NMI Version 1.01 Have you ever seen the message 'PARITY CHECK' on your display? If you have, you've probably noticed that it always seems to happen at the most inopportune time. You're cruising along putting the finishing touches on something that you've worked long and hard, just to look up from the keyboard to a blank display with only the words 'PARITY CHECK' in the upper left corner. Your heart sinks into your shoes, your face gets flush, and palms begin to sweat, as you try franticly to figure a way out of this one... but you KNOW that you've lost everything! Your only option is to power off and start over. The IBM-PC is a great machine, and parity checking is a wonderful concept, but I've always wished that I had been given a choice before the machine was put to sleep. Even more, I've wished that somehow the machine could have told me where it hurts before it died. I've never gotten much of anything by wishing, so I sat down to attempt to remedy the situation. NMI is a resident program to replace the non-maskable interrupt handler that the PC uses to handle parity errors. In the event of a parity error, NMI will interrupt the currently executing program(s) and begin testing memory, looking for the error. It will continue testing until either it locates a failing address, or you interrupt it by pressing a key. If a failure is located, NMI will display the failing 64K bank, exact memory address, and bit(s). You have the option to continue or terminate testing. If you interrupt the memory test, or elect not to continue testing after an error is detected, you are given the options to return to the interrupted program(s) with or without parity checking enabled, or to reboot the system. The memory test is of a non-destructive type, in that it will preserve the current contents of memory. It can be executed any time without regard for concurrently executing processes. It's also a very thorough test, reading and writing each byte of memory with several different bit patterns and checking for parity errors following each read or write operation. Every memory location is tested, including the locations used by NMI. This all sounds a bit like insurance doesn't it? It doesn't do you a bit of good unless you have a tragedy on your hands. Well, I like programs that you can play with, and since you've got this neat little memory test just sitting there in memory, why not be able to use it? NMI will execute the memory test any time you like, as many times as you like! If you're like me (a memory miser), the question in your mind right now is... 'Just how much memory does this thing eat?'. The answer, approximately 1.8K including the Program Segment Prefix and a normal size environment. In English, that means about 1.8K TOTAL! Even I can afford that. NMI (Non-Maskable Interrupt) PROGRAM LOADING --------------- NMI /N/I/T#### | | | | | Test memory #### times. | | | | If the number of test repetitions is 0 or blank, then the | | memory test loops indefinitely until an error is detected | | or you interrupt it by pressing a key. This option may be | | used to invoke the memory test from the command line or a | | batch file. NMI need not be installed to run the memory | | test. | | | Install NMI. | | This option must be selected the first time NMI is executed | if you desire to replace the current non-maskable interrupt | handler. It need not, and should not be specified again. | | No program logo display. This option is selected to suppress display of the program logo when NMI is installed. * If no parameters are specified, or the specified parameters are invalid, NMI will display the program logo followed by a summary of valid parameters. * Parameters need not be separated, however, they may be separated by one or more spaces, tabs, commas, slashes, or semi-colons. * Alphabetic parameters may be upper or lower case. * Parameters may be entered in any order or combination so long as the number of test repetitions follows the 'T' parameter and they are not separated by another parameter. NMI (Non-Maskable Interrupt) +--------------------------+ | TESTING ####K MEMORY | | Bank Bit Address | | 00 P76543210 0000:0000 | | 0000 Passes | +--------------------------+ When the memory test is invoked, either as the result of a parity error, or from the command line, the above window will be displayed in the upper left corner of the screen. This window indicates the size of the memory being tested, as sensed by NMI, the 64K bank currently being tested, and the number of successful test repetitions. In the interest of speed, the 'Address' field is updated only for each 64K bank in this window. The 'Bit' field is used to indicate failing bits and will not change unless an error is detected. If the 'Esc' key is pressed, the memory test will be terminated with parity checking enabled, and control passed to the interrupted process. Any other key will exit the memory test and display the termination options. The memory test takes approximately 35 seconds per 64K bank, longer depending upon other system activity. +--------------------------+ | ** ERROR DETECTED ** | | Bank Bit Address | | 00 P76543210 0000:0000 | | Continue testing? _ | +--------------------------+ In the event that an error is detected during the memory test, the above window will be displayed. This window indicates the failing 64K bank, exact address, and bit(s). Depending upon the display type and video mode active at the time of the failure, the failing bit(s) will be indicated either by blinking, or non-display. These bit(s) directly correspond to the failing chip(s) in the indicated 64K bank. Pressing 'Y' (or DDY) will continue the memory test starting with the next byte. The number of successful passes will be reset to zero. Pressing 'N' will exit the memory test and display the termination options. NMI (Non-Maskable Interrupt) +--------------------------+ | 1) Continue with parity | | 2) Continue w/o parity | | 3) Reboot system | | Enter selection: _ | +--------------------------+ The termination option window is displayed as a result of any key other than 'Esc' being pressed during the memory test, or electing not to continue testing following an error detection. Selecting option 1 will return to the interrupted process with parity checking enabled. Selecting option 2 will resume the interrupted process without parity checking. Option 3 will reboot the system. Pressing DDY will restart the memory test. RETURN CODES ------------ If NMI is invoked from within a batch file, or by another process, a return code may be retrieved indicating any errors. For batch files, the return code may be interrogated using the 'IF ERRORLEVEL' batch subcommand. The following table indicates possible return codes for the current release. 0 - Successful, no errors detected. 1 - Error detected in parameters, program aborted. 2 - NMI was invoked with the 'I' parameter, but is already installed. Installation aborted. 4 - Memory test already active, no memory test performed. This error can occur in multi-partition systems if memory test is called while active in another partition. It is possible for more than one error to be returned in the return code. For example, a return code of 6 indicates both errors 2 and 4 occurred. PROGRAM DESIGN CONSIDERATIONS ----------------------------- Certain performance and function trade-offs are made in the design of every program. These trade-offs are usually made in an attempt to provide the most function within the constraints imposed by the operating environment in which the program must run. In this case, the following criteria were taken into consideration. 1) All resident programs must use the smallest possible amount of memory. 2) During the memory test, it is assumed that a memory failure has occurred. It is possible that the memory failure occurred in the resident portion of DOS. For this reason, DOS function calls must not be used within the resident NMI code. No attempt is made to save and restore the contents of graphics screens. This would have required additional code and buffer space within the resident portion of the program. If a disk file had been used to save the video buffer, DOS function calls would have been required, increasing the possibility of program check during the memory test. 3) The memory test should first att...
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