## Input/Output Device Driver Reference

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### ROM BIOS

During OS/2 system initialization processing, the APM ROM BIOS support is queried. The BIOS query is performed using the INT 15h, APM Installation Check Function (AH=53h AL=00h). The returned information is saved and made available to device drivers by way of the DevHelp_GetDOSVar function call ( index parameter value = 16). The returned data is checked (if an APM BIOS is present) for the types of interface connections supported (real mode, protect-mode 16 bit, or protect-mode 32 bit). OS/2 will attempt to connect to the APM BIOS using the protect-mode 16-bit interface, if available. If the protect-mode 16-bit interface is not available then OS/2 will try to connect to the APM BIOS using the protect-mode 32-bit interface. If neither of the protect-mode options are available then OS/2 will end the connection processing.

When the APM Subsystem (APM.SYS PDD) loads (with either a /B option or a failed /D option) it issues the DevHelp_GetDOSVar (index = 16) request to determine if the APM ROM BIOS is available. If so, APM.SYS will check which interface connection mechanism was established with the APM BIOS and begin using it to setup the OS/2 environment variables. (See the OS/2 Required APM BIOS functions defined in the previous table.