COM1 and COM3 were sharing IRQ4 (a fairly common real situation). If we tried to use both COM ports at the same time OS/2 would refuse to allow the second one to start. A well written OS/2 communications program would see and report the error from OS/2 that the port could not be opened. A DOS application, however, will likely be unprepared to respond to this strange situation and it may simply hang there waiting forever for the port that will never open. The solution for all of this is to make sure that all of your hardware adapters have their own unique I/O addresses and IRQ assignments. Unfortunately, on an ISA machine, OS/2 has no way to query the computer to find out what these settings are. Therefore, after checking and setting the adapters according to the instruction manuals you must also tell OS/2 what you've done by placing explicit information into the CONFIG.SYS file. TO SUMMERIZE ------------ *) Even though there is some flexibility for Printer & Comm. port assignment try to stick to the standard assignment as shown in IRQ table at the beginning of document. *) Available interrupts, in order of priority, are: IRQ10, IRQ11, IRQ12, IRQ15, IRQ3 (if not used for COM2), and IRQ5 (if not used for LPT2). *) Addresses and interrupts can be assigned in OS2 to comm ports as described in info apar II06069. Standard assigment is as follows: COM1 - 3F8 - IRQ 4 (default) COM2 - 2F8 - IRQ 3 (default) COM3 - 3E8 COM4 - 2E8 There is no OS/2 default setting for COM3 and COM4. It must be specified by the device=com.sys statement in config.sys. *) Printer port addresses and IRQ levels are hardcoded in OS/2 as follows: 3BC and 378 ==> IRQ7 (LPT1) 278 ==> IRQ5 (LPT2) Unlike the the comm ports, where the addresses and the interrupts can be modified by the device=com.sys in config.sys, the printer port addresses and IRQ shown above are fixed. OS2 assigns LPT1 to the highest printer port address being used. The printer address is specified in the printer adapter board. With OS/2 you can not use both addresses 3BC and 378 as printer port addresses. Both LPTs would be sharing IRQ7. Unlike DOS, OS/2 uses interrupts for printing. The interrupt is triggered by a signal line from the printer, ACK. If the IRQs are not configured correctly, or the printer cable is missing the ACK line the printer may work under DOS and have problems under OS/2. An example of address and interrupt assignment is follows: COM1 - 3F8,IRQ4 COM2 - 2F8,IRQ3 COM3 - 3E8,IRQ5 (IRQ5 not being used by lpt2) COM4 - 2E8,IRQ10 LPT1 - 378,IRQ7 *) If interrupt devices are occasionally losing data, try moving to a higher priority unused interrupt.