Booting from a BIOS Extension

If the target has a BIOS but no mass storage device such as a diskette, hard disk, or EPROM disk, RTTarget-32 can be booted from a BIOS extension. After the BIOS has completed its initialization, it scans the address range 0C8000h to 0DFFFFh in 2k increments for an expansion ROM signature. Such a signature is produced by the Locate BIOSVector command in the configuration file. If the BIOS finds an expansion ROM, it transfers control to it.

A BIOS extension signature consists of the following byte sequence: 55h, AAh, and the number of 512 byte blocks in the BIOS extension. At offset 3, the actual BIOS extension code starts. The BIOS reads the signature and then calculates the checksum over the complete BIOS extension. Control is passed to the entrypoint only if the checksum is zero. The Locate BIOSVector ensures that the BIOS extension has the required format and provides the correct checksum.

To boot from a BIOS extension, BIOSVector, BootCode, and BootData must be specified in Locate commands. The BIOSVector must be located at an address searched by the BIOS. All program entities having an image must be located in ROM.

The boot code Biosboot.exe delivered with RTTarget-32 is suitable for booting by this method for targets with PC compatible hardware.

Left Running a Program on the Target

Booting from Disk

Down Booting from the CPU Reset Vector