Troubleshooting HIMEM.SYS (78609)

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HIMEM.SYS Auto-detection Strategy

Selection of A20 handlers is based on an auto-detection strategy. HIMEM runs through all of its auto-detection schemes trying to find a match. If found, the particular handler is installed. If no match is found, the default A20 handler is installed. There are a number of different auto-detection routines and these are done in a certain order. Sometimes, due to the non-standard nature of the machine, the auto-detection fails and the default handler is installed. However, the default handler does not work too well. In this case, the user has to give an explicit override and ask HIMEM to install a particular A20 handler. In other cases, HIMEM may identify a machine incorrectly and install the wrong A20 handler. Basically, the machine identification strategies are not, and cannot be, foolproof. Troubleshooting this kind of problem can be done only by trial and error.

HIMEM.SYS and /INT15=xxxx Switch

When HIMEM starts, it grabs all of the INT 15h memory present in the system. It then provides extended memory to different applications via the XMS interface. However, there are some old applications that do not understand XMS and need some INT 15h memory to be present. To support these applications, HIMEM.SYS provides a /INT15=xxxx switch on its command line. Using this, you can ask HIMEM.SYS to leave xxxx Kbytes of extended memory on the INT 15h interface so that applications can use INT 15h to allocate extended memory. For example, Pharlap DOS Extender needs this switch. The use of this switch is documented in the version 5.0 "Microsoft MS-DOS User's Guide and Reference."